Efficiency (OCR AS Physics): Revision Note
Exam code: H156
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:
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
Step 2: Rearrange for the power input
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
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.
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