Efficiency: (HT only) Students should be able to describe ways to increase the efficiency of an intended energy transfer.

The idea

Efficiency compares the useful energy transferred by a device with the total energy supplied to it. No device is perfectly efficient because some energy is always dissipated to the surroundings, often by heating due to friction or resistance. To increase efficiency, you must reduce these unwanted transfers. Practical methods include lubricating moving parts to reduce friction, tightening loose parts to stop vibration, using thermal insulation to cut heat loss, and streamlining shapes to lower drag. For example, a motor is made more efficient by oiling its bearings, while a hot water tank's efficiency improves by adding thermal insulation to reduce heat loss. Improving efficiency means a greater proportion of the input energy becomes useful, so less energy is wasted for the same useful output.

What earns the marks

• State that efficiency is the ratio of useful energy transferred to total energy supplied, often expressed as a percentage.
• Identify a named unwanted transfer, such as energy dissipated by friction between moving parts or by heating of wires.
• Describe a specific method that reduces that transfer, for example lubricating bearings, tightening loose components, adding insulation or streamlining a shape.
• Explain that reducing wasted energy raises the useful proportion of the total input, so efficiency increases.
• Apply the idea to a device, such as a motor, vehicle or hot water tank, naming the intended useful transfer and the unwanted transfer being reduced.

Common mistakes and how to put them right

• Saying a machine can be made 100% efficient.

  Put it right: Some energy is always dissipated to the surroundings, so efficiency stays below 100%.

• Confusing efficiency with power or with the total energy supplied.

  Put it right: Efficiency is a ratio of useful output to total input, not a quantity of energy or a rate.

• Suggesting that adding more energy input increases efficiency.

  Put it right: Adding input without changing useful output lowers efficiency; the wasted transfer must be reduced instead.

Exam tips

• Name the device and the intended useful energy transfer before describing the improvement, so your answer is clearly focused.
• Use precise vocabulary such as lubricate, insulate and streamline rather than vague words like 'make it better'.
• Link each method to the wasted transfer it reduces, then state the effect on the proportion of useful energy.