Section 1 of 7Efficiency
Meaning
- Meaning Efficiency compares the useful output with the total input.
- A high efficiency means most of the energy is useful.
Energy equation
- Energy equation efficiency = useful energy output ÷ total energy input
Power equation
- Power equation efficiency = useful power output ÷ total power input
Percentage form
- Percentage form Multiply the ratio by 100 to give a percentage.
- Efficiency has no unit, and cannot exceed 100%.
Section 2 of 7Power
| Quantity | Equation | Unit |
|---|---|---|
| Power | P = E ÷ t | watt (W) = joule per second |
| Power (work form) | P = W ÷ t | W |
| Gravitational potential energy | ΔE = m × g × Δh | J (g = 10 N/kg) |
| Efficiency (percentage) | (useful ÷ total) × 100 | %, no unit |
Section 3 of 7Calculating efficiency
1
Identify the useful output and the total input, using either energy or power consistently.
2
Write the equation: efficiency = useful output ÷ total input.
3
Substitute values with matching units, for example 60 J useful out of 240 J supplied.
4
Calculate: 60 ÷ 240 = 0.25.
5
Convert to a percentage if asked: 0.25 × 100 = 25%.
6
Wasted energy = total input − useful output = 180 J.
Section 4 of 7Worked example: lifting motor
1
A motor lifts a 7.2 kg load through 5 m in 3 s and is 35% efficient.
2
Useful energy output = m × g × Δh = 7.2 × 10 × 5 = 360 J.
3
Useful power output = 360 ÷ 3 = 120 W.
4
Rearrange: input power = useful output power ÷ efficiency = 120 ÷ 0.35.
5
Input power ≈ 343 W (not joules, as power is measured in watts).
Section 5 of 7Improving efficiency
1Reduce friction
- Reduce friction Lubricate moving parts so less energy is wasted by heating.
2Reduce resistance
- Reduce resistance Use shorter or thicker wires and low-resistance components.
3Insulate
- Insulate Prevent thermal losses from heating systems.
4Higher efficiency
- Higher efficiency More of the input becomes useful output, saving fuel and money.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Efficiency | The fraction (or percentage) of the energy supplied that is transferred usefully. |
| Power | The rate of transfer of energy, in watts. |
| Watt | A rate of energy transfer of one joule per second. |
| Useful output | The energy or power that does what the device was designed to do. |
| Wasted energy | Energy transferred to unwanted stores, usually the thermal store of the surroundings. |
Section 7 of 7Exam technique
1
Giving an efficiency of more than 100% or with units: it is a ratio or percentage.
2
Dividing input by output instead of useful output by total input.
3
Using joules for powerpower is measured in watts.
4
Forgetting to convert the final ratio to a percentage when the question asks for one.