Energy can be transferred usefully, stored or dissipated, but cannot be…

The idea

Conservation of energy is the rule underneath the whole topic: energy can be transferred usefully, stored for later or dissipated to the surroundings, but the total never changes. Nothing is created and nothing is destroyed. That is why the word wasted is only ever shorthand. When a wind turbine turns, most energy goes usefully to the generator, but some is dissipated by friction in the bearings into the thermal store of the parts and the air. Oil the bearings and less work is done against friction, so a greater proportion is transferred usefully and the efficiency rises. The energy that was wasted has not disappeared; it has simply spread out, too thinly spread to be recovered.

What earns the marks

• Stating that energy is transferred rather than created or destroyed.
• Naming the useful transfer and the dissipated transfer in the example given.
• Linking reduced friction to less energy dissipated to the thermal store.
• Concluding that the proportion of energy transferred usefully, and so the efficiency, increases.

Common mistakes and how to put them right

• Writing that energy is lost or destroyed instead of dissipated to the surroundings.

• Saying oiling the bearings creates more electricity rather than increasing the proportion usefully transferred.

• Naming friction on its own without saying that less work is done against it.

• Claiming a device can be made 100% efficient by removing friction entirely.

Exam tips

• Use the word dissipated rather than lost; the mark schemes accept wasted but never lost energy as an explanation.
• For an explain mark, give the chain: less friction, less energy dissipated, higher proportion transferred usefully.
• Do not answer with more electricity is generated on its own, as it is ignored without the efficiency link.