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The frozen planet
The frozen planet

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4.1 What is energy?

Energy is a word in common use but with a variety of everyday meanings. It does however have a precise scientific meaning and is an important concept.

  • Think of two or three phrases in which you include the word ‘energy’ in an everyday sense.

  • Possible examples are: ‘Where do children get their energy from?’, ‘I haven’t got the energy to get up’ or ‘Sweets are full of energy’.

None of the everyday uses of the word ‘energy’ is very precise but they all encapsulate the notion that energy enables activity to take place. This is also at the heart of the scientific notion of energy.

Energy is a physical property possessed by a substance and it is a measure of its capability to ‘make things happen’. In order for things to happen, some of the energy within the substance must be transferred to another object and a common consequence of the transfer of energy to an object is to cause its temperature to rise. For example, if I use energy to rub my hands very hard 10 times, they will be warmer when I have finished. Another example is when you burn fuel under a pan of water: energy is transferred from the flame to the water and, as a result, the temperature of the water increases. The heat is increasing the random motion of its constituent atoms or molecules and so the temperature of a substance is a measure of how much energy of motion – so-called kinetic energy – the atoms or molecules within it have. A high temperature corresponds to a high rate of internal motion and high kinetic energy of the substance’s particles.

The ultimate source of almost all the energy reaching the Earth’s surface is the Sun. (There is some heat energy flow from the interior of the Earth to the surface, but the amount is small compared with the solar contribution.) The incoming energy is transmitted across space by solar radiation (the Sun’s rays), and when it reaches the Earth it is transferred to its surface, or to your skin which, as you will know from experience, warms up. How much the Earth or your body heats up is, as in the next section, down to the energy balance.