Practical: Measuring the output power of a small electric motor
Apparatus:
12 V electric motor, power pack, connecting leads, string, known weight, stop clock, felt tip pen, two cardboard pointers.
The power pack is connected to the 12 V motor using the connecting leads.
The 12 V motor is attached to the known weight by a length of string wound round the spool.
Record the weight in newtons in a table.
Two pointers A and B are attached to the bench close to the string.
Measure the distance between the two markers with a metre rule and recorded in metres.
A suitable distance is between 1 m and 2 m.
A felt tip pen is used to mark a clearly visible dot on the string.
When the motor is switched on the weight is lifted at a constant speed.
Start the stop clock when the dot passes pointer A.
Stop the stop clock again when the dot passes pointer B.
Record the time in seconds in the table.
Repeat this process twice more and calculate the average time taken.
This helps to reduce error due to reaction time when starting and stopping the stop clock.
The work done in lifting the weight is calculated using:
work done W = Fd
The power of the motor is then calculated using:
power = \(\frac{\text{work done}}{\text{time taken}}\)
The process can be repeated for a range of increasing weights.
To ensure that this is a fair test the height the weight is raised (i.e. the distance between the pointers) and the voltage are kept constant.
Results
| Weight lifted in N | Vertical height h in m | Work done in J | Time taken in s | Time taken in s | Time taken in s | Average time taken in s | Power of motor in W |
| 6.0 | 1.75 | 10.5 | 2.9 | 3.0 | 2.8 | 2.9 | 3.6 |
| Weight lifted in N | 6.0 |
|---|---|
| Vertical height h in m | 1.75 |
| Work done in J | 10.5 |
| Time taken in s | 2.9 |
| Time taken in s | 3.0 |
| Time taken in s | 2.8 |
| Average time taken in s | 2.9 |
| Power of motor in W | 3.6 |