Quick answer: Electromagnetic induction is the creation of an emf in a circuit when the magnetic flux linked with it changes. This Physics 11-12 lab on electromagnetic induction gives the aim, the equipment, the safety points, a step-by-step method, sample results and 6 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
Does a faster magnet induce a bigger emf? You will drop a small magnet through a coil from several heights and record the peak emf. The heights give the speeds, so you can test Faraday’s law directly with simple equipment.
Level: Physics grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB D.4
Aim
To find how the peak emf induced in a coil depends on the speed of a magnet falling through it.
Equipment
- coil of about 500 turns of enamelled copper wire wound on a short plastic former
- small neodymium or ceramic magnet that fits freely in a clear plastic tube
- voltmeter with a peak-hold function, or a data logger, with leads
- clear plastic tube 60 cm long, clamp stand and metre rule
- soft cloth or foam pad to catch the magnet
Safety
Strong magnets can pinch skin and wipe bank cards, so keep them apart and handle one at a time. Place the foam pad under the tube so that the falling magnet cannot chip the floor or the coil. Clear away any magnet that falls on the floor.
Method
- Clamp the tube vertically so that it passes through the middle of the coil, and place the foam pad below it.
- Connect the coil to the voltmeter or data logger.
- Hold the magnet in the tube with its lower end 5 cm above the top of the coil and release it. Record the peak emf.
- Repeat from 10, 20, 30 and 40 cm, taking three readings each time and using the mean.
- Calculate the speed at the coil for each height from v = √(2gh), ignoring air resistance.
Results
Each emf value is the mean of three drops:
| Height h / cm | Peak emf / mV | Speed v / m/s (calculated) |
|---|---|---|
| 5 | 34 | 0.99 |
| 10 | 48 | 1.40 |
| 20 | 67 | 1.98 |
| 30 | 84 | 2.43 |
| 40 | 95 | 2.80 |
Analysis questions
Question 1 Plotting
Plot peak emf (vertical) against speed v (horizontal) and draw the best-fit line through the origin.Question 2 Pattern
Describe the relationship between emf and speed shown by the graph.Question 3 Gradient
Find the gradient of your line, with units.Question 4 Explanation
Use Faraday’s law to explain why a faster magnet induces a larger emf.Question 5 Sign of the pulse
With a data logger you see a positive pulse followed by a negative pulse. Explain why.Question 6 Evaluation
Suggest two reasons why the real speed at the coil is smaller than √(2gh), and one way to improve the experiment.
