What it shows
Faraday’s law says the emf in a coil equals the rate of change of magnetic flux. A magnet falling faster through the same coil gives a larger voltage pulse.
The science
Faraday’s law gives emf = −N dΦ/dt: a bigger rate of change of flux gives a bigger voltage. A magnet released from height h reaches the coil at a speed v = √(2gh) (if air drag is small), so the peak emf should rise in proportion to √h. The sign of the pulse reverses as the magnet leaves the coil, showing the Lenz’s law effect. The area under the pulse stays the same because the total flux change is the same.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A coil of about 500 turns of enamelled copper wire on a plastic pipe or a thick pencil tube
- A small strong magnet that slides freely in the tube
- A multimeter with a MAX or peak-hold function on its mV range, or a phone to film the display
- A tall plastic pipe or rolled card tube to guide the fall
- A ruler and a soft cushion
- Tape, a marker and a notebook
Safety
For this project: Neodymium magnets are brittle and can pinch; an adult must handle them. Catch the magnet on a cushion so it does not chip.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
7 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Wind the coil on the tube with neat layers and scrape the enamel off the ends to connect the multimeter on the mV range.
Stand the tube upright with the coil about halfway down and a cushion at the bottom.
Drop the magnet from just above the coil (h = 5 cm) and record the largest voltage shown or filmed on the multimeter.
Repeat for h = 10, 20, 40 and 80 cm, taking three drops at each height and noting the highest value for each.
Calculate the average peak emf and compute √h for each height.
Plot the peak emf against √h and see whether the points lie close to a straight line through the origin.
Repeat with a coil of half the turns to show the emf drops to about half.
What you should see
The peak emf increases with the drop height, and the graph of peak emf against √h is close to a straight line. A coil with half the turns gives about half the emf.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is Faraday’s law?
The induced emf equals the rate of change of magnetic flux linked with the coil.
2Why is the emf larger from a greater height?
The magnet is moving faster, so the flux changes faster.
3Why does the second pulse have the opposite sign?
The flux falls as the magnet leaves, so the induced emf reverses, as Lenz’s law predicts.
4How does the number of turns matter?
The emf is proportional to the number of turns N.
Take it further
Time the fall with a phone video and compare the measured speed with √(2gh) to estimate the drag.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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