There are changes in the way energy is stored when a system changes.

The idea

Every change in a system is a redistribution between stores, never a creation or destruction of energy. Lift a 100 kg mass of water 500 m and about 490 kJ moves from the chemical store of the fuel powering the pump to the gravitational potential store; heat the same water by 80 °C and roughly 33 600 kJ moves into its thermal store. Comparisons like that are common: state where energy started, where it ended, and how much. Storing energy by heating holds far more per kilogram but leaks steadily through the insulation, while pumping uphill holds less yet wastes a smaller percentage and needs mountains. So the useful question is always which store, how much, and how much of it is recoverable.

What earns the marks

• Identifying the store energy is transferred from and the store it is transferred to.
• Quantifying the change, for example the energy stored per 100 kg by each method.
• Using the percentage wasted to work out the useful energy that could be recovered.
• A genuine comparison that links the two methods rather than describing them separately.

Common mistakes and how to put them right

• Writing that energy is created by the pump or used up by the heater instead of transferred between stores.

• Naming heating, a pathway, as the final store rather than naming the thermal store.

• Listing features of each method in turn, which caps a compare question at the lower level.

Exam tips

• In a compare question, write sentences that hold both options together: method A stores more per kilogram but wastes 40% against method B at 25%.
• Give the magnitude of any difference when the data allows it; the top level asks for the size of the difference, not just its direction.