Efficiency (OCR GCSE Physics A (Gateway)) : 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 energy output from a system to its total 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 can be represented as a decimal or as a percentage
The equations for efficiency are:
Since power is the energy transferred per unit time, power can also be used to calculate efficiency
Worked Example
The blades of a fan are turned by an electric motor. In one second, 300 J of energy is transferred electrically from the mains supply. 85 J is wasted due to friction and sound.
Calculate the percentage efficiency of the motor.
Answer:
Step 1: List the known quantities
Total energy input = 300 J
Total wasted energy = 85 J
Step 2: State the equation
Step 3: Determine total energy output
Due to the conservation of energy:
total energy input = total energy output
Therefore, total energy output = 300 J
Step 4: Calculate the useful energy output
Due to the conservation of energy:
Total energy output = useful energy output + wasted energy
Useful energy output = total energy output − wasted energy
Useful energy output = 300 − 85
Useful energy output = 215 J
Step 5: Substitute these values into the efficiency equation
Examiner Tips and Tricks
Efficiency can be given in a ratio (between 0 and 1) or percentage format (between 0 and 100 %)
If the answer is required as a percentage, remember to multiply the ratio by 100 to convert it:
if the ratio = 0.25
the percentage = 0.25 × 100 = 25 %
Remember that efficiency has no units
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