This energy is often described as being ‘wasted’.

The idea

Wasted is the everyday word for energy that has been dissipated into stores nobody can use. It is a label about usefulness, not about existence: when brakes get hot, the energy in the thermal store of the brake discs is every bit as real as the kinetic store it came from, but nothing in the car can collect it back. In a wind turbine the wasted share warms the bearings and the air; in a kettle it warms the body of the kettle and the kitchen; in a runner it warms the muscles. Because the wasted energy spreads into the surroundings at a temperature close to everything around it, it becomes impossible to concentrate again. So an exam answer should say the energy is wasted or dissipated to the thermal store of the surroundings, never that it is lost.

What earns the marks

• Identifying which transfers in the device are wasted rather than useful.
• Naming the thermal store of the surroundings or of the component as the destination.
• Stating that the total energy is unchanged even though part is wasted.
• Linking a reduction in wasted energy to a rise in efficiency.

Common mistakes and how to put them right

• Writing that wasted energy is lost or destroyed, which mark schemes do not credit.

• Calling all thermal energy wasted, even in a kettle or heater where heating is the useful output.

• Giving sound as the main waste from a motor when the thermal store takes far more.

• Treating wasted energy as a fixed property of a device rather than something lubrication or insulation can reduce.

Exam tips

• Decide what the device is for first, because the same thermal transfer is useful in a heater and wasted in a drill.
• Pair the word wasted with a destination, such as wasted to the thermal store of the surroundings.
• Unqualified references to friction are ignored, so always say less work is done against friction.