Energy Transferred by Working
Energy and Thermal Physics

What’s wrong with ‘forms of energy’?

Teaching Guidance for 14-16 PRACTICAL PHYISCS

Many textbooks and teaching schemes talk of ‘transforming’ energy, or of ‘converting energy from one form to another’.

This is a very common way of talking, but it has its problems. Particularly, it is in danger of saying nothing at all. For example, “A torch converts chemical energy in the battery to light energy”. All this says is that a chemical reaction happens and light comes out.

It is easy to teach and learn the language of ‘transforming’ energy. Students can translate throwing a ball into ‘muscle energy is changed into kinetic energy’. But it is dangerously close to being no more than a game of words.

Describing chains of energy transfers, for example with a steam engine or electric motor lifting a load, particularly tends towards this kind of talk. Yet the energy in the motion of the moving parts is irrelevant to an understanding of the overall process. If, for example, a drive belt was replaced with a lighter one that otherwise had the same mechanical properties, it would have less energy – but the process would continue as before. The energy of the moving parts is not a useful quantity to know.

It is more useful to focus on the initial and final energy stores. This puts the emphasis on where the energy is and why, not on renaming it once it goes from one thing to another.

In some situations, the ‘forms of energy’ approach can easily lead to incorrect analyses of processes. For example, many textbooks discuss energy transfers in a car moving along a level road. It is common to show stored energy of the petrol being transferred to the car (kinetic energy) and heating parts of the car and the surroundings. This is correct for the period when the car is speeding up. But once the car is going at a steady speed (and so has constant kinetic energy), all of the stored energy is ultimately heating parts of the car and the surroundings, in part by pushing air out of the way.

The really important thing is to work from very early on with actual quantities of energy, to do plenty of simple sums about amounts of energy and rates of delivery. This is where there is real payoff; where something is actually being said, and understanding has something to get a grip on.

Energy Transferred by Working
appears in the relation dU=dQ+dW
is used in analyses relating to Working Engines Thermionic Emission
is a special case of Work
has the special case Potential Energy Kinetic Energy
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