Conservation of Energy
Energy and Thermal Physics

Helpful language for energy talk

Teaching Guidance for 14-16 PRACTICAL PHYISCS

Some ways of talking about energy are clearer and more helpful than others.

Energy stores

It is helpful to talk about energy stores. A spring, or a rubber band, can rather obviously store energy. You do work to stretch them (or to squash the spring), and you can get back pretty much the same amount of energy when they relax. These then are two of the best iconic examples for grasping what ‘potential energy’ is all about. It is energy in a mechanical store.

Many students find the term ‘potential’ confusing. They think ‘potential energy’ is somehow different from actual energy. Talking about energy stores offers a way of deferring the term ‘potential energy’ until later, for students who choose to continue studying physics.

You can similarly feel energy being stored when magnets are pushed together or pulled apart.

The example nearly all textbooks give of potential energy is perhaps the most difficult of all. It is the gravitational energy of a lifted mass. Now the energy is said to be ‘in’ the lifted object – as for a spring it is said to be ‘in’ the spring. If you have the courage, you could say that the energy is stored between the Earth and the lifted object (in the gravitational field). The trouble is of course that an external examiner might score that truthful answer as wrong because specialist understanding is not required at this level.

Another kind of energy store is a mixture of fuel and oxygen. In this case bonds between carbon and oxygen atoms can snap shut, releasing energy in a fire or explosion. It is common to talk about just the fuel – for example petrol – as the energy store, but do remember that for this chemical spring to snap shut, there must be oxygen too.

There are a limited number of energy stores:

  • chemical (e.g. fuel + oxygen)
  • kinetic (in a moving object)
  • gravitational (due to the position of an object in a gravitational field)
  • elastic (e.g. in a stretched or compressed spring)
  • thermal (in a warm object)
  • magnetic (in two separated magnets that are attracting, or repelling)
  • electrostatic (in two separated electric charges that are attracting, or repelling)
  • nuclear (released through radioactive decay, fission or fusion)

Energy carriers (or pathways) and energy transfers

It is often helpful to think of energy being carried from one place to another. For example, light carries energy from the Sun to the Earth. Light is not itself ‘energy’ – it is after all an electromagnetic wave, or a stream of photons (however you care to look at it). But energy does travel with the light. The same is true of radio waves. In a microwave oven microwaves carry energy from the microwave generator to the interior of the food. Other kinds of waves carry energy too, for example ocean waves.

Electric current in a circuit is another energy carrier. It is helpful to think about a power circuit as a way of moving energy from one place to another. The National Grid distributes energy from a number of power stations, via the wires and cables, to homes and factories.

It is often handy to think of moving matter as carrying energy, too. A strong wind delivers energy to a wind turbine. But, equally often, it is better to think of the moving mass as storing energy. A train has to be given energy to get it moving, and energy has to be taken from the train to stop it. This is what we call kinetic energy.

Energy carriers (or pathways, or transfers)

  • mechanically (when a force moves through a distance)
  • electrically (when a charge moves through a potential difference)
  • by heating (because of a temperature difference)
  • by radiation (e.g. light, microwaves)

With all of these, we are interested in the rate at which energy is being transferred and not the amount stored anywhere.

You can use flow diagram representations to strengthen the distinction between energy stores and carriers, for example:

There are some very important scientific ideas in this way of looking at things. Among them are:

  • that energy tends, in most cases, to spread from a more concentrated store to more dispersed stores; and that this makes it less useful for doing anything more
  • that the energy often ends up warming the environment

Visit School Science Review for two useful papers: Richard Boohan Making sense of energy and Robin Millar 'Teaching about energy: from everyday to scientific understandings':

School Science Review

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