Quick answer: Electromagnetism links electricity and magnetism: a current produces a magnetic field, a current-carrying wire in a field feels a force F = B I L, and a changing magnetic field induces a voltage. This Physics 9-10 lab on electromagnetism gives the aim, the equipment, the safety points, a step-by-step method, sample results and 7 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
Does a real transformer follow the turns-ratio rule, and how much energy does it lose? In this lab you change the primary voltage of a demountable transformer, measure the secondary voltage and compare it with the voltage predicted from the numbers of turns. It takes about 30 minutes.
Level: Physics grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: HS-PS2-5, GCSE P7
Aim
To test whether the voltages across the two coils of a transformer follow the turns-ratio rule Vp ÷ Vs = Np ÷ Ns, and to see how close a real transformer gets to the ideal.
Equipment
- demountable transformer kit with 200-turn and 400-turn coils and a laminated iron core
- low-voltage a.c. supply, 2–12 V
- two a.c. voltmeters
- connecting leads
Safety
Use only the low-voltage a.c. supply and never connect the coils to the mains. Keep the primary below 12 V. Switch off between readings, because the coils can become warm.
Method
- Fit the 200-turn coil (primary) and the 400-turn coil (secondary) on the core and clamp the top bar firmly.
- Connect the supply and a voltmeter across the primary and a second voltmeter across the secondary, as shown.
- Set the supply to 2.0 V a.c., switch on and read both voltmeters.
- Record Vp and Vs, switch off and repeat for 4.0 V, 6.0 V, 8.0 V and 10.0 V.
Results
Sample readings from one group:
| Primary voltage Vp (V) | Secondary voltage Vs (V) |
|---|---|
| 2.0 | 3.8 |
| 4.0 | 7.7 |
| 6.0 | 11.5 |
| 8.0 | 15.3 |
| 10.0 | 19.0 |
Analysis questions
Question 1 Planning the readings
State which quantity you change, which you measure, and two things kept constant during the lab.Question 2 Predicted value
Use the turns ratio to predict Vs when Vp = 6.0 V. Compare it with the measured value and calculate the percentage difference.Question 3 Graph of Vs against Vp
Plot Vs on the y-axis against Vp on the x-axis and draw the best-fit line. Calculate its gradient using the first and last points.Question 4 Ratio for each row
Calculate Vs ÷ Vp for every row and find the mean. Compare it with the ideal value from the turns and calculate the percentage difference.Question 5 Why is Vs low?
Suggest two reasons why the measured secondary voltages are lower than the ideal values.Question 6 Efficiency
With Vp = 10.0 V a lamp is connected to the secondary. The primary draws 0.65 A and the secondary supplies 0.30 A at 19.0 V. Calculate the efficiency.Question 7 Making it more reliable
Suggest two changes that would make the results more reliable or the transformer more efficient.
