Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

Measuring the power of a motor

Class experiment

Measuring the power input to a motor.

Apparatus and materials

For each group of students

LV power supply, variable DC

Motor/generator unit

Ammeter (0 - 1 amp), DC

DC voltmeter (0 - 5 volt)


Health & Safety and Technical notes

Read our standard health & safety guidance




Power input

a Set up the circuit as shown in the diagram with no load on the motor other than friction.

A circuit

Adjust the voltage supply to give the appropriate voltage for the motor.
b Note the readings of the ammeter and voltmeter.
c Calculate the number of joules transferred each second.
Efficiency of the motor

 Connecting lampsThe experiment can be extended by using the motor to drive a generator which lights 1, 2, or 3 lamps in a lamp unit. Connect the voltmeter across the voltage supply and use the ammeter to measure the current to the motor.   Connect 1, 2 or 3 of the lamps into the circuit. Note the change in input power. Now place the voltmeter across the generator terminals, and the ammeter in series with the lamps. Record the results in a table and calculate output power.



Teaching notes

1 The power transferred to the motor is calculated by measuring the current in the circuit for various potential differences across the motor. You might wish to use the heading of ‘charge flowing per second, in coulombs per second’ as well as ‘current in amps’. Similarly you might want to use ‘energy transferred in joules by each coulomb’ as well as ‘potential difference in volts’. 

Electrical power is calculated from
P = potential difference x current
= VI
The power, transferred to the motor from the power supply, is then expressed in joules/second or watts.
2 In step b, the efficiency of the energy transfer is calculated from

Efficiency equation
This experiment was safety-checked in January 2006


Related guidance

Power and energy

Helpful language for energy talk

Measuring energy transfers


Related experiments

Measuring the power of a lamp


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