Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

Energy: common knowledge, hard concept

The word ‘energy’ is used in everyday life, in news reports, public information leaflets and by politicians and policy-makers. For example:  

  • “My, you’re full of energy today” 
  • “A bite of X gives you instant energy” 
  • “What renewable energy sources do we need?”  

Students will hear the word used in many contexts, and feel more or less comfortable with its different meanings. In consequence, teachers can talk about energy without being challenged, as long as they use the word in one of these socially understood ways. From that point of view, getting started in teaching about energy seems to be no problem at all. 
But energy and power (the rate of transfer of energy) are scientifically rather sophisticated terms. A transfer of energy from one thing to another has a not well-known unit (the joule). It isn’t obvious that multiplying a force by a displacement in the direction of the force gives you anything sensible, let alone an amount of energy transfer. Nor does the name ‘work’ for this help much, given the everyday uses of that word. 
Amounts of energy are supposed to be conserved – always to stay the same – seemingly in direct conflict with everyday usage where energy is gained, produced, lost, used up, saved, wasted, etc. In consequence, a teacher talking about energy in the scientific sense is soon challenged or misunderstood. So starting to teach about energy does get to be a problem after all. 
We start from three propositions: 
1 Exploit to the full the everyday use of the idea, in getting started. There is nothing wrong with learning how people use a word, and only gradually getting to understand it more fully. It is useful to take stuff from everyday discussion, to work on in science lessons. 
2 Science lessons need to add doing sums with energy values, to go beyond everyday talk. Energy and power are calculated quantities (indeed you pay for them by amount). Science teaching needs to add this quantitative aspect, beginning early. 
3 The full ‘scientific concept of energy’ is remarkably sophisticated. A science student will go on extending her or his idea of energy throughout school and university. But teachers need to know more than they will tell students, to see where an idea will head in the future. That is one of the purposes of these Guidance pages.