What it shows
A magnet dropped through a coil induces a voltage pulse, and one dropped through a copper pipe falls slowly because of eddy currents. The project records both events and measures how induced voltage depends on speed and how Lenz’s law slows the fall.
The science
As the magnet enters the coil, flux through it grows and an EMF opposes the change, and as it leaves, the EMF reverses. The area under each pulse equals the change in flux linkage divided by resistance. In the copper pipe, the moving magnet induces circulating currents whose magnetic field opposes the motion, so the magnet reaches a slow terminal speed, much like drag.
Materials
Cost: under Rs 500. 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 wire on a plastic pipe
- A neodymium magnet that slides inside the pipe
- A copper pipe about 50 cm long and a plastic pipe of the same size
- A 3.5 mm audio jack and a 10 kΩ resistor in series
- A laptop with free audio-recording software such as Audacity
- A stopwatch or phone slow-motion video
Safety
For this project: Strong magnets can pinch skin and snap together hard. An adult must handle them. Never connect the coil directly to a phone input without the series resistor.
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
10 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 about 500 turns on a short piece of plastic pipe or buy a bobbin, scrape the ends and attach them to the audio jack through the 10 kΩ resistor.
Record on the laptop at 44.1 kHz and drop the magnet through the coil inside a plastic tube.
Zoom in on the waveform and note the two pulses of opposite signs and the time between them.
Drop from three heights (10, 20, 30 cm) and measure the peak voltage in relative units and the time between pulses.
Calculate the speed from the coil length divided by pulse time and plot peak height against speed.
Time the fall of the same magnet through the plastic pipe and the copper pipe with slow-motion video, five times each.
Calculate the average speed in each pipe and compare with free fall time.
Repeat with a non-magnetic steel dummy of the same size to show that the copper has no effect on it.
Explain the results using flux and Lenz’s law with sketches.
Write the report with waveform screenshots and tables.
What you should see
The recording shows a positive and then a negative pulse, with larger pulses for faster falls. The magnet takes several times longer to fall through the copper pipe than through the plastic one, while the steel dummy falls normally.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why are the two pulses opposite in sign?
Flux first increases and then decreases as the magnet enters and leaves the coil.
2Why does the magnet fall slowly in copper?
Eddy currents set up by the moving field create an opposing magnetic force.
3Will the effect change in an aluminium pipe?
It will be weaker because aluminium has higher resistivity than copper, so smaller eddy currents flow.
4Where are eddy-current brakes used?
In some trains and roller coasters, and in energy meter discs.
Take it further
Cut a slit along the copper pipe and show that the magnet now falls faster because eddy currents are broken.
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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