Units
Use the following units: Kelvin, degree Celsius (°C), joule (J), newton (N), watt (W), kilowatt (kW), megawatt (MW)
EFFICIENCY & DISSIPATION
For any device which converts energy:
efficiency =(useful work or energy output) / (total energy input)
Whenever energy is transferred, only part of it is transferred to where it is wanted and in the form it is wanted. The rest of the energy is transferred in some non-useful way and so is 'wasted'.
Candidates should be able to describe the intended energy transfers and the main energy wastages which occur when using a range of everyday devices.
Both the energy that is ‘wasted’ during energy transfers and the energy which is usefully transferred end up being transferred to the surroundings, which become warmer.
In many processes, energy is dissipated as heat in the surroundings.
The energy becomes increasingly spread out (dissipated) and becomes increasingly more difficult to use for further useful energy transfers.
The more of the energy supplied to a device that is usefully transferred the more efficient we say the device is.
Candidates should be able, when provided with appropriate information, to evaluate methods of reducing wasteful transfers of energy, e.g. ways in which energy transfer from a house is reduced.
SPECIFIC HEAT CAPACITY
energy transfer = mass x specific heat capacity x temperature change
Appreciate some of the effects of materials having different specific heat capacities.
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Energy Resources and Energy Transfer
Thermal Energy Transfer
Thermal energy just means heat. It refers to the jiggling around of atoms and molecules in a substance. Heat is the energy that an object has because of the motion of its molecules, which are continuously jiggling and moving around. When energy is added to an object, its molecules move faster, creating more heat. Compared to a warm object, the molecules in a cold object have less molecular motion. Heat (i.e. thermal energy) is the total energy of molecular motion in a substance.
When different parts of a substance are at different temperatures, energy is transferred by the substance from places where the temperature is higher to places where the temperature is lower. Heat can be transferred from one place to another by three methods: conduction in solids, convection of fluids (liquids or gases), and radiation through anything that will allow radiation to pass. The method used to transfer heat is usually the one that is the most efficient.
Transfer of energy by a substance, without the substance itself moving, is called conduction. Metals are very good conductors. Nonmetals are usually poor conductors (insulators). Gases are very poor conductors.
Liquids and gases can flow and so can transfer energy from places where the temperature is higher to places where the temperature is lower. Transfer of energy by liquids or gases moving in this way is called convection.
Energy is continually being transferred to and from all objects by radiation, even through empty space (a vacuum).
Hot bodies emit mainly infra red radiation.
The hotter an object is, the more energy it radiates.
Dark, matt surfaces emit more radiation than light, shiny surfaces at the same temperature. Particles of matter are not involved. Dark, matt surfaces are good absorbers (poor reflectors) of radiation. Light, shiny surfaces are good reflectors (poor absorbers) of radiation.
Candidates should be able:
• to describe various ways in which heat energy is transferred from buildings;
• to describe and explain ways in which the rates of these energy transfers can be reduced.
Given appropriate information, evaluate the effectiveness and cost-effectiveness of methods used to reduce energy consumption in buildings.
Conduction occurs in metals because the hotter the metal is the more kinetic energy the vibrating ions in the metal structure have. This energy is transferred to cooler parts of a piece of metal by free electrons as they diffuse through the metal and collide with ions and with other electrons.
Convection currents occur in liquids and gases because their particles move faster when they are hot causing the liquid or gas to expand. Warm regions are then less dense that cold regions. The warm regions rise up through the colder regions and colder regions replace the warmer regions.
Thermal radiation is the transfer of heat energy by electromagnetic waves. Waves from all parts of the electromagnetic spectrum carry heat energy, but in our everyday lives, most heat energy is radiated via the infra-red part of the spectrum.
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Energy transfer
Candidates will be assessed on their ability to:
• describe energy transfers involving the following forms of energy: thermal,
light, electrical, sound, movement (kinetic), chemical, nuclear and potential
(elastic
and gravitational)
• understand that energy is conserved
• recall that efficiency is the proportion of energy transferred to useful
work and apply this to everyday situations
• describe a variety of everyday and scientific devices and situations
explaining the fate of the input energy in the above terms including their representation
by flow
diagrams (Sankey diagrams)
• describe how insulation is used to reduce energy transfers from buildings
and the human body
• understand that many insulating materials make use of the insulating
properties of air that is not free to form convection currents