Efficiency (AQA A Level Physics)

Revision Note

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Katie M

Last updated

7 November 2024

Efficiency

The efficiency of a system is a measure of how successfully energy is transferred in a system
Efficiency is defined as:
The ratio of the useful power output from a system to its total power input
If a system has high efficiency, this means most of the energy transferred is useful
If a system has low efficiency, this means most of the energy transferred is wasted

Determining which type of energy is useful or wasted depends on the system
- When electrical energy is converted to light in a lightbulb, the light energy is useful, and the heat energy produced is wasted
- When electrical energy is converted to thermal energy in a heater, the heat energy is useful, and the sound energy produced is wasted

Efficiency is represented as a percentage, and can be calculated using the equation:

Efficiency equation 2, downloadable AS & A Level Physics revision notes

Where power is defined as the energy transferred per unit of time

The efficiency equation can also be written in terms of energy:

The energy can be of any form e.g. gravitational potential energy, kinetic energy

Worked Example

An electric motor has an efficiency of 35 %. It lifts a 7.2 kg load through a height of 5 m in 3 s.

Calculate the power of the motor.

Answer:

Step 1: Write down the efficiency equation

$Efficiency = \frac{Power output}{Power input} \times 100$

Step 2: Rearrange for the power input

$Power input = \frac{Power output \times 100}{Efficiency}$

Step 3: Calculate the power out

The power output is the energy ÷ time
The electric motor transferred electric energy into gravitational potential energy to lift the load

Gravitational potential energy = mgh = 7.2 × 9.81 × 5 = 353.16 J

Power = 353.16 ÷ 3 = 117.72 W

Step 4: Substitute values into power input equation

$Power input = \frac{117.72 \times 100}{35} = 336 W$

Examiner Tips and Tricks

Efficiency can be in a ratio (between 0 and 1) or percentage format (between 0% and 100%).

If the question asks for efficiency as a ratio, give your answer as a fraction or decimal. If the answer is required as a percentage, remember to multiply the ratio by 100 to convert it: if the ratio = 0.25, percentage = 0.25 × 100 = 25 %.

Remember that efficiency has no units. It is a ratio with both the numerator and denominator with the same units.