Quick answer: Work done is the energy a force transfers, W = F s cos θ, measured in joules. This Physics 11-12 lab on work energy theorem gives the aim, the equipment, the safety points, a step-by-step method, sample results and 8 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
How efficiently can a small motor turn electrical energy into gravitational potential energy? In this lab you lift loads through a fixed height, time each lift, read the meters and compare the useful energy with the energy supplied.
Level: Physics grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB A.3
Aim
To measure the efficiency of a small electric motor as it lifts loads of different mass through the same height, using work done, electrical input and time.
Equipment
- Small DC motor (3 V) with a spool, fixed at the top of a retort stand
- Slotted masses (0.050 kg to 0.250 kg) and a hanger
- Variable DC power supply set to 3.0 V
- Ammeter, voltmeter and connecting leads
- Metre rule and stopwatch
- Thread (about 1.2 m)
Safety
Keep feet and hands away from the falling load and clamp the stand to the bench. Switch off at once if the motor or wires feel hot. Do not exceed 3.0 V.
Method
- Fix the motor at the top of the stand. Tie the thread to the spool and to the hanger, then mark a lift height of 0.80 m with the rule.
- Set the supply to 3.0 V. Connect the ammeter in series and the voltmeter across the motor.
- Add 0.050 kg to the hanger. Start the stopwatch as you switch on and stop it when the load reaches the 0.80 m mark.
- Read the current while the load is moving.
- Repeat for 0.100, 0.150, 0.200 and 0.250 kg. Use fresh batteries or let the motor cool between runs.
Results
Sample results from Kofi’s school lab in Accra. The mass of the hanger is included in m.
| m (kg) | I (A) | t (s) | useful energy m g h (J) | input energy V I t (J) | efficiency (%) |
|---|---|---|---|---|---|
| 0.050 | 0.32 | 4.1 | 0.39 | 3.94 | 10.0 |
| 0.100 | 0.34 | 5.0 | 0.78 | 5.10 | 15.4 |
| 0.150 | 0.36 | 6.0 | 1.18 | 6.48 | 18.2 |
| 0.200 | 0.38 | 7.3 | 1.57 | 8.32 | 18.9 |
| 0.250 | 0.40 | 8.6 | 1.96 | 10.32 | 19.0 |
Analysis questions
Question 1 Useful energy
Show how the useful energy 0.39 J in the first row was calculated. State why the hanger mass is included.Question 2 Input energy
Calculate the electrical energy supplied for the 0.150 kg load and explain why V I t gives energy.Question 3 Efficiency values
Check the efficiency for the 0.250 kg load and state the trend in the table.Question 4 Graph
Describe a graph of useful energy against input energy and say what its gradient represents.Question 5 Where the energy goes
Name two places where the ‘wasted’ electrical energy goes.Question 6 Uncertainty in time
The stopwatch reaction time is about 0.2 s. Calculate the percentage uncertainty in the 4.1 s measurement and its effect on the input energy.Question 7 Improvement
Suggest two improvements that would make the efficiency values more reliable.Question 8 Conclusion
State a conclusion and say whether efficiency of 100 % could ever be reached.
