Efficiency (Oxford AQA IGCSE Physics)
Revision Note
Efficiency
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
Efficiency is represented as a percentage, and can be calculated using the following equations:
Worked Example
An electric motor turns the blades of a fan. In one second, 300 J of energy is transferred electrically from the mains power supply. 85 J of energy is wasted due to friction and sound.
Calculate the efficiency of the motor.
Answer:
Step 1: List the known quantities
Total energy in = 300 J
Total wasted energy = 85 J
Step 2: State the equation
Step 3: Determine the total energy out
Due to the conservation of energy:
Step 4: Calculate the useful energy out
Due to the conservation of energy
Step 5: Substitute these values into the efficiency equation
Examiner Tip
Efficiency can be given as a ratio (between 0 and 1) or as a percentage (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 (only %)
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