| EARTH AND SPACE The Earth, planets and stars The Sun is a star that gets its energy from nuclear reactions. It is at the centre of the solar system, which contains planets, moons, comets and asteroids. The Milky Way is the galaxy containing our Sun. The distances between stars in a galaxy are so huge we measure them in light-years, rather than in kilometres. The Earth is meanwhile about 4.5 thousand million years old, about a third of the age of the Universe. The Earth has not remained the same all that time. How big is our galaxy? Continental drift Alfred Wegener proposed his theory of continental drift in the early 20th century. It offered an explanation of the existence of similar fossils and rocks on continents that are far apart from each other. But it took a long time for the idea to become accepted by other scientists. Wegener said continental drift happens when tectonic plates (huge segments of the Earth’s crust and upper mantle, sometimes forming continents) move. They shift at a few centimetres per year, with earthquakes and volcanoes often occurring around their edges. Wegener’s theory Outer space The Universe is thought to have started about 14 thousand million years ago in a hot Big Bang. The stars it contains do not stay the same forever. We can measure the distance to them using the parallax method, or by studying their relative brightness. Hubble’s Law describes the relationship between the distance a galaxy is from us and how quickly it is moving away. It is evidence that space is expanding. There is, as yet, no evidence of life on other planets. But many scientists think it is likely somewhere in the Universe. Stars RADIATION AND THE ENVIRONMENT Electromagnetic radiation The electromagnetic spectrum contains radiation including gamma rays, X-rays, ultraviolet light, visible light, infrared light, microwaves and radio waves. A beam of electromagnetic radiation delivers energy in photons. The more photons that are delivered per second, and the more energy each photon carries, the greater the intensity of the beam. The intensity decreases the further from the source you go. Some types of electromagnetic radiation are ionising. Different types of electromagnetic radiation have different uses. What is a spectrum? Radiation hazards Radio waves, microwaves and infrared light are all forms of non-ionising radiation. They heat up materials when they are absorbed. Visible light is also non-ionising. While microwave radiation is used for cooking and telecommunications, there is concern that the use of mobile phones could be harmful, although no conclusive evidence. We do know, however, that ultraviolet light can damage the skin and eyes. It causes cataracts, sunburn, skin cancer and premature ageing of the skin. Non-ionising radiation The Sun and global warming The Earth’s ozone layer absorbs ultraviolet light from the Sun and prevents it from reaching the surface. This protects living things from its harmful effects. Greenhouse gases such as methane, water vapour and carbon dioxide absorb infrared radiation emitted from the Earth. This increases the temperature of the planet. The concentration of greenhouse gases is increasing, leading to global warming. Carbon dioxide is removed from the atmosphere by photosynthesis, but returned by respiration and the combustion of fuels. Processes like these form the carbon cycle. The ozone layer ENERGY AND RADIOACTIVITY Nuclear radiation An atom consists of a nucleus containing protons and neutrons, surrounded by electrons. Radioactive elements give out ionising radiation. There are three types: alpha, beta and gamma. They have different abilities to penetrate materials. The half-life of a radioactive substance is the time taken for half of its atoms to decay, releasing radiation. An isotope of an element contains different numbers of neutrons. A different element is left behind when an unstable nucleus emits radiation. Atomic structure and radiation
Radiation can damage living cells. Doses are measured in sievert (Sv). The higher the dose, the more damage is done. This dose depends on both the amount of radiation and its type. Background radiation is all around us, and most of it is natural. Gamma radiation can be used to kill cancer cells, and to sterilise surgical instruments and certain foods. People at risk from radiation are monitored to ensure their dose does not exceed recommended limits. Radiation and living cells Electricity Electricity is a convenient secondary energy source. It can be generated using renewable energy resources such as wind or water power, or non-renewable resources such as fossil fuels and nuclear fuels. Each method has its advantages and disadvantages. But in all cases some energy is lost to the environment. Fossil fuels produce carbon dioxide when they are used. Nuclear fuels generate harmful radioactive waste - different levels of which are stored in different ways. Generating electricity
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