Edexcel IGCSE Physics 4.4 Efficiency equation

Practise using the efficiency relationship to calculate useful, total or wasted energy and interpret what less than 100% efficiency means.

Exam points

substitute useful and total energy into the efficiency equation, with or without percentage form
calculate wasted energy after finding the total energy output from an efficiency value
link efficiency below 100% to an increased thermal store in a device

[Maximum number: 3]

A student uses this apparatus to investigate how the efficiency of an electric motor varies with its input voltage.

Figure for Question 4.4 Efficiency equation question 1 — Edexcel IGCSE Physics

This is the student's method.
- connect the electric motor to a d.c. power supply and a joulemeter
- attach a 1.0 kg mass to the electric motor using a length of string
- set the voltage of the power supply to 10 V and switch on the power supply
- switch off the power supply when the mass has been lifted through a distance of 50 cm
- record the input energy to the motor from the joulemeter
- calculate the energy transferred to the gravitational store of the mass
- calculate the efficiency of the motor

The student repeats this process, setting the power supply to a different voltage each time.

The table shows the student's results.

Table for Question 4.4 Efficiency equation question 1 — Edexcel IGCSE Physics

Calculate the motor efficiency when the power supply is set to a voltage of 10 V.

efficiency = ............ %

Marking points:
- efficiency formula seen
- substitution
- evaluation

Example:
$\text{efficiency}=\frac{\text{useful energy output}}{\text{total energy input}}\times100\%$ $\text{efficiency}=\frac{5}{12.7}\times100\%$ $\text{efficiency}=39.4\%$

Accept:
- $39\%$
- $39.37\ldots\%$

Ignore:
- significant figures

Reject:
- unsupported incorrect answer

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4.4 Efficiency equation question 1