Is energy always conserved?
Teaching Guidance for 11-14
Accounting for friction
Wrong Track: Energy can't be conserved, and this proves it! When a pendulum swings back it doesn't go as high.
Right Lines: If you take into account the energy of the pendulum and that of the surrounding air, then energy is conserved. It is true that the pendulum does not swing back to its original height, but some of the energy from the gravitational store of the pendulum (in the Earth's field) is shifted to the thermal energy store of the surrounding air. In other words, as the pendulum travels through the air there is some heating of the air due to friction. Overall, there is as much energy around at the end of the process as there was at the start.
Energy is always conserved
Thinking about the learning
Pupils need to get hold of the idea that energy is always conserved.
Thinking about the teaching
Energy is conserved. But showing that this is so requires rather careful measurement. A good approach to take is to emphasise at all points that energy must be conserved. If some process happens in which there seems to be an energy loss, the cry should go up:
Let's play hunt the energy! In virtually all of these cases, where there appears to be a loss of energy, something will be warmed (with the energy previously unaccounted for turning up in a thermal energy store).
You can help them in this by always talking of the changes of energy in a store, and avoiding qualifying the energy in any way, such as:
Just talk about the