Resource 5: Understanding energy

Energy suffers from a problem. We often treat it as a real substance of some kind, but Richard Feynman, the Nobel Prize-winning physicist, had this to say about energy (Feynman et al, 1964):

There is a fact, or if you wish a law, governing all natural phenomena that are known to date. There is no exception to this law – it is exact so far as is known. The law is called the conservation of energy. It says that there is a certain quantity, which we call energy, that does not change in the manifold changes which nature undergoes. That is a most abstract idea, because it is a mathematical principle; it says that there is a numerical quantity, which does not change when something happens. It is not a description of a mechanism, or anything concrete; it is just a strange fact that we can calculate some number and when we finish watching nature go through her tricks and calculate the number again, it is the same.

Here are some ideas about energy that might be common. How would you respond to these ideas?

  1. There is energy in food. When we eat food, the energy goes in to our bodies so that they can work.
  2. Energy can come in different types. Some of these types are sound, light, chemical and kinetic.
  3. When you drive a car, the energy in the petrol is used up, which is why you need to refill the car with petrol.
  4. Energy can be stored in different ways, like in the chemicals in batteries or in an object that has a specific position in a gravitational or electric field.
  5. Energy can be transferred from one place to another by a range of mechanisms such as electric current, sound and light.
  6. Energy is not real. We should accept that it is a mathematical idea that helps us understand the world around us.

Which do you think are the most accurate statements from a physics point of view?


  1. Energy is stored in the chemicals in food. This energy is transferred to our bodies so that they can function by using the energy in chemical reactions.
  2. This is a common method of thinking, but in recent years teachers have been encouraged to think of energy as being in a single form that is transferred from one place to another through various pathways such as electric current, light and sound.
  3. This is not good thinking. Energy is never ‘used up’, but it can be transferred from one place to another in such a way that it becomes dissipated and less useful. The rule of conservation of energy prevents energy being used up. In a car engine, energy is wasted through transfers to the environment by sound and heat. Some of this heat is generated in combustion; some is generated in the friction of the various moving engine parts and in the contact with the ground.
  4. This is good thinking and reflects current teaching ideas that are being promoted in schools.
  5. This is good thinking and reflects current teaching ideas that are being promoted in schools.
  6. This kind of thinking is potentially controversial. However, looking at Feynman’s definition of energy it would seem this is the case. We like to give things some sort of concrete existence, but in physics many ideas are just that; ideas or models.

Resource 4: Using questioning to promote thinking

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