Glossary of GCSE Physics Terms

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

A

If we cool a substance, its molecules move less. Absolute zero is the temperature at which molecular activity is at a minimum; it is the lowest possible temperature theoretically achievable by a system. It is denoted by 0 K, which is equivalent to -273.15°C. In practice, it seems like absolute zero is impossible to reach - but we can get to less than a billionth of a Kelvin of it.

Acceleration *

Change of velocity per second (in metres per second per second, ms^-2). Deceleration tells you how much slower it gets every second. The acceleration due to gravity is roughly 10 ms^-2. It is often given the symbol 'g'. See also weight and mass.

Accommodation

(1) The process in which the eye to changes its focal length in order to focus on objects at varying distances. To focus on a close object, the eye lens will be fat (having a short focal length). To focus on a far object, the lens will be pulled into a thin shape having a long focal length. Failure of accommodation gives rise to long-sightedness or short-sightedness. (2) In ordinary English usage, accommodation means where people live.

Acid Rain *

Rain that is acidic due to dissolved gases, such as sulphur dioxide, in the atmosphere. These dissolved gases are present because of the burning of fossil fuels.

Activity *

The number of atoms of a radioactive sample that decay each second. The units are becquerels (Bq).

Aerodynamic

Aerodynamic objects have very little air resistance: they are streamlined. This means that they can move at greater speed through the air. As objects go faster the aerodynamic drag increases. Therefore, an aerodynamic object will be able to travel faster than a non-aerodynamic one.

Alpha (α) Particle *

A helium nucleus (two protons and two neutrons) ejected from an unstable nucleus. α particles are stopped by a few centimetres of air, or a sheet of paper. They are strongly ionising radiation, due in part to its relatively large mass and double positive charge- it is about 8000 times as massive as beta particles.

Alpha (α) Radiation *

The stream of alpha particles emitted by certain unstable radioactive isotopes.

Alternating Current (a.c.) *

Electrical current that continually changes direction (as opposed to direct current (d.c.)) In the UK, mains electricity is a.c., with a frequency of 50 Hz and an r.m.s voltage of 230 V.

Alternator

A device for generating a.c. electricity. It consists of rotor coils (with a d.c. supply) and stator coils which produce the a.c. electricity.

Ammeter *

A device for measuring current. An ammeter is always connected directly into a circuit in series with the components through which the current is flowing. A good ammeter will have virtually no resistance.

Amp / Ampere *

The SI unit of electrical current. At a simple level it can be defined as the number of coulombs of electrical charge flowing per second.

Amplifier *

A component of an electronic system, such as a radio or TV, that makes all the input signals louder. Ideally, an amplifier will increase the amplitude of all input signals by the same factor. Amplifiers require a power supply, which can be battery or mains. The voltage gain or the power gain can be used to describe how much amplification is taking place.

Amplitude *

The height of a wave, measured vertically from the centre line to a crest or a trough.

Amplitude Modulation

The changing of the amplitude of a radio-frequency or microwave frequency carrier wave. The other method of modulation is called frequency modulation.

Analogue Signal.

A signal that varies continuously in amplitude or frequency between a minimum and a maximum value. A microphone is an analogue input device, because it can produce a range of voltage levels. Contrast analogue signals with digital signals

AND Gate

A logical device. It usually takes two inputs (although more are possible) and produces only one output. The logic state of the output depends on the logic state of the inputs. This is shown in the truth table below. A and B are the inputs and Z is the output state. The output is on only if both inputs are both on.

Angle of Incidence *

The angle between the normal and a ray incident to a surface.

Angle of Reflection *

The angle between the normal and a ray reflected from a surface.

Angle of Refraction *

The angle between the normal and a ray refracted at a surface

Antenna

Radio arial.

Armature

A multi-coil rotating assembly, used in commercial motors instead of the single rotating coil used in simple classroom motors. A segmented commutator is used to connect each of the armature windings to the power supply in turn, thus providing smoother rotation of the motor.

Asteroid

A lump of rock which orbits around the sun. (The biggest asteroids are about 100km across, although many are 1km or less across. Asteroids orbit the Sun in the asteroid belt, between the orbits of Mars and Jupiter.

Astronomy *

The study of objects (such as stars, planets, comets, asteroids, nebulae, star clusters and galaxies) and happenings outside the Earth's atmosphere. It includes the study o the formation and development of the universe.

Atmosphere *

All the gas (mostly air) that surrounds the Earth, extending from the Earth's surface to outer space.

Atom *

The smallest possible particle of any chemical element. For example, an atom of carbon is the smallest possible unit of carbon. The size of an atom is approximately 10^-10m. It consists of a nucleus (containing protons and neutrons) surrounded by electrons, which can simplisticly be thought of as 'orbiting' the nucleus. The reason why atoms of oxygen are different from those of carbon (or any other element) is down to the structure within the atom itself. Atoms of different elements have different numbers of protons. Atoms of the same element have the same number of protons. For example, atoms of carbon always have six protons. Atoms of oxygen always have eight protons. The nucleus is very small, approximate diameter is 10^-14m. Atoms are usually electrically neutral, i.e. they the same number of protons as electrons so that the positive and negative charges cancel out. Atoms that have lost or gained are know as ions.

Atomic Nucleus *

Where neutrons and protons are tightly bound at the centre of an atom. . Nuclei are many thousands of times smaller than the atom itself. For example, if an atom was the size of a football stadium, the nucleus would be comparable to a pea.

Atomic Number Z / Proton Number Z*

The number of protons in a nucleus.

Audio Signal *

An electrical representation of a sound.

Average Speed *

This is usually calculated over the whole distance and time of the journey. For example a train may cover a distance of 240 km in a time of 3 hours. The average speed can be worked out as follows:

In kilometres per hour: speed = 240 km / 3 hr = 80 km/hr

In metres per second: speed = 240 000 m / 10 800 s = 22.2 m/s

Average speed is different from instantaneous speed.

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B

Background Radioactivity *

The radioactive radiation which occurs naturally in the rocks, air and water around us.

Balanced *

Equal in magnitude (size), but opposite in direction, therefore summing to zero. Example: balanced forces.

Battery *

A number (or battery) of cells, connected in series with the positive terminal of one cell connected to the negative of the next. Batteries provide a d.c.(direct current) source of electrical energy.

Becquerel (Bq)*

This is a measure of the activity of a radioactive sample. One Bq means one nucleus decay per second. So a sample with an activity of 12 Bq is undergoing an average of 12 nuclear decays per second.

Beta (β) Particle *

A fast moving electron ejected from a certain type of unstable nucleus. A β particle is stopped by a few millimetres of aluminium. They are ionising particles, but not as strongly ionising as α (alpha) particles.

Beta (β) Radiation *

Beta particles emitted by certain unstable isotopes.

Big-bang *

Cosmological theory maintaining that the Universe began as an unimaginably huge explosion, when space, time and matter came into being. Evidence for the big-bang includes redshift measurements and cosmic microwave background radiation studies of the universe.

Billion *

A thousand million.

Binary

A system of counting in twos instead of in tens (counting in tens is called decimal). Here is a comparison of the decimal and binary counting system:

Decimal			Binary
Hundreds	Tens	Units	Eights	Fours	Twos	Units
100s	10s	1s	8s	4s	2s	1s
	1	2	1	1	0	0

You should notice that each column (or place) in decimal is ten times the value of the previous one, whereas in binary, each column is only twice the value of the previous one. Also in decimal, you can have any digit from 0 up to 9 in any of the columns, whereas in binary, you can have either a zero or a one in any of the columns. In the example given, you can see that 12 in decimal is the same as 1100 in binary.

Biomass Fuel

Fuel from plant or animal waste.

Black Hole

A collapsed star of such huge density that nothing - not even light - can escape, due to the intense gravitational field.

Block Diagram

In electronics, a block diagram provides a simple way of analysing how a system works. Block diagrams don't show details of how components work, but simply show how information or signals should travel through the system. They are usually drawn using rectangles and arrows.

Boiling *

The process by which liquids change into gases at temperatures at or above the boiling point. It occurs throughout the whole of the liquid.

Braking Distance *

The distance travelled by a vehicle during the time that its brakes act.

Brownian Motion

The continuous, random, jerky motion of pollen grains or similarly sized particles due to air molecules colliding with them.

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C

Cable

Two or three insulated wires surrounded by an outer sleeve of rubber or plastic. A cable is not designed to be flexible or to be moved about.

Camera

An instrument for photographing an object.

Cancer

Cancer is a growth of out-of-control cells. Localised cancer growths are called tumours. a, b, g or X-ray radiation can be used to diagnose and treat cancer.

Capacitor

A capacitor is a device which can store electrical charge. Capacitors are often used to introduce a time delay in electronic circuits. The 'capacitance' of a capacitor is the amount of charge it can hold for each volt. The SI unit of capacitance is the farad (F).

Carbon Dating / Radiocarbon Dating

This is a form of radiometric dating that analyses the proportion of Carbon-14 in an organic sample.

Carrier Wave / Carrier Signal

This is used to carry information from transmitter to receiver. The carrier wave will be radio or microwave frequency. There are two methods of using the carrier wave to carry information - amplitude modulation and frequency modulation. In radio transmissions for example, the carrier is of a much higher frequency than the audio wave which is to be transmitted. The higher the frequency of the carrier, the higher the quality of the transmitted signal.

Cathode

(1) In an electrolytic cell, the cathode attracts and reacts with positively charged ions. (2) In a cathode ray tube (and vacuum tubes in general) the cathode collects and conducts electrons. (3) The term 'cathode' applies to electrodes in other devices too.

Cathode Ray Tube (CRT)

This is an evacuated tube containing an electron gun (a source of electrons) and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam. The image may represent electrical waveforms (oscilloscope), pictures (television, computer monitor), radar targets, etc.

Cell *

(1) An electrochemical cell is a source of d.c. electricity: chemical energy is converted to electrical energy. An electrochemical cell can be made using two different metals inserted into a salt solution, for example. The metals react with the salt solution at different rates and in so doing, generate a small electrical current. Several cells can be combined to produce a battery of cells.
(2) An electrolytic cell is the opposite of an electrochemical cell, in that electrical energy is converted to chemical energy (electrolysis takes place.)
(2)In biology, cells are the basic building blocks of all living things. Plant and animal cells alike are surrounded by a cell membrane which controls the entry and exit of substances. The inside of the cell is filled with a liquid called 'cytoplasm' and it contains a very small nucleus which controls the behaviour of the cell. These cells can be damaged or destroyed by ionising radiation such as α, β, or γ. The amount of cell damage caused by radiation depends on:

the type of radiation
the type of tissue or cells
the total amount of energy absorbed

D

Decay / Disintegration *

(1) See radioactive decay / radioactive disintegration. (2) In ordinary English usage, decay means 'decompose'.

Deceleration *

Slowing down - reducing speed. Like acceleration, deceleration is measured in ms^-2.

Decibel (dB) Scale *

The decibel scale is used to measure the volume (intensity or loudness) of sound. 80dB is considered to be the danger level, at which hearing damage starts to occur.

Decoder

This part of a radio removes the carrier wave from the modulated signal, leaving only the audio wave, which is then amplified. In TVs there are two separate decoders - one for the picture and one for the sound.

Degree

(1) A degree Celsius (°C) and a degree Kelvin (°C) are units of temperature. (2) In mathematics, a degree is a measure of angle. (3) In ordinary English usage, it often means 'amount'.

Density

The mass per unit volume of a substance.

Deflection Plates

These are located in the cathode ray tube of an oscilloscope and are used to deflect the electron beam to the required parts of the screen. One set of parallel plates control horizontal movement of the beam; a second set control vertical movement. Each set of plates controls the beam by having a certain voltage applied across the plates.

Diffraction *

Diffraction is a wave phenomenon - i.e. all types of wave can diffract. Diffraction means the bending of wave (change in direction) and it occurs whenever a wave passes near to an obstacle. Longer wavelength waves diffract through a greater angle than shorter wavelength waves. Do not confuse diffraction with refraction.

Digital

Digital can refer to electronic signals or to components of an electronic system. Digital signals have only two voltage levels, described as 'HI' and 'LO', or 'OFF' and 'ON'. In a real component, 'HI' might be represented by +5V d.c. and 'LO' by 0V d.c. A digital component is one which works with only digital signals. Here are some examples of digital components: LED (output), photodiode (input), switch (input). Contrast digital signals with analogue signals.

Diode *

A diode is a semi-conductor device. Diodes will allow current to pass through them in only one direction.

Direct Current (d.c.) *

Electrical current only flows in one direction from a d.c. power supply (as opposed to alternating current (a.c.))

Discharge Lamps

See under Gas Discharge Lamps

Displacement

Distance moved in a specified direction. A vector quantity.

Distance

Distance moved without considering direction. A scalar quantity.

Diverging Lens / Concave Lens

A lens that causes parallel light rays to diverge (spread out) in such a way that they appear to have come from a single point.

Dose Equivalent

This is the biological risk due to exposure to radiation. The SI unit of dose equivalent is the sievert (Sv). The damage caused to cells depends on three major factors (see under cells).

Double Insulation

An electrical appliance which is double insulated does not have an earth wire fitted. The appliance is designed in such a way that the electrical parts can never come into contact with the outer casing of the device. Common double insulated appliances are hair dryers, radios and cassette players.

A wet double insulated appliance is exceptionally dangerous - water is a good conductor of electricity and will easily reach the live electrical components within the case. Any human user touching the casing will then receive an electric shock. It is important to note that the human conductivity is increased when hands are wet, allowing a greater current to flow through the casualty.

For this reason, do not operate a mains radio, hairdryer or double insulated appliance in any wet area - especially a bathroom

Do not touch any person who has been electrocuted until you are sure that the electricity supply has been shut off.

Drag / Viscous Drag *

Drag is a type of friction force usually associated with movement through a fluid like air or water. Drag forces generally increase at high speeds.

Dynamo *

A device (a tranducer) which converts kinetic energy into electrical energy. Dynamos are often used to produce a.c. electricity on a push-bike. Unlike a full-blown generator, the rotor coil is replaced with rotating permanent magnets. These magnets rotate near to the stator coil, (wrapped around a soft iron core) and cause a small current to flow (via electromagnetic induction).

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E

Earth (planet) *

Planet Earth has the third most distant orbit from our star the Sun. To the best of our knowledge it is the only place in the entire universe where we know that life exists (well, some people have made this claim at least) - although this may change as we explore Jupiter's moon Europa. It is the only place in the Solar System where large quantities of liquid water exist (oceans).

Earth (wire) *

If fitted, the earth core of a flex connects the casing of the electrical appliance to ground via the earth terminal of the plug. Its insulation is coloured green and yellow. In the event of an appliance malfunction resulting in the casing becoming live, the earth wire provides a route to ground for the current. This will help prevent electric shock. If the earth wire is not present and the casing is live, any person touching the casing will receive a shock as the current flows to earth through them. If the earth wire is intact, its low resistance will result in a very high current flowing through live. This should cause the fuse in the plug to blow and remove any fire hazard.

Note that some devices are double insulated and have no earth. These devices must not be used near water or steam

Do not touch any person who has been electrocuted until you are sure that the electricity supply has been shut off.

Efficiency *

No machine can be 100% efficient. Efficiency is defined as 'Useful Energy Out' / 'Total Energy In'. It is a fraction, sometimes written as a percentage. For example, if an electric motor in a winch consumes 2000J of energy per second, and does 420J of useful work per second, then the efficiency of the winch is, eff = 420/2000 = 0.21 = 21%. Note that efficiency has no units. Note that since 2000J per second is the same as 2000W power, you can do the efficiency calculations with power too.

Elastic *

Able to return to its original size and shape after having been deformed.

Elastic Limit *

The point beyond which stretching the object will deform it in such a way that it won't return to its original shape once the stretching force is removed.

Electrode *

An electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. an electrolyte, a vacuum or a semiconductor). An electrode can be either a cathode or an anode.

Electrolysis *

A method of using a direct electric current (DC) to drive a chemical reaction. Electrolysis is commercially highly important in the separating elements from ores.

Electrolyte *

Any substance containing free ions that make the substance electrically conductive.

Electromagnet *

An electromagnet is a solenoid with an iron core inserted into it. If a current flows in the coil, a magnetic field is generated. All the randomly oriented domains of the iron core then align in the presence of the field of the solenoid. Thus, the core greatly enhances the strength of the electromagnet.

Electromagnetic Induction *

If a conductor is moved within a magnetic field, then a current will be induced (caused) in the conductor. If the magnetic field is reversed or if the direction of movement is reversed, then the direction of the current flow will also reverse.

This is a very similar effect to the movement of a current carrying wire in a magnetic field: a non-magnetisable copper wire placed in a magnetic field can be made to move if a current is passed through it. If either the direction of the magnetic field or the direction of the current is reversed, then the movement will be in the opposite direction.

Electromagnetic Radiation (EMR) *

This is a form of energy emitted and absorbed by charged particles, which exhibits wave-like behavior as it travels through space. It has both electric and magnetic field components. In vacuum, electromagnetic radiation travels at the speed of light, which is 3 x 10⁸ ms^-1.

Electromagnetic Spectrum / EM Spectrum *

This is a list of all the different types of EM radiation. Starting with the lowest frequency and longest wavelength radiation, the spectrum consists of

Radiowaves
Microwaves
Infrared Radiation
Visible Radiation
Ultraviolet Radiation
X-Ray Radiation
Gamma Radiation

F

Far-sighted

See under long-sighted.

Farad

The farad (F) is the SI Unit of capacitance. Since the farad is a massive unit, most capicitances are given in micro-farads.

Fibrescope

A fibrescope (sometimes called a flexible borescope) is a flexible bundle of optical fibres with an eyepiece at one end, and an objective lens at the other. It allows the examinination of the inside of things that cannot easily be otherwise accessed . Fibrescopes used by doctors to view inside a patient without cutting them open are usually called endoscopes. There are two separate fibre bundles - one for taking light down into the patient, called the lightguide. The other is for forming an image and is called the image guide. The image guide must be a coherent fibre bundle.

Field

A region of space that exerts a force on any object in that space. The field can be a gravitational field, electrostatic or magnetic.

Filament Lamp *

These lamps rely on an incandescent metal filament to produce light. The interior of the lamp is filled with an inert gas in order to stop the filament from corroding. An electrical current, passed through the filament makes it extremely hot. When sufficiently hot, the filament itself will emit light radiation. Tungsten metal is chosen for the filament due to its high melting point of about 3956 celsius.

Note that filament lamps produce both light and unwanted heat. As such they are less efficient than fluorescent tubes

Fissile

An unstable material whose atomic nuclei can easily be split.

Fission / Nuclear Fission / Atomic Fission *

This usually refers to nuclear fission, where an atomic nucleus splits apart into smaller pieces. Fission should not be confused with fusion. When a heavy, unstable nucleus undergoes fission, energy is released as heat. In nuclear power stations, uranium is often used as the fissile material, where a nuclear chain reaction is controlled to release energy.

Fleming's Left Hand Rule / Motor Rule *

The rule, devised by Fleming, which uses the left hand to work out the direction of the force produced on a wire carrying a current in a magnetic field that is perpendicular to the wire.

Flex *

A flexible insulated cable that allows electrical current to flow into domestic appliances from the mains socket. A flex consists of two or three insulated wires surrounded by an outer sleeve of rubber or plastic. The flex should be correctly wired to a plug.

Fluid *

Something that can flow and take on the shape of a container. Its molecules are not rigidly bound to each other. Often mistaken as being another word for liquid; in fact gases, plasmas and liquids are all fluids.

Fluorescent *

Able to give off visible light when bombarded with radiation - such as electromagnectic radiation, electrons, nuclear particles etc.

Flourescent lamps produce light by passing electrical current through a mercury vapour at low pressure. The electrical current excites ("energises") the electrons of the mercury atoms. These electrons quickly give up their energy as light. The light which is produced is invisible ultra-violet and would be useless. To get round this problem, the inside of the tube is coated with fluorescent paint, which converts U.V. light to visible light.

Note that fluorescent lamps are more efficient than filament lamps as they waste less energy as heat.

Focal Length

The focal length of lens is measured in metres. It is the distance from the centre of the lens to the point at which rays passing through the lens meet (i.e. focus). It is related to lens power.

Force *

Forces can cause an object to speed up, slow down, change direction or change shape (if they are unbalanced). Forces are either push forces or pull forces. The SI unit of force is the newton.

Fossil Fuel *

Fossil fuels are coal, oil and gas. They are non-renewable resources. Fossil fuels come from ancient remains of plants and animals, compressed and heated over many millenia. The combustion of fossil fuels is now (nearly) universally accepted as causing global warming. They are well known to produce acid rain. Most of the world's energy is still produced from fossil fuel. Here in Shetland, we paid the price in the form of the Braer tanker disaster.

Free Electrons/ Conduction Electrons *

Electrons that move about freely inside a metal and are not bound to any individual atom.

Frequency *

The frequency of a regular event is the number of times the event occurs in a given time. e.g. The Sun rises with a frequency of once per day.

The frequency of a wave is the number of complete wavelengths which pass any point in one second. The SI unit of frequency is the hertz (Hz) (equivalent to 'per second' or s^-1).

The concept of wave period is closely related to frequency.

G

Galaxy *

A grouping of about approximately 100 billion stars, held together by their own gravity. We live in the Milky Way galaxy, visible on any clear, moonless night away from Lerwick street-lights. Within a galaxy, there are many different types of star, some older, younger, brighter, dimmer, or more/less massive than the Sun. Galaxies do not all look the same - astronomer's classify them according to their appearance. Ask your teacher to show you the Andromeda galaxy - easily visible on a clear, dark night as a large misty patch. Andromeda is one of the nearest galaxies to our galaxy (the Milky Way), at a distance of two million light-years.

Gamma (γ) Radiation / Gamma Rays *

γ (gamma) radiation is high frequency electromagnetic radiation. It is a part of the electromagnetic spectrum. Gamma radiation carries no mass, no charge, travels at the speed of light. (It can be thought of as being made of particles called photons or as being a wave). It can be stopped by several centimetres of lead or a few metres of concrete. Gamma radiation is an ionising radiation, but is not as strongly ionising as a radiation. It is released during the disintegration of an unstable nucleus.

For the purposes of treating cancer, medical gamma rays from cobalt-60 are used.

Gamma Camera

This is a device used to detect the gamma radiation from a patient during an examination using a radioactive tracer. Since gamma radiation passes straight through most materials, it is not possible to build a camera with glass lenses or mirrors. Instead an array of holes drilled in a thick layer of lead is placed between the actual gamma detectors and the patient. This allows an image to be built up. Scintillation counters are used to detect the radiation after it has passed through the holes in the commutator.

Gas *

One of the states of matter, in which the substance does not resist change of shape and will expand to fill any container. The atoms or molecules are far apart and have little infuence on each other.

Gas Discharge Lamps

These lamps produce light by passing electrical current through a gas at low pressure. The electrical current excites ("energises") the electrons of the atoms in the gas. These electrons quickly give up their energy as light. Sodium is often used (in street lights) or an inert gas for colourful advertising lights. Note that discharge lamps are more efficient than filament lamps as they waste less energy as heat.

Geiger-Muller tube (GM tube) *

This is a device used to detect the presence of radiation. It relies on ionisation to produce tiny bursts of current which can be counted by a ratemeter. A popular exam question, and worth checking your class notes.

Geostationary *

A geostationary satellite is one that situated in space above the equator and orbits the Earth once per sidereal day (very close to 24 hours). As a result, it in effect "hovers" above a chosen point on the Earth's surface. Used in communications etc. A geostationary satellite is a special class of geosynchronous satellite.

Geothermal Energy *

This is a renewable energy resource, unlike fossil fuels. In some parts of the world, hot subsurface rock heats water which reaches the surface and can be used for heating or electrical power generation.

Generator / Electrical Generator *

A machine which uses motion to produce electrical energy. Sometimes called an alternator. A generator is like a dynamo, but with the permanent rotor magnets replaced with (d.c) rotor coils (electromagnets). The stator coils have an a.c. induced within them.

Gold Grain

Radioactive gold grains are used in the treatment of cancer. Gold (which is unreactive) doped with radioactive gamma-emitting nuclei is implanted in a patient, near or in a tumour. The radiation will destroy or damage cells near the grain. The radioactive material chosen will have a short half-life so that the grain's activity level will drop to background levels after several days.

Gradient *

The slope of a graph line measured as the increase of the y-axis variable divided be the increase of the x-axis variable. Can also refer to physical objects, such as a hill or a plank.

Gravitational Field *

The Earth generates a gravitational field which attracts all other masses. It is a called a force field - the word field means that the force changes as you move away from the surface of the Earth (in fact the force becomes steadily less as you move away from the surface of the Earth). It is the combined mass of all particles on Earth which cause the Earth's gravitational field.

Gravitational Field Strength (g) *

The force in newtons exerted per kilogram of mass by the gravitational field. At the Earth's surface, this is approximately 10 N/kg.

Gravitational Potential Energy *

If a mass is lifted up then its potential energy is given as follows:

E_p = mgh

E_p is the potential energy

m is the mass in kilograms

g is the strength of the gravitational field (10ms^-2 on Earth)

h is the height through which the object is lifted

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H

Half-life *: The half-life of a radioactive material is the time taken for its activity level to drop by a half. Half-life is measured using any unit of time (seconds, hours etc). For example, if the activity of a sample is 20MBq at 12:00 pm, falling to 10 MBq at 2:00 pm, then the half-life is 2 hours. At 4:00 pm the activity will be 5 MBq, etc. There is a dramatic range in the half-life from element to element. For example the half-life of ¹¹³Cd (cadmium) is 9 x 10¹⁵ years (nine million billion years), whilst the half-life of ²¹³At (astatine) is only 0.11 millionths of a second!
Hard Magnetic Materials: Materials that retain ther magnetism well.
Heat / Heat Energy: At a simple level, this is assumed to mean the same as thermal energy. (In the branch of physics called 'thermodynamics, however, the term 'heat' is used only when some kinds of energy transfer take place.)
Heat Exchanger: A device used to transfer heat from one fluid to another without direct contact of the fluids.
Heat Pump: This is a device which can move heat energy from a colder object to a hotter one, but requiring an external energy source to do it. A fridge is an example of a heat pump. Due to the second law of thermodynamics, a heat pump can never be 100% efficient.
Heater *: The operation of a bar heater is very similar indeed to a filament lamp. The difference is in the thickness of the 'filament' or element of the heater. Curiously, heaters operate at a lower temperature than lamps, but produce far more heat - as infra-red radiation. (Remember that temperature and heat are very different concepts). Note that some energy is wasted as (red) light.
Hydroelectric Power *: Power produced using the gravitational potential energy of water stored in reservoirs in mountainous regions.
Hydroelectricity *: Electriciy produced by generators using hydroelectric power.
Hyperopia or hypermetropia (hyperopic or hypermetropic eye): See under long-sighted.

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I

Inelastic *: Materials that are unable to return to their original shape after they have been deformed .
Inert *: A chemical element is inert if it does not react with anything. Examples are helium, neon, argon, krypton, xenon and radon. Inert gases can be used in filament lamps to protect the filament from destruction due corrosion.
Inertia *: Inertia is a property of matter. It describes how difficult it is to either get something moving or to stop it when it is moving. So an elephant has a lot more inertia than a paper clip. The SI unit for inertia is the kilogram. It's important to realise that, e.g. when in a spacecraft in orbit, the effects of gravity can be ignored, but inertia cannot - an astronaut can still be crushed by a stray satellite! See also: Mass.
Infrared *
: Infrared radiation is invisible. It is a form of electromagnetic radiation, with a wavelength just longer than that of visible red light. Infrared light is use in telecommunications (via optical fibres). It is used in medicine to diagnose illness (thermograms) and accelerate healing in physiotherapy. Loosely speaking, infrared radiation can be thought of as 'heat rays'.
Input Device: The first part of an electronic system, an input device typically processes requests from a human user. For example in a CD player, the 'ON' button and volume controls (potentiometer) are all input devices. Other input devices include switches, thermistors, LDRs, photodiodes.
Instantaneous *: This means an event which occurs over a very short (infinitesimal) period of time. It usually refers to speed, i.e. instantaneous speed as opposed to average speed.
Instantaneous Speed *: The speed at any particular stage or time during a journey. Instantaneous speed can be calculated by measuring short distances and time intervals. For example a train coming into a station may move forward by 0.2 m in a time of 0.5 s, giving an instantaneous speed of 0.4 m/s. Compare this with the average speed of the train over a whole journey.
Insulation *: (1) Thermal insulation is the reduction of heat transfer between objects in thermal contact or in range of radiative influence. (2) Electrical insulation is reduction of electical currents between two conductors. (3) Acoustic insulation (soundproofing) is any means of reducing the sound pressure with respect to a specified sound source and receptor.
Insulator *: (1) A thermal insulator is a material which does not conduct heat well. Ceramics, stone and plastics are good thermal insulators. Metals are poor thermal insulators as they will quickly heat up or cool down. (2) An electrical insulator, such as air, is a material that is a bad conductor of electricity. It has very high resistance. Given a suitably high voltage, insulators can be forced to conduct - take for example lightning during a thunderstorm. See also conductors and semi-conductors. (3) One form of acoustic insulator is rubber foam, which absorbs sound.
Inverter (NOT gate): A processing device within an electronic system, the NOT gate is a single input logic gate which 'inverts' its input. A logic 'HI' input will become a logic 'LO' output and visa-versa.
Ion *: An ion is an electrically charged atom. In a neutral atom, the number of protons in the nucleus is balanced by the number of electrons around the nucleus. If an electron is knocked away from the nucleus (perhaps as a result of radiation), then the atom will be left with a net positive charge. The process of creating ions is called ionisation.
Ionisation *: An atom or molecule is said to be ionised when it has gained or lost one or more electrons. If an atom or molecule gains an electron, it is negatively charged (see anion*); losing an electron makes it positive.
Isotope: Isotopes of a chemical element are versions of that particular element in that have the same number of protons and electrons but different numbers of neutrons. All isotopes of a particular element have the same chemical properties.

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J

Joule *: The joule (J) is the SI Unit of energy and of (mechanical) work. One joule is the same as one newton-metre (1 J = 1 Nm).
Jupiter: Planet Jupiter has the fifth most distant orbit from our star the Sun. It is the largest planet of our solar system, with a diameter which is 11 times that of the Earth. A so-called 'gas giant', most of the planet is made of swirling clouds of toxic gas, with wind speeds up to 450 mph. (For comparison, typical wild winter storms in Shetland which reach well over 100mph, would be very tame).

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K

Kelvin (K) *

The kelvin is the SI Unit of absolute temperature. A 1K temperature change is the same as a change of 1°C. 0K is defined to be theoretical temperature which is absolute zero; 273.16K is defined to be the temperature of the triple point of pure water (the temperature and pressure at which ice, water and water vapour can exist together i.e. 0.01°C). 273.15 K is the same as 0 ° Celsius and 373.15 K is the same as 100 ° Celsius.

Kilogram *

The kilogram is the SI unit of mass. It is defined as the mass of a particular cylinder of platinum which is kept at the International Bureau of Weights and Measures at Sevres, near Paris.

Kilowatt-hour (kWh) *

This is a unit of energy, usually used in electrical situations. Electricity companies bill their customers on the number of kWh used. One kWh means that energy has been used at a rate of 1000 W (Js^-1) for one hour. Using this, the kWh can be related to the joule as follows:

E = P t

1 kWh = 1 000 Js^-1 x 3 600 s

1 kWh = 3 600 000 J = 3.6 MJ

kWh is a unit of electrical energy and NOT power!

Kinetic Energy *

Kinetic energy is the energy of motion. It is given by the formula:

E_k=0.5 * mv²

where, E_k is the kinetic energy in joules

m is the mass of the object in kilograms

v is the velocity of the object in metres per second.

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L

Lamp *

A device intended to produce light energy. Modern lamps convert electrical energy to light energy via one of two main mechanisms - the filament lamp or the gas discharge lamp. Note that filament lamps are less efficient than discharge lamps (i.e. fluorescent lamps) as they produce more waste heat energy.

Laser *

The word laser was originally an abbreviation for Light Amplification by the Stimulated Emission of Radiation. Lasers have countless applications, many in medicine and communications. Lasers produce light which is monochromatic, coherent and tightly focussed. Lasers are used in the technique of photocoagulation (used to treat retinae).

Latent Heat *

Latent heat is heat energy used or released during change of phase.

When matter changes phase, energy must be given out or taken in. Change of phase takes place at constant temperature. For example, if a bunsen is used to bring 100ml of water to the boil, it is easy to understand that the heat energy from the bunsen increases the heat energy of the water - because the water gets hotter. During boiling however, the temperature of the water does not increase. The energy from the bunsen no longer causes an increase in temperature, but is now carried away in the steam. The energy is used to break the bonds between the water molecules instead of increasing the temperature. (Remember: heat and temperature are different concepts!)

Lens

A ground or molded piece of glass, plastic, or other transparent material with opposite surfaces either or both of which are curved, by means of which light rays are refracted so that they form a real or virtual image. A lens can either be converging or diverging.

Light Dependent Resistor (LDR) *

LDRs are designed so that their resistance depends on the intensity(brightness) of light falling on them.

When in low intensity illumination (dark!), the resistance of an LDR is high.

When in bright illumination, the resistance of an LDR is low.

Light Emitting Diode (LED *

LEDs are simply diodes which emit light when connected correctly to a power supply.

Light Waves *

Electromagnetic Waves that we can see, i.e. can be detected by the human eye.

Light-year *

The light-year (ly) is a unit of distance which is useful in astronomy. It is the distance which light (or any other form of electromagnetic radiation) travels in one year. Note that real astronomers do not use light-years, but use a unit called the parsec instead.

It is easy to work out how far a light-year is as follows:

distance = speed x time

1 ly = 3 x 10 ⁸ x 365x24x60x60 metres

1 ly = 9.46 x 10 ¹⁵ metres

To appreciate the scale of this unit, recall that the distance from the Earth to the Sun is 1.5 x 10 ⁸ metres. This is about 8 light-minutes. The distance to the next-nearest star (Proxima Centauri) is about 4 ly!

Line Spectrum *

Plural: Line Spectra. A spectrum of light with only certain wavelenghts or colours present. (See also continuous spectrum). If you use a direct vision spectroscope to view light from a (yellow) sodium vapour street-lamp, you will see that only a few wavelengths are present. This is an emission line spectrum and is unique to sodium. Sodium can also absorb these wavelengths too, so if a continuous spectrum of white light were to be shone through sodium vapour and then observed through a spectroscope, certain colours or wavelengths would be missing. This is called an absorption spectrum. These facts can be used to identify which elements are present in stars' atmospheres, by matching lines in their spectra with the spectra of elements tested on Earth.

Liquid *

One of the states of matter. The particles in a liquid are free to flow, so while a liquid has a definite volume, it does not have a definite shape. It is difficult to compress it. The molecules are close together, but not bound to each other.

Liquid Crystal Display (LCD)

This is an output device. They run on almost no power since they don't produce their own light, but work by reflecting room or daylight.

Logic Diagram

A simplified diagram of an electronic system, using rectangles to represent input and output devices along with the usual logic gate symbols for the AND, OR and NOT gate etc. Inputs and outputs are given letters (A,B,C,.. for inputs; Z for output) to help layout a truth table.

Logic Gate

Apart from AND, OR and NOT gates, other common gates are the NAND, NOR and XOR (or EOR). Logic gates allow an electronic system to make decisions based on a number of inputs. For example an OR gate can be used to illuminate a warning light if engine temperature is too high or if it is over-revved.

Logic State

In a digital system, there are two logical states. One state is 'ON', 'HI' or 'logic 1' (all mean the same). The other is 'OFF', 'LO', 'logic 0' (again all mean the same).

Long-sighted

This refers to a problem with the accommodation ability of an eye. Basically, the eye can focus on far away objects with no problem, but can't properly focus on nearby objects. There can be two causes of this. The first occurs in older people, where the aging eye lens becomes less flexible and so cannot adopt the highly curved shape needed to focus at short range. This condition is called presbyopia. The second cause is that retina is too near to the lens - so that the focal point falls behind the retina. This is called hypermetropia or hyperopia. Long-sightedness can be treated by wearing spectacles which are convex in shape and therefore help to focus the image properly by reducing the overall focal length of the glasses and eye.

Longitudinal Waves

Waves in which the particles of the medium move backwards and forwards along the same line as the direction of transfer of energy.

Loudness

The strength of a sound. Loudness depends on the amplitude of the sound wave..

Lubricant

A lubricant is a substance which reduces friction between two surfaces. Examples are oil, water, air, bananas, seaweed.

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M

Magnet *

A simple permanent magnet like a bar magnet has two poles, called 'North' and 'South'. Like poles repel and unlike poles attract. A magnet sets up a magnetic field in three dimensions around itself. Field lines can be drawn to try to represent this magnetic field. The arrows on a field line point from North to South and show the direction a North pole would move. Field lines never cross. Where they are closest together, the field is strongest.

Magnetic *

Able to attract iron and its compounds.

Magnetic Field *

A region of space that has a magnetic effect on suitable objects.

Magnetism *

Magnetism is a force that can be attractive or repulsive.

Mars

Planet Mars has the fourth most distant orbit from our star the Sun. It is slightly smaller than Earth and has a thinner atmosphere, made of carbon dioxide. Mars has carbon dioxide ice caps at its north and south poles.

Mass *

Mass refers to the quantity of matter present in an object. The SI unit of mass is the kilogram. In actual fact there are two kinds of mass - inertial mass and gravitational mass. Gravitational mass describes how an object responds to (and creates) a gravitational field.

Matter *

You can think of matter as being the "stuff" that everything is made from. Where there is something, there is matter. Otherwise, there is a vacuum.

Mechanical Waves

Waves that require a material medium to travel through.

Medium / Transmission Medium

A substance that allows energy waves to travel through. For example, the transmission medium for sound received by the ears is usually air, but solids and liquids may also act as transmission media for sound.

Mercury (metal) *

Mercury is a metal which is liquid at room temperature. It is often used in liquid in glass thermometers. It is a toxic substance. Chemicool web-page for mercury.

Mercury (planet)

Planet Mercury has closest orbit to our star the Sun. It is much smaller than Earth and has no atmosphere. It is very hard to see Mercury from Earth, due to it being very close to the Sun.

Meteor

Commonly called 'shooting stars', meteors are tiny specks of dust which the Earth sweeps into in its orbit. These dust grains enter the Earth's atmosphere at up to 70km per second - very fast! They cause the atoms in the atmosphere which they hit to glow with the heat produced as the meteor slows down. The meteor is destroyed by this process. Many meteors are particles which are left behind in the orbit of comets as they pass through the inner solar system.

Meteorite

This is a lump of material which manages to reach Earth's surface, having only partially burned up on the way down.

Meteroid

These are small lumps of material found in space. They range from less than 1mm across, up to tens of metres. Only if a meteroid enters Earth's atmosphere it is called meteor.

Metre *

The metre is the SI unit of distance. It is defined as distance light travels in the fraction 1 / 299 792 458 of a second.

Microphone *

A microphone is an input device in an electronic system. It converts sound energy into electrical energy. It is therefore a transducer.

Microwaves *

Microwaves, along with light, UV, x-rays and gamma radiation form part of the electromagnetic spectrum. The wavelength of microwaves is longer than that of infra-red radiation, but shorter than that of radio waves. Microwaves are used commonly in mobile phones and in microwave ovens. See also Cosmic Microwave Background Radiation.

Milky Way *

This is the name of the galaxy we live in. There are about 100 billion stars in the Milky Way. The Milky Way is a spiral galaxy, made up of several arms. Our arm is imaginatively called the 'local arm' (by us at least). The Milky Way actually has a few smaller satellite galaxies, two of them are the Large and Small Magellanic Clouds, visible from the southern hemisphere.

Miniature Circuit Breaker (MCB) *

Instead of using a fuse to protect an electrical flex, a miniature circuit breaker (MCB) can be used instead. MCBs are sometimes known as Earth Leakage Circuit Breakers (ELCBs) or Residual Current Devices (RCDs). The MCB has several advantages over fuses as follows:

MCBs can be reset once the fault is eliminated, whereas a fuse must be replaced
MCBs cut-off the current very quickly in comparison to fuses.
Some designs of MCB are tamper proof in that the cut-off current can not be altered. With a fuse, it is always possible to use a too-highly-rated fuse.

Disadvantages of the MCB are as follows:

MCBs cost more than fuses
MCBs require regular testing to ensure they will work
The cut-off current of the MCB can be affected by temperature

The Australian government's website has more information on RCDs, useful for background

Moderator *

In a nuclear fission reactor, the moderator slows fast neutrons down. Slower moving neutrons are more effective at keeping the chain reaction going by causing more uranium nuclei to break apart. Each fission event releases fast neutrons - the moderator is therefore required to slow them down.

Modulation

Modulation means change. In physics, this is usually used to describe changes made to the properties of a radio-wave or microwave carrier wave in order to carry information. There are two types of modulation - amplitude modulation and frequency modulation.

Molecule *

A molecule is the least amount of a compound that can exist and still remain a compound.

Momentum / Linear Momentum *

This quantity is given by momentum = mass of an object x its velocity.

Moons

Natural satellites held in orbits around planets by the force of gravity.

Motor *

Motors convert electrical energy into kinetic energy, along with some unwanted sound and heat energy.

A simple motor, which can run on d.c. electricity, has a rotor coil and permanent field magnets. As the rotor coil spins, the direction of the current flowing in the coil is reversed by means of a split-ring commutator. Carbon brushes allow the commutator to slide whilst maintaining an electrical connection. Carbon brushes are used as carbon is a good conductor and is hard-wearing.

In commercial motors, the permanent field magnets are replaced with electromagnets called field coils. This allows a more powerful magnetic field to be created than would be possible using permanent magnets. Also, the single rotor coil of the simple motor is replaced by a number of rotor coils (called a multi-coil or armature) and a segmented commutator replaces the split ring commutator. Each coil of the armature is connected to a pair of contacts in the segmented commutator. This allows for a smoother rotation of the motor. Note that in a commercial motor, because the magnetic field is produced by electromagnets, the motor will work equally well with a.c. or d.c. electricity. This is because a reverse current in the field coil will coincide with a reversal in the armature current. Therefore the armature will be forced round in the same direction.

Motor Rule *

See Fleming's Left Hand Rule.

Mutation / Cell Mutation

A changel in the function of a living cell, sometimes caused by ionising radiation.

Myopia (Myopic eye)

See under short-sighted.

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N

National Grid *

This is a network of power lines which connect together all the mainland UK power stations. Grid controllers can monitor the demand for electrical energy across the UK and allow more power to be diverted to areas with high demands.

Near-sighted

See under short-sighted.

Neptune

Planet Neptune usually has the eighth most distant orbit from our star the Sun. It is gas giant, although smaller than Jupiter or Saturn. It was discovered after observing abnormalities in Uranus' orbit. Calculations were made and Neptune was observed where the figures said it had to be. Neptune has three very faint rings (not visible from Earth). Sometimes Pluto is closer to the Sun than Neptune.

Neutral *

Having no overall electric charge. Atoms and neutrons are netral.

Neutron *

Neutrons are sub-atomic particles. They are uncharged
They have a mass of m_n = 1.675 x 10^-27 kg.

Neutron Star

The remnant of a dead star after it has exploded in a supernova. Neutron stars are extremely dense and have intense graviational fields, so strong that all atoms are crushed, forcing their nuclei together. Electrons and protons from the smashed atoms are crushed together to form neutrons. Neutron stars are a few kilometres across and have masses comparable to that of the Sun! (100 million tonnes per teaspoonful)

Newton *

The newton (N) is the SI unit of force. An unbalanced force of 1 newton will cause a mass of 1 kg to accelerate at 1ms^{-2. The newton is named
after Sir Isaac Newton, 1642-1727. Newton tried farming before going on to
develop his three Laws of Motion (see below). Newton also contributed to other
branches of science and maths.}

Newton Balance *

This is a spring balance (called a pundler in Shetland). It can be used to measure force, including weight forces. Remember all forces are measured in newtons!

Newton's First Law

If the forces acting on an object are all balanced, then the object will not change in its motion. It will not speed up, slow down, change direction or shape. IF it is not moving, then it will stay stationary.

Newton's Second Law

If forces are not balanced on an object then it will either speed up, slow down or change direction - this means it will have to accelerate. Remember that a change of direction is also an acceleration, even if the speed does not change. The formula for this law is:

F = ma

F is the force in newtons, m is the mass in kilograms and a is the acceleration in ms^-2

Newton's Third Law

For any force in a situation, there will be an equal and opposite force. For example if you stand on some grass, then your weight, which is a force, pulls you down and the grass pushes you up with the same force, but in the opposite direction. This means that the forces are balanced - so by Newton's first law - you don't move.

Non-conductor*

See insulator.

Non-Renewable Energy Sources *

Fossil fuels are termed non-renewable sources because they will run out one day. See also renewable energy sources.

Normal *

In optics and wave applications, the normal (line) is an imaginary line drawn at right angles to a surface. If the surface is curved, then the tangent to the surface must be drawn first, with the normal being drawn at right angles to the tangent.

Normal Reaction *

A contact force acting at right angles to a surface.

NOT gate (inverter)

See under inverter.

Nuclear Chain Reaction *

In a typical nuclear power station (or in a nuclear fission bomb) an unstable nucleus of uranium is hit by a neutron, causing it to break up, releasing a small quantity of energy plus several more neutrons. These new neutrons go on to hit more nuclei, causing them to break up, releasing even more energy and neutrons. This quickly builds up to a nuclear chain reaction. In a power station, control rods are used to regulate the rate of energy release. In a fission bomb, the chain reaction is allowed to proceed out of control with horrendous consequences.

Nuclear Energy *

Nuclear energy is the energy contained within the nucleus of an atom. It can be released in nuclear fission or nuclear fusion. Much of the world's electricity is produced using nuclear fisson power stations. Fusion power remains elusive, although progress is slowly being made.

Nuclear Reaction

A process in which two nuclei, or smaller particles such as a proton, or high energy electron, collide to produce different products.

Nuclear Reactor

A device to maintain a sustained nuclear chain reaction. Usually they are used for generating electricity and for the propulsion of ships. Sometimes they are used for producing isotopes for medical and industrial use, and some are run only for research.

Nucleon

Any of two particles normally found in an atomic nucleus. i.e. protons and neutrons.

Nucleus *

See Atomic Nucleus.

Nuclide

A version of an element with a unique number of protons and neutrons. The term 'isotope' is very often used instead of 'nuclide' (although strictly speaking, it has a subtly different meaning.)

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O

Objective Lens *

In a refracting telescope the objective lens is the large lens which collects the dim light from objects in the night sky. The purpose of the objective is to collect as much light as possible. It does not magnify the image (see eyepiece). With a larger objective, the image will be brighter and fainter objects will become visible.

Ohm (W) *

The ohm is the SI unit of resistance. The resistance of an electrical component is one ohm (1 W) if, when a voltage of 1 V applied across it causes a current of 1 A to flow through it.

Ohm's Law *

Georg S. Ohm discovered that for a conductor at constant temperature, the current which flows through the conductor is proportional to the voltage across it. The constant of proportionality is called the resistance of the conductor. The law is usually written as:

V = I R

Open Circuit *

This is one of two types of circuit fault (the other being a short circuit). In an open circuit fault, there is a break in the circuit. This break will prevent any current from flowing. Open circuit faults register on an ohmmeter with an extremely high (ideally infinite) resistance.

Optical Fibre *

Optical fibres rely on the principle of total internal reflection for their operation. Optical fibres are made from extremely thin, flexible hairlike strands of glass. They allow light to be sent over great distances and round corners. This makes possible devices such as the medical endoscope (or fibrescope) and various high capacity data communications. In communications applications, fibre optics have several advantages over copper wires - they are lighter in weight, more difficult to eavesdrop and require far fewer booster stations.

OR Gate

An OR gate is a logical device. It usually takes two inputs (although more are possible) and produces only one output. The logic state of the output depends on the logic state of the inputs. This is shown in the truth table below. A and B are the inputs and Z is the output state.

A	B	Z
0	0	0
0	1	1
1	0	1
1	1	1

From this it is easy to see that the output is on if at least one of the inputs is on.

Orbit *

The curved path, described by a planet, satellite, spaceship, etc., around a celestial body such as the Sun. This path is usually an elliptic, though sometimes it approximates quite well to a cirlce.

Ore *

A type of rock that contains minerals with important elements - including metals. The ores are extracted from the ground through mining; the valuable elements are then extracted from the ores.

Oscillation *

A to-and-fro movement.

Oscilloscope *

The oscilloscope provides a visual way of investigating electrical signals. The vertical scale shows the voltage of the signal - its scale can be modified by changing the Y-gain setting (in V/div). The voltage can then be deduced by calculating V = Y-gain x div, where 'div' is the number of divisions on the oscilloscope screen. The horizontal scale shows time and its control is called the timebase (or X-gain) (in ms/div). The period T of a wave-like signal can be calculated from T = timebase x div, where 'div' is the number of divisions taken to draw one full cycle.

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P

Parallel Beam *

A parallel beam is one which does not diverge or converge - its width stays the same as the beam travels out from its source. Parallel beams are produced using curved (parabolic) reflector dishes (e.g. in car headlamps, satellite transmitter dishes, mobile phone mast relay dishes). Parallel light beams can also be produced using converging lenses (e.g. in binoculars or telescopes). Parallel beams can be used to send waves over much greater distances than would otherwise be possible. This is because the wave energy stays much closer together when reaching its destination. However, this would be no use for sending a signal out which was intended to cover a wide area.

In astronomy, the light and other electromagnetic radiation from stars has come from such a great distance that the starlight does effectively form a parallel beam travelling down the tube of a telescope.

Parallel Circuit *

In a parallel circuit, each component is connected directly across the terminals of the power supply. Therefore, the voltage across any component is the same as the voltage across the power supply terminals. The rules for current, voltage and resistance in a parallel circuit are as follows:

V_supply = V₁ = V₂ = V₃ = ...

I_supply = I₁ + I₂ + I₃ + ...

1/R_total = 1/R₁ + 1/R₂ + 1/R₃ + ...

Note that the calculation for total resistance in parallel circuits requires an extra step to get the final answer. Here is an example calculation for a 30 ohm and a 40 ohm resistor connected in parallel:

1/R_total = 1/R₁ + 1/R₂

1/R_total = 1/40 + 1/30

1/R_total = 3/120 + 4/120 (change to common denominator)

1/R_total = 7/120

R_total/1 = 120/7 (extra step)

R_total = 17.14 W (remember the unit)

Calculators with a reciprocal button can make these calculations easier. See also: Series circuits

Parent Nuclide *

An unstable nucleus that decays or splits into one or more lighter nuclei - which are called daughter nuclei.

Particle *

A particle is a very small piece of something. Examples: a grain of sand, an atom, a proton, an electron.

Peak *

This refers to waves. The peak of a wave (also called the crest) is simply the part of the wave with the greatest amplitude.

Peak Voltage *

This the maximum voltage reached by an a.c. source in each cycle. It is very similar to the peak or crest of a wave. Compare with the r.m.s voltage of an a.c. source.

Period *

(1) The time taken for one complete oscillation cycle. (2) Wave period is the time taken for one complete wave to pass a point. The SI unit of wave period is the second (s). A closely related concept is that of frequency. (3) Satellite period is the time taken for a satellite to make one complete orbit around its parent body (e.g the time taken for the Moon to orbit the Earth is called the Moon's period). Satellite period is usually measured in hours or days.

Perpendicular *

At right angles (at 90 degrees).

Persistence of Vision

This is the effect which allows the human brain to be fooled into thinking that the 25 frames per second of TV is actually continuous motion. The retina retains each image for a fraction of a second and so provided that the changes between the frames are small enough, smooth motion will be seen. (The phosphor dots which make up the screen also continue to emit light for a short time after the electron beam has passed - this effect also helps to give the impression of a smooth motion)

Photographic_film *

Photographich film can be used to detect visible light, x-rays, or alpha, beta and gamma radiation. These radiations will blacken film due to a chemical reaction. Photographic film badges are used by medical and nuclear workers to monitor their level of exposure to radiation.

Physics *

A natural science which studies matter and its motion through space-time, along with related ideas such as energy and force in order to understand how the universe behaves.

Pitch *

How high a musical note is: the higher the frequency of the sound wave, the higher the pitch of the resulting note.

Planet *

Our solar system has nine planets, including the Earth. (Pluto is no longer considered to be a planet.) The table below shows some data for the planets. A planet does not produce its own light but shines by reflected star-light (sunlight!). The word 'planet' literally means 'wanderer' due to the way the planets move across the night sky. From Earth, we can easily observe Mars, Venus, Jupiter and Saturn using just binoculars or the naked eye. With 20x60 binoculars, firmly mounted it is also easy to observe Jupiter's four Galilean moons - a treat well worth the effort- they will be visible as tiny pin-points of light all in a row (but you may not see all four at once). These moons move around pretty rapidly and if you sketch their positions, you should easily see them shift position after several hours.

Name	Approximate Distance from Sun compared with Earth's Distance	Time taken to orbit the Sun	Length of the planet's "day"	Mass/ mass of Earth	Radius / radius of Earth
Mercury	0.4	88 Earth days	58.6 Earth days	0.055	0.382
Venus	0.7	226 Earth days	243 Earth days	0.815	0.949
Earth	1.0	1 Earth year	23.9 hours	1.000	1.000
Mars	1.5	1.9 Earth years	24.6 hours	0.107	0.533
Jupiter	5.2	11.9 Earth years	9.93 hours	318	11.2
Saturn	9.5	29.5 Earth years	10.7 hours	95.2	9.45
Uranus	19.2	84.0 Earth years	23.9 hours	14.6	4.1
Neptune	30.1	165.0 Earth years	17.8 hours	17.2	3.9

Mercury, Venus, Earth and Mars are called 'inner planets'. Jupiter, Saturn, Uranus and Neptune are 'outer planets'.

Plasma

A state of matter, similar to gas, in which some of the particles are ionised. Plasmas generated in laboratories can have temperatures ranging from near absolute zero to about 100 million kelvin.

Plug *

A plug is correctly wired as follows:

Green/Yellow insulated core to the earth terminal (top)

Brown insulated core to the live terminal. The live terminal can be identified by the fuse. The fuse and the switch should always be connected to live as it is the live which supplies electrical energy to the device. If the fuse and switch were connected to neutral, the appliance could appear to be off, yet still be connected to live.

Blue insulated core to the neutral terminal.

Pluto

Planet Pluto usually has the furthest orbit from our star the Sun. It is the smallest planet, really just a large lump of rock. Its status as a planet has been challenged by some, due to its similarity to other similar sized objects. Currently the IAF still list Pluto has a planet. Due to its highly elliptical orbit, Pluto is nearer to the Sun than Neptune for part of its orbit.

Potential Difference (P.D.) *

Think of this as meaning the same as voltage (an oversimplification actually).

Potential Divider (potentiometer)

A variable resistor, when used with all three of its terminals, it is called a potentiometer or potential divider. This arrangement is for controlling voltage. See also: Rheostat.

Potential Energy *

Potential energy means stored energy. Some examples are chemical, elastic, nuclear and gravitational - all forms of potential energy.

Power *

Power is the rate of transfer of energy. The SI unit of power is the watt (W). One watt is the same as one joule per second

The formula which relates power (P), energy (E) and time (t) is:

E = P x t

In electrical applications, there are other equations for power, which involve current, voltage and resistance. They are:

P = I V

P = I²R

P = V² / R

Power (of a lens)

Lens power describes the curvature of a lens. High power lenses have a short focal length and are highly curved. A positive lens power indicates a convex lens and negative power indicates a concave lens. Lens power, P, is measured in Dioptres (D) and is related to the focal length, f, in metres through the formula:

P = 1 / f

Power Gain

Power gain has no units of its own. It expresses how many times more power the output signal from a process has compared to its input power. A closely related concept is that of voltage gain. Power Gain = Output Power / Input Power.

Power Station / Generating Station / Power Plant

An industrial facility for generating electric power.

Presbyopia (presbyopic eye)

See under long-sighted.

Pressure

Force acting per unit area.

Primary Colour

There are three primary colours (of light): red, green and blue. Any two of these colours can be mixed to give a secondary colour as follows:

Red + Green = Yellow

Red + Blue = Magenta (pink)

Green + Blue = Cyan (Turquoise)

If all three colours are mixed together, white light can be obtained.

Note that these are the primary and secondary colours of light. The primary and secondary colours of pigment (paint) are not the same.

Primary Mirror

In a reflecting telescope, the primary mirror collects the light from the objects under study in the night sky. Reflectors can be made much bigger than the objective lenses of a refracting telescope. They are also easier to make, since their is only one optical surface to prepare. The primary mirror itself is usually made from glass, with an aluminium coating on its forward face (unlike the mirror in your bedroom). The mirror is made as large as possible for the same reasons as for the objective lens of a refractor. See also eyepiece.

Principle of Reversibility

This applies to light rays passing through any system of mirrors or lenses. Basically, it means if a ray of light follows a path when going, say, from left to right, then another ray of light going from right to left will follow the same path in reverse.

Proton *

Protons are sub-atomic particles. They carry a positive electrical charge (q_p = +1.602 x 10^-19 C). They have a mass of m_p = 1.673 x 10^-27 kg.

Pumped Storage

At times of low electricity demand, typically at night, the National Grid has overcapacity. Since it is difficult and expensive to shut down power stations, this extra energy would have to be wasted. However, by using it to pump water back uphill into reservoirs, a small percentage of this energy can be stored as gravitational potential energy of the water. The process is inefficient, but dumping the excess power would be 0% efficient.

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Q

Quasar: Quasar means 'quasi-stellar radar source'. These are objects which look like stars but are not. (They are only visible in big telescopes, because of their great distance from us. A typical quasar differs most noticeably from a star because it gives off the same intensity of light as would one million million suns. This is quite impressive, but even more impressive is the fact that quasars spew out much more radiation at radio wavelengths. Quasars are amongst the most distant objects observed from Earth.

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R

R.M.S. Voltage *

This is a kind of average voltage of an a.c. source (averaged over a complete cycle). It is the effective voltage delivered by the source and can be used in power calculations (P=IV etc). Compare rms voltage with the peak voltage of an a.c. source. In the UK, the rms voltage of a.c. mains is quoted at 230 V.

Radar Gun

A hand-held device used by police to measure the speed of passing cars.

Radiation *

This can refer to electromagnetic radiation (infrared radiation is often referred to as heat radiation) or to alpha and beta (or other) particles released from radioactive decay. Some types of radiation can cause ionisation of neutral atoms, by knocking away electrons. This can damage or kill cells. In this way, radiation can cause cancer and be used to treat cancer.

Radio *

A radio is an electronic system which allows a listener to hear programmes sent from a transmitter. The main components of a radio (in order) are: the aerial, tuner, decoder, amplifier and loudspeaker.

Radio Telescope *

Astronomers use radio telescopes to help build up a picture of the Universe. Since radio waves can penetrate Earth's atmosphere, we can detect them by building large antennae or dishes. The radio part of the spectrum provides an alternative and complimentary view of the Universe - importantly radio waves convey information about much lower energy events than visible light does, much of what a radio telescope 'sees' will be not appear in visible light images.

Radio Transmitter *

A source of radio waves. Radio transmitters are usually large vertical conducting masts which emit radio waves. Moving electrons in the metal of the mast radiate the radio waves.

Radio Waves*

Radio waves are a type of electromagnetic radiation with wavelengths longer than infrared light, ranging from 1 millimeter to 100 kilometres. Like all other electromagnetic waves, they travel at the speed of light. Naturally occurring radio waves are made by lightning, or by astronomical objects. Artificially generated radio waves are used for radio communication, broadcasting, radar and other navigation systems, satellite communication, computer networks and many other applications.

Radioactive *

A material is said to be radioactive if it emits alpha, beta or gamma radiation from its atomic nuclei. Radioactive materials have unstable nuclei which have a tendency to decay. It is when a decay occurs that radiation is released from the nucleus. Important related concepts are the activity and half-life of a source. The term 'radioactive isotope' is often shorted to 'radioisotope', and 'radioactive nuclide' is often shorted to 'radionuclide'.

Radioactive Decay / Radioactive Disintegration *

This refers to the decay of an unstable nucleus. The number of decays per second taking place in the sample is called the activity of the sample. Radiation is given out when a disintegration takes place.

Considering each nucleus on its own, radioactive decay is a random process. There is no way of knowing when a particular nucleus will decay - it may decay within the next millionth of a second, or it may not decay for another billion years! This isn't a measurement problem - the information just doesn't exist.

However, when you take a very large number of nuclei, the number of nuclei decaying (i.e. the activity) becomes predictable. Once a nucleus has decayed and given out radiation, the remains of the nucleus will still be in whatever material it started in. However that nucleus will not be able to decay in the same way again. (You can only smash a cup once, but you can go back over and stamp on all the bits again!) This means that the level of activity will gradually drop away over time as the nuclei all decay. For some elements, the activity will drop away to background very rapidly, for other elements activity drops more slowly. Half-life is a measure of how quickly the activity level drops.

Radioactivity *

The decay (disintegration) of atomic nuclei, giving rise to alpha, beta and/or gamma radiation. See Activity.

Radiometric Dating

This is the name given to the general technique of using half-life to find out the age of samples of rock, fossil or material which is organic in nature. (Radio)carbon dating is used to date organic material up to about 30 000 years.

Random

Unpredictable.

Reaction

(1) A nuclear reaction. (2) An equal and opposite force exerted by an object against a force acting upon it. (3) A normal reaction force - used in mechanics (4) A chemical reaction.

Reaction Time

The time taken for a human conscious response to some event.

Reactor

This usually refers to a Nuclear Reactor.

Real Image

An image formed when light rays actually meet.

Receiver

An electronic device that receives radio waves and converts the information carried by them to a usable form.

Redshift

This refers to the fact that the spectrum of distant galaxies is shifted towards the long wavelength 'red' end of the electromagnetic spectrum. (Redshift is caused by Doppler shift). Bigger redshifts mean that the object is moving away from us faster. An astronomer by name of Edwin Hubble discovered that the more distant galaxies are moving away from us the most quickly (he measured the greatest redshifts for the farthest galaxies). This means that the Universe is expanding and leads directly to the currently popular Big-Bang theory of the origin of the Universe.

Reed Relay

A reed switch, controlled by an electromagnet. Relays can be used for remote control of electrical circuits in dangerous places. For example, a circuit within a nuclear reactor can be switched on or off with a reed relay. In general, relays can be used to switch high current circuits using low current circuits. In this way, control panels can be made safer for their operators because the more dangerous high current circuits can be separated from the control panel itself.

Reed Switch

A simple switch, controlled by a magnet. The reed switch can be designed either to make or break contact when a magnetic field is present. Reed switches can be used (with permanent magnets) as door switches, for example to switch on an oven light when the oven door is opened, or to switch off a microwave oven when the door is opened. In a reed relay, an electromagnet is used to control switch.

Reflection *

All waves can be reflected from an obstacle - for example, water waves can reflect off a harbour wall and light waves can reflect off a mirror. The law of reflection is very simple:

The angle of incidence is equal to the angle of reflection.

Refraction *

All waves can be refracted. Refraction means bending. Light rays refract when they pass through a medium of different density (e.g. when light travels from air into glass). When light passes into a region of increased density (e.g. air to glass) it bends towards the normal. When light passes into a region of reduced density, it bends away from the normal.

Renewable Energy Sources *

Solar, wind, geothermal, tidal and wave power plants are examples of renewable energy resources. They will not run out, unlike fossil fuels. Currently political will exists to develop these sources and good progress has been made. Shetland already uses solar and wind plants to generate non-trivial power. There are plans to develop an off-shore wave power station too. For solar power, the following link is noteworthy, although a quick search on google will yield more. Solar power link.

Resistance *

The electrical resistance of a component (e.g. a lamp) is the component's opposition to the flow of current. The SI unit of resistance is the ohm (W). See also: Ohm's Law

Resistor *

A resistor is an electronic component which opposes the flow of current. See resistance and variable resistors.

Resultant Force *

The combined effect of all the forces acting on an object.

Rheostat *

A variable resistor, when used with just two of its terminals, it is called a rheostat. This arrangement is for controlling current. See also: potential divider.

Ring Circuit

This is a special type of parallel circuit, used in household wiring. The advantage of the ring circuit is that thinner, cheaper cable can be used. This is because there are two separate paths to any socket. Therefore, if each cable is capable of carrying up to 18 A, then the maximum current the ring can draw from the consumer unit is 36 A.

Rocket Engine

Rockets are propelled upwards by ejecting exhaust gases. Because of Newton's third law, as the rocket pushes the exhaust gases down, the exhaust exerts an equal and opposite upwards force on the rocket. A rocket stores the oxygen it needs for combustion in a tank, whereas other jet engines get their oxygen from the atmosphere. See also two-stage rocket.

Rotor Coil

In an a.c. generator, the rotor coil is a rotating electromagnet, with a d.c. power supply. It replaces the permanent rotating magnets of a dynamo.

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S

Sankey Diagram

Diagram representing the relative size of energy conversions taking place in a system.

Satellite *

A satellite is any object which is in orbit about a much more massive object. The force required to keep the satellite in orbit is the gravitational attraction between the satellite and the massive object. The Moon is an example of a natural satellite of the Earth; the International Space Station is an example of an artificial satellite of Earth. Earth is a natural satellite of the Sun.

Saturn

Planet Saturn has the sixth most distant orbit from our star the Sun. It is slightly smaller than Jupiter and is also a gas giant planet. Saturn's ring system is easily seen in a small telescope from Earth.

Scalar

Quantity with magnitude (size) but no specific direction. Examples: mass, energy, temperature.

Second *

The second is the SI unit of time. It is defined as the time taken to produce 9 192 631 770 complete wavelengths of electromagnetic radiation from a particular event in cesium atoms.

Semiconductor *

A material whose ability to conduct electricity is roughly half way between that of an insulator and that of a conductor. Semiconductor materials are the foundation of modern electronics, including radio, computers, telephones, transistors, solar cells, and integrated circuits.

Scintillation Counter

These devices detect gamma rays and form part of the gamma cameras used in radioactive tracer examinations. Scintillations are flashes of light given out by fluorescent materials (e.g zinc sulphide. When radiation is absorbed by zinc sulphide, it causes the zinc sulphide to produce a small burst of light. These can be electronically counted.

Semiconductor

A material that is half way between a non-conductor and a conductor in its ability to carry an electric current. It has special conduction properties.

Series Circuit *

In a series circuit, each component is connected to the other to form a 'chain' between the terminals of the power supply. The rules for current, voltage and resistance in a series circuit are as follows:

V_supply = V₁ + V₂ + V₃ + ...

I_supply = I₁ = I₂ = I₃ = ...

R_total = R₁ + R₂ + R₃ + ...

The mnemonic VISA may help to remember the rules for voltage and current in series and parallel circuits:

VISA: Voltages In Series Add

Short Circuit *

This is one of two types of circuit fault (the other being an open circuit). A short circuit fault is often caused by wires touching, allowing current to bypass a component in the circuit. Short circuit faults register on an ohmmeter with almost zero resistance.

Short-sighted

This refers to a problem with the accommodation ability of an eye. Basically, the eye can focus on near-by objects with no problem, but can't properly focus on far-away objects. This is called myopia. This is because the retina is too far from the lens - so that the focal point falls short of the retina. Short-sightedness can be treated by wearing spectacles which are concave in shape and therefore help to focus the image properly by extending the overall focal length of the glasses and eye.

SI Units *

SI stands for Systeme International. This is the standard system of units used worldwide and includes the metre, the second, the kilogram, the ampere, the kelvin, and the coulomb.

More information is available from the University of Alberta, CA or the University of Exeter, UK.

Sievert (Sv)

The sievert is the SI unit of dose equivalent. Since 1 Sv is a massive dose of radiation, units of microsieverts are often used instead.

Soft Magnetic Materials

Materials that are easy to magnetise (make magnetic) and demagnetise.

Solar Cell *

A solar cell is made of a semiconductor material which converts light energy into electrical energy. Solar cells are transducers.

Solar Power *

Power obtained from the energy transferred by the Sun's electromagnetic waves.

Solar System *

The Sun and everything that orbits it - planets and their moons, asteroid belt, comets, etc.

Solenoid *

A coil of wire with an electrical current flowing through it. An soft iron core, inserted into the coil, has the effect of increasing the strength of the magnetic field produced. An increased current or a greater number of turns on the coil will also increase the strength of the magnetic field. See also: Electromagnet

Solid *

A state of matter in which particles are arranged so that shape and volume of the substance is relatively stable. The constituents of a solid are packed together more closely than in a gas or liquid, and the intermolecular bonds are relatively strong. Examples: brick, penny, wood, aluminum.

Sound / Sound Wave *

A longitudinal wave which travels through a medium, usually air. Sound also travels through liquids (e.g. water) and solids (e.g. steel). The volume (loudness) of sound is measured using the decibel (dB) scale.

Specific Heat Capacity

The energy required to increase the temperature of 1kg of a material by 1 degree celsius. This is also the energy released during cooling. Water has a high specific heat capacity at 4200 Jkg^{-1 o}C^-1. Metals have much lower heat capacities. Physically, this is why metals heat up and cool down more rapidly than water - less energy is required to bring about a given change in temperature.

Specific Latent Heat of Fusion

This the latent heat per kilogram of material required to change its phase from solid to liquid (i.e. heat energy required to melt 1 kg of material). It is also the heat energy released (in joules) if 1 kg of material 'freezes' from liquid to solid.

Specific Latent Heat of Vapourisation

This the latent heat per kilogram of material required to change the material's phase from liquid to gas (i.e. heat energy required to boil or vapourise 1 kg of material). It is also the heat energy released (in joules) if 1 kg of material condenses from solid to liquid.

Spectroscope

A device using the diffraction of light to split the visible spectrum up into the rainbow of colours. The spectroscope is used in astronomy to study the spectra of light from the stars.

Spectrum *

(Plural: spectra). Usually refers to visible spectrum, which itself is part of the electromagnetic spectrum.

Speed *

Speed is the rate at which distance is covered. The SI unit of speed is the metre per second. Other units include feet per second, kilometres per hour and miles per hour.

The simple formula for movement at constant speed is: distance = speed x time.

For waves, the formula v = f l can be used.

Speed-Time Graph *

This is a graph with time plotted along the horizontal axis, and with speed up the vertical axis. You need to be able to interpret these graphs. The area under the graph is the total distance travelled.

Stable

(1) A stable nuclide is one that stays as it is for a long time, without decaying into a different nuclide. (2) In ordinary English, stable means 'not easily changed).

Star *

A star is a hot ball of gas which shines by producing its own light. The closest star to Earth is called the Sun. The next closest is Proxima Centauri at around four light-years distant. Other types of stars include red giants, white dwarfs and neutron stars.

States of Matter *

The forms that matter take. Solid, liquid and gas are the most common states of matter on Earth. However, much of the matter of the universe is in the form of plasma, and plasma can be created in laboratories..

Stator Coil

In a dynamo, the stator coil is the the fixed coil which has an electric current (a.c.) induced in it by the motion of the nearby rotor (permanent magnet).

Stethoscope

The stethoscope is a device which allows a doctor or nurse to listen to sounds coming from within a patient's body. The sounds can be used to diagnose medical conditions. The two bells of a stethoscope are connected to the earpieces via a flexible hollow tube. The bells can be selected by turning a valve which allows only sound from one of the bells to enter the tubing at a time. The open bell is used to listen to lower frequency (e.g. heart) sounds and the closed bell is used to listen to higher frequency (e.g. breathing sounds).

Streamlined *

Streamlined objects are designed to reduce fluid drag forces. All rough or angular edges are removed, leaving only smooth curved surfaces. Fish, dolphins, falcons, air interceptors and racing cars are good examples of streamlined objects.

Sun *

The Sun is our nearest star. It is a dim yellow dwarf star, not particularly big or bright by astronomical standards. It has a mass of 2x10³⁰kg and diameter 110 times that of the Earth. As an example for comparison, Betelgeuse in Orion has around 20 times the mass of the Sun, and a diameter 290 times that of the Sun. Also in Orion, the bright star Rigel is intrinsically 60 000 times brighter than the Sun.

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T

Telecommunication *

Communication (by any means) over a distance

Telescope *

A telescope is a device used in astronomy, including radio and optical telescopes. In a refracting telescope, an objective lens is used to collect light. The biggest optical telescopes are of the reflecting type, using a primary mirror to collect light. The aperture of the telescope determines how good it will be for observing faint objects. For use by a human, all optical telescopes also require an eyepiece lens.

Tension *

The force instretched materials.

Thermal Energy *

This is is the kinetic energy of all the particles in a substance moving in a random way. These particles may be atoms, molecules, electrons, etc. In a solid, these movements will be vibrations of the particles. In a liquid, gas or plasma, the particles will be moving from one place to another. The more thermal energy an object has, the higher is its temperature - though it is not the same as temperature. Thermal energy is very often referred to as heat (energy), though strictly speaking this is not the correct term in physics.

Thermistor *

Thermistors are designed so that their resistance depends on temperature.

When subjected to low temperatures, the resistance of a thermistor is high.

When subjected to a high temperature, the resistance of a thermistor is low.

Thermocouple

When two different metals are joined together, they will convert heat energy into electrical energy. This is the basis of the thermocouple. Thermomcouples are transducers.

Thinking Distance

The distance travelled by a moving vehicle in the time it takes for the driver to react to an emergency before applying the brakes.

Tidal Power

Power obtained from the rise and fall of the oceans (the tides).

Total Internal Reflection (TIR) *

Total internal reflection occurs when light, instead of being refracted at a boundary between media of different densities, the light is totally reflected off the inside surface. The usual case is inside glass, where the light ray bounces off the inside of the glass instead of escaping out. This effect is the basis of fibre optics - used in communication and medicine.

Total internal reflection occurs only when the angle of incidence is above a certain value called the critical angle.

Tracer *

Radioactive tracers are injected into a patient to examine the function of organs and blood supplies within the body. Technetium is often used as it emits gamma radiation which can be detected outside the patient's body. A gamma camera is used to detect the radiation as it travels around the parts of the body of interest.

Transducer *

Any device which converts one form of energy into another. For example, a microphone, a TV, a car or a solar cell.

Transformer *

A device which consists of two separate coils. Transformers only work with a.c. and can be used to step voltages up or down. A popular exam topic, transformers are well worth some study time.

Transistor *

A semiconductor device which can be used as an electronic switch (popular exam question). The two types of transistor are NPN and PNP.

Transmitter / Radio Transmitter *

An electronic device which, with the aid of an antenna, produces radio waves.

Transverse Waves

Waves in which the particles of the medium move at right angles to the direction of transfer of energy.

Trough *

This refers to waves. The trough of a wave is simply the part of the wave with the least (most negative) amplitude.

Truth Table

A table showing the output logic states of a logic gate or logic system for all given combinations of input logic states. Input states should be listed systematically to avoid confusion. As examples, have a look at the truth tables for the AND, OR and NOT gates. As a further example, here is the truth table for a 3 input XOR gate (exclusive OR), showing how to list the inputs in the correct order.

A	B	C	Z
0	0	0	0
0	0	1	1
0	1	0	1
0	1	1	0
1	0	0	1
1	0	1	0
1	1	0	0
1	1	1	0

Tuner (Radio or TV)

The tuner in a radio or TV allows the user to select one channel from the many channels which arrive at the aerial. The output signal from the tuner section is then passed to the decoder.

Tumour

Localised growth of cancer cells. Tumours are sometimes treated using radiotherapy.

Two-stage Rocket

Essentially, two rockets one on top of the other. The top rocket will not ignite until the first rocket is finished and jettisoned. The advantage over a big single stage rocket is that all the dead weight of the empty first stage can be cut away and left to fall back to Earth. This can greatly reduce the total mass of fuel required to reach orbit.

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U

Ultrasound *

Ultrasound is sound which has a frequency (or pitch) greater than 20 kHz. This is above the maximum frequency which a human can hear. Many other animals however can hear sounds with a frequency above 20 kHz. Ultrasound can be so high pitched that no animal can hear it. Here are several application areas of ultrasound:

Animal navigation
Submarine navigation
Fishing - detecting shoals
Medical - looking inside the body without need of surgery. Often used during pregnancy.

All these applications rely on the fact that ultrasound is reflected when the density of the material through which they pass changes (in other words, they echo back when passing from skin to bone).

Ultraviolet *

Ultraviolet (UV) radiation is invisible. It is a form of electromagnetic radiation, with a wavelength just shorter than that of visible violet light. UV radiation can be used to treat skin conditions like acne. Humans use UV light from the Sun to produce vitamin D3 (used in the kidneys and liver). UV light causes sun-tans, but too much can cause skin cancer.

Unbalanced

Not adding up to zero. So for example, unbalanced forces have a non-zero resultant force.

Unstable

(1) An unstable nuclide soon decays into another nuclide. (2) (In ordinary English, stable means 'easily changed, easily unbalanced').

Universe *

The Universe in which we live consists of several hundred billion galaxies. These galaxies are grouped as superclusters and clusters. Astronomers currently favour the Big-Bang theory of the origin of the Universe, with recent results (of cosmic microwave background radiation) tending to support this view.

Upthrust

The upward force that acts on an object because it has displaced a volume of fluid.

Uranus

Planet Uranus has the seventh most distant orbit from our star the Sun. It is slightly smaller than Jupiter and Saturn and is also a gas giant planet. Uranus has very faint rings around it (not visible from Earth)

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V

Variable Resistor *

Variable resistors are resistors whose resistance can be altered, usually by means of a rotating or sliding contact. They can be used as potential dividers or rheostats.

Vector *

A quantity that has both magnitude (size) and direction. Examples: velocity, acceleration, force.

Velocity *

Velocity is speed in a stated direction. The unit of velocity is m/s. Velocity may alternatively be defined as rate of change in displacement. The formula for velocity is velocity = displacement ÷ time. See also speed.

Venus

Planet Venus has the second most distant orbit from our star the Sun. It is slightly smaller than Earth. In many ways it is our sister planet - it receives similar heat energy from the sun (twice what Earth receives), with a similar internal structure to Earth. Its atmosphere is quite different, with 90 times the pressure of ours, and consisting of very corrosive gases. Its surface temperature can be well over 400 degrees celsius, due to a runaway greenhouse effect.

Vibrate

To move continuously to and fro. Atoms in a solid vibrate.

Virtual Image

An image that is not the actual source of real light rays, for example the image seen in a mirror appears to be behind the mirror.

Visible Spectrum / Visible Radiation *

Usually refers to the 'rainbow' of colours seen by the human eye: Violet, Indigo, Blue, Green, Yellow, Orange, Red (in order of increasing wavelength). Vioet light has a wavelength of roughly 400 nm and red light's wavelength is approximately 700 nm. All other colours have a wavelength between these two limits. It is important to remember that this visible spectrum is only a tiny part of the full electromagnetic spectrum.

Volt *

The volt is the SI unit of voltage. In fact 1 V = 1 JC^-1 (i.e. a charge of one coulomb will gain or lose one joule of potential energy when moved through a potential difference of one volt).

Voltage *

In electrical circuits, voltage is a measure of the electrical energy available between two points to each unit of charge. Voltage must be measured between two points in a circuit - e.g. "The voltage between point X and point Y is 3 volts". Voltages are measured using a device called a voltmeter. It is not necessary to break into a circuit in order to use a voltmeter.

It is wrong to say: "The voltage flows into the lamp". Voltage does not flow. It is current that flows. A difference in voltage between the terminals of a lamp causes current to flow through the lamp. (An analogy for voltage is the difference in heights between two points on a hillside. This difference in heights (voltage) causes water to flow downhill (electrical current). One would not speak of height flowing into a water-mill!).

Voltage, current and resistance are related through Ohm's Law

Voltage Divider

A simple circuit branch consisting of two resistors connected in series. The voltage accross the terminals of the branch is shared between the two resistors, with the bigger resistor getting the biggest share. Because the resistors are connected in series, the current through each is the same. The easiest way to calculate the voltage division for given resitances is to use division in ratios as follows:

Problem: Given resistors of 100R and 200R, connected in series, calculate the voltage division when connected to a 12V power supply.

Solution: 100:200 is equivalent to 1:2, so split the 12V supply voltage into 1+2=3 parts. Hence 1 part = 12/3 = 4 V and then 2 parts = 8 V. Do a quick check to see that 4+8=12V and then the answer can be given: the 100R resistor has a p.d. across it of 4V, the 200R p.d. is 8V.

Voltage Gain

Voltage gain has no units of its own. It expresses how many times greater the output signal voltage is (from some component) compared to the input voltage. A closely related concept is that of power gain. Voltage Gain = Output Voltage / Input Voltage.

Voltmeter *

A device for measuring electromotive force or potential difference.

Volume *

1. The amount of space occupied by a three-dimensional object or region of space. The SI unit of volume is m³ (cubic metres)
2. A measure of the loudness or intensity of a sound.

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W

Watt *

The watt (W) is the SI Unit of power. It is equivalent to one joule per second (Js^-1). Another common unit for power is the horsepower (hp). 1 hp = 746 W.

Wave *

A travelling disturbance, carrying energy from place to place. Examples are water waves and sound waves where it is the movement of particles which constitutes the wave motion. In the example of light waves and other electromagnetic waves it is changing electric and magnetic fields which carry energy from place to place.

Wave Power *

Power obtained from ocean waves.

Wavelength, l (lambda) *

This is the length of a wave between any two equivalent points. It is easiest to measure from crest to crest or from trough to trough. The SI unit of wavelength is the metre, although kilometres, millimetres, micrometres and nanometres are also used.

X

X-rays *

X-rays were discovered accidentally by Wilhelm Röntgen in 1895. X-ray radiation is a part of the electromagnetic spectrum. With wavelengths ranging from about 0.001nm to about 100 nm, X-rays are much shorter than ultraviolet. They travel at the speed of light.

X-rays are produced in an X-ray tube (soft X-rays for diagnosis) or in a linear accelerator (hard X-rays for treatment).

X-rays can be detected using photographic film or scintillation counters.

In medicine, 'soft' X-rays can be used to take photographs of broken bones. Bone absorbs X-rays more than soft flesh and therefore shows up white on a photographic plate. A break in the bone will show up dark. (X-rays blacken the photographic chemicals on the film).

Alternatively, scintillation counters can be used to detect x-rays and display an image on a TV type screen. This method is used with barium meals for investigating the gut.

For a three dimensional view, computed tomography can be used where the X-ray source and detector rotate around the patient to build up a series of image slices.

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Y

Y-Gain: The height of the waveform on an oscilloscope.

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Z

Zenith *: The zenith is the point in the sky directly above an observer's head. This term is often used in astronomy.

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Accuracy An accurate measurement is one which is close to the true value.
Calibration This involves fixing known points and then marking a scale on a
measuring instrument, between these fixed points.
Data This refers to a collection of measurements.
For example: Data can be collected for the volume of a gas or the type of rubber.
Datum The singular of data.
Errors,
- random
- systematic
- zero
These cause readings to be different from the true value. Random
errors may be detected and compensated for by taking a large number
of readings.
For example: Random errors may be caused by human error, a faulty technique
in taking the measurements, or by faulty equipment.
These cause readings to be spread about some value other than the
true value; in other words, all the readings are shifted one way or the
other way from the true value.
For example: A systematic error occurs when using a wrongly calibrated
instrument.
These are a type of systematic error. They are caused by measuring
instruments that have a false zero.
For example: A zero error occurs when the needle on an ammeter fails to return
to zero when no current flows, or when a top-pan balance shows a reading when
there is nothing placed on the pan.
Evidence This comprises data which have been subjected to some form of
validation. It is possible to give a measure of importance to data
which has been validated when coming to an overall judgement.
Fair test A fair test is one in which only the independent variable has been
allowed to affect the dependent variable.
For example: A fair test can usually be achieved by keeping all other variables
constant.
Precision The precision of a measurement is determined by the limits of the
scale on the instrument being used. Precision is related to the
smallest scale division on the measuring instrument that you are using.
It may be the case that a set of precise measurements has very little
spread about the mean value.
For example, using a ruler with a millimetre scale on it to measure the thickness of
a book will give greater precision than using a ruler that is only marked in
centimetres.
Reliability The results of an investigation may be considered reliable if the results
can be repeated. If someone else can carry out your investigation and
get the same results, then your results are more likely to be reliable.
One way of checking reliability is to compare your results with those
of others. The reliability of data can be improved by carrying out
repeat measurements and calculating a mean.
True Value This is the accurate value which would be found if the quantity could
be measured without any errors at all.