Energy and Thermal Physics

Comparing the powers of electric motors 2

Practical Activity for 14-16 PRACTICAL PHYISCS


Comparing the power output of two motors.

Apparatus and Materials

  • Small electric motors of different powers, 2
  • Line shaft unit
  • Knife switch
  • Mass, 1 kg
  • Power supply, LV
  • Rubber band or driving belt
  • Cord
  • Stopwatch & metre rule

Health & Safety and Technical Notes

The person controlling the switch must stop the motor before the load reaches the line shaft. (If the load spins round the shaft, the string may break and the load go flying.)

In all activities where loads may fall on toes, precautions (such as using cardboard boxes full of waste material) should be taken.

Read our standard health & safety guidance

The motors suggested are part of the Malvern Energy Conversion Kit, now obtainable from the supplier: Beecroft and Partners.


  1. Clamp the two electric motors to the bench on either side of the line shaft unit, so that the driving belt can be connected to either of them. The line shaft is used for lifting the load of 1 kg.
  2. Link one of the motors to the line shaft and connect it to the power supply.
  3. Measure the time taken to raise the load from the floor to the bench.
  4. Repeat for the second motor.

Teaching Notes

  • The same amount of work has to be done in both cases. The same amount of energy will be stored gravitationally in the raised weight in both cases. The energy is transferred electrically from the power station by the power supply and motor to the raised weight, but the faster motor is the more powerful.
  • Labels on the motors may indicate the horse-power or wattage. The motor does not always supply this power, (the maximum recommended). Instead the motor adjusts itself to the load. If more power than that is demanded the motor will overheat and perhaps burn out. Its efficiency is also likely to be lower.
  • It may be worth indicating that animals, including humans, adjust to the load as well; watch how slowly someone staggers when they have to lift something heavy. Safety rules try to prevent people hurting themselves. (Injuries to backs may not manifest themselves for many years.)

This experiment was safety-tested in January 2006

appears in the relation P=VI P=I^2R P=V^2/R ΔQ=PΔt
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