Efficiency (Cambridge (CIE) O Level Physics): Revision Note
Exam code: 5054
Efficiency of Energy Transfer
The efficiency of a system is a measure of the amount of wasted energy in an energy transfer
Efficiency is defined as:
The ratio of the useful power or energy output from a system to its total power or energy 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
The overall efficiency of a typical thermal power station is approximately 30%
This means that 70% of the energy transferred from the power station to the National Grid is wasted energy
In the production of electricity:
Energy is used to heat water to produce steam
The steam turns a turbine
The turbine turns a generator
The generator produces electricity
At each stage of this process, energy is dissipated to the surroundings
Sankey Diagram of a Power Station

Sankey diagram showing the efficiency of a gas-fired power station
Calculating Efficiency
Efficiency is represented as a percentage, and can be calculated using the equation:
The efficiency equation can also be written in terms of power:
Where power is defined as the energy transferred per unit of time
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
Step 2: Rearrange to make power input the subject
OR
Step 3: Calculate the power output
ΔE is equal to the change in gravitational potential energy as the load is lifted
Therefore,
Step 4: Substitute the values into the power input equation
OR
Examiner Tips and Tricks
Efficiency can be given 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
Unlock more revision notes. It’s free!
By signing up you agree to our Terms and Privacy Policy.
Already have an account? Log in
Did this page help you?