Quick answer: Electromagnetic induction produces an emf whenever the magnetic flux Φ = BA cos θ through a loop changes. This AP Physics 2 lab on faraday's law 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.
How does the speed of a magnet affect the voltage induced in a coil? By dropping a magnet through a coil from different heights and recording the voltage pulse, you can test Faraday’s law with a simple stack of equipment and see how flux change connects to speed.
Level: AP Physics 2 (algebra-based) · Curriculum: AP P2 Unit 12
Aim
Investigate how the peak emf induced in a coil depends on the speed of a magnet falling through it, and compare the results with Faraday’s law.
Equipment
- coil of about 500 turns on a plastic former
- strong cylindrical magnet that fits inside a plastic tube
- plastic tube about 60 cm long, clamped vertically through the coil
- voltage sensor and data logger (or oscilloscope with a voltage-input app)
- metre rule, soft cloth or foam for the landing
Safety
Strong magnets can pinch fingers and damage phones or cards. Catch the falling magnet on a soft pad, and keep the magnet away from anyone with a pacemaker.
Method
- Clamp the tube vertically with the coil near its lower end. Place a soft pad under the tube.
- Connect the coil to the voltage sensor and start recording.
- Release the magnet from h = 0.05 m above the top of the coil. Read the peak emf from the first pulse on the graph.
- Repeat for h = 0.10, 0.20, 0.30 and 0.40 m. Repeat each height three times and use the mean peak value.
- Calculate v = √(2gh) as the speed on entering the coil, using g = 9.81 m/s².
Results
Sample means of three trials, coil of 500 turns:
| Drop height h (m) | Speed v = √(2gh) (m/s) | Peak emf (V) |
|---|---|---|
| 0.05 | 0.99 | 0.21 |
| 0.10 | 1.40 | 0.30 |
| 0.20 | 1.98 | 0.42 |
| 0.30 | 2.43 | 0.52 |
| 0.40 | 2.80 | 0.59 |
Analysis questions
Question 1 Entry speed
Calculate the speed for h = 0.30 m and explain the assumption in using v = √(2gh).
Question 2 Graph
Plot the peak emf (y-axis) against v (x-axis). Describe the shape of the graph.
Question 3 Slope of the graph
Calculate the slope using the first and last points and give its unit.
Question 4 Meaning of the result
Explain with Faraday’s law why the peak emf is roughly proportional to the speed.
Question 5 Area under a pulse
For one release the area under the emf pulse was 2.1 × 10⁻³ V·s. Calculate the flux change through one turn, ΔΦ.
Question 6 Two pulses
Explain why the recorded emf has a positive pulse followed by a negative pulse as the magnet passes through.
Question 7 Limitations
Give two reasons why the real speed is below √(2gh) and one way to reduce the effect.
