What it shows
Moving a magnet through a coil changes the magnetic flux through it and induces an emf (Faraday’s law), which lights an LED with no cell at all. Faster shaking and more turns give a brighter flash.
The science
Faraday’s law says the induced emf equals the number of turns multiplied by the rate of change of magnetic flux, e = N dphi/dt. As the magnet enters the coil the flux rises and a voltage appears; as it leaves, the flux falls and the voltage reverses, so the output is alternating. Two LEDs wired in opposite directions light on alternate halves of each shake. Lenz’s law explains why the magnet feels a slight drag as it passes through the coil: the induced current opposes the motion that creates it.
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 15 cm length of PVC or a thick ballpoint pen barrel, 15 to 20 mm wide
- About 40 m of enamelled copper wire, 32 to 36 SWG (about Rs 150)
- A strong neodymium rod or stack of disc magnets that slides freely inside the tube
- Two LEDs (red works best, it needs only about 1.8 V)
- Two bottle caps or rubber stoppers to close the ends
- Tape, sandpaper and a multimeter set to AC volts
Safety
For this project: Keep neodymium magnets away from phones, cards and small children. Sand the enamel outdoors or over a bin. No cells are needed, so there is nothing to short.
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
8 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.
Close one end of the tube with a cap and check the magnet slides from end to end freely but without rattling sideways.
Wind about 1000 turns of fine wire in the middle 4 cm of the tube in neat layers, taping each layer, and leave 20 cm tails.
Sand the enamel off the tails and connect the two LEDs across the coil in opposite directions (anti-parallel).
Close the other end, hold the tube level and shake it along its length so the magnet passes through the coil; the LEDs should flicker.
Connect a multimeter on AC volts across the coil and record the peak reading for slow, medium and fast shakes in a table.
Wind a second coil of 500 turns on a similar tube and repeat the readings to show the effect of the number of turns.
Drop the magnet through the coil from a measured height and note that a bigger drop gives a bigger flash.
Mount the finished torch on a card with a diagram of the field lines and the formula e = N dphi/dt.
What you should see
The LEDs flash on every pass of the magnet, with the two LEDs alternating, and the meter shows a higher voltage for faster shakes and more turns, typically 1 to 3 V peak for 1000 turns.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What produces the emf in the coil?
The changing magnetic flux as the magnet moves, by Faraday’s law of electromagnetic induction.
2Why do the two LEDs light alternately?
The induced current reverses direction when the magnet enters versus leaves the coil, so the output is AC.
3What does Lenz’s law predict here?
The induced current opposes the motion of the magnet, so there is a slight drag.
4How can the output be increased?
More turns, a stronger magnet, or faster motion.
Take it further
Add a diode bridge and a 1 F supercapacitor so the torch stores charge and stays lit after shaking stops.
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
Science projects
Electricity and Magnetism Experiments
Hands-on experiments with cells, wires, bulbs and magnets
Science projects
Physics Working Models Explained
Twenty classic models, how each works and how to make it