What it shows
Bicycle dynamos and wind turbines make electricity from a moving magnet near a coil. The project builds a model generator and tests how peak voltage depends on the speed of rotation and the number of turns, which is Faraday’s law in action.
The science
By Faraday’s law, the voltage in a coil equals the number of turns times the rate of change of magnetic flux. Faster rotation changes the flux faster, so peak voltage rises in proportion to speed, and more turns give a proportionally larger voltage. Alternating poles give an alternating output whose frequency equals rpm times magnet pairs divided by 60.
Materials
Cost: under Rs 1,500. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A cardboard or wooden disc with four neodymium magnets fixed at the edge
- A motor or a hand crank to spin it (a 12 V DC motor of 500 to 1000 rpm, with speed set by changing the battery voltage in steps)
- Three bobbins wound with 100, 200 and 400 turns of enamelled wire
- A sound-card scope app or a multimeter with AC range
- An IR tachometer or phone app to measure rpm
- A stand, glue, tape
Safety
For this project: Neodymium magnets are very strong and can pinch fingers or shatter. Keep them away from phones, cards and pacemakers. Glue and tape the magnets firmly, because a loose magnet can fly off a spinning disc, and stand clear while it turns. An adult must handle the magnets and the motor.
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.
Step 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 the three coils on identical bobbins, counting the turns carefully, and scrape the ends clean.
Fix four magnets on the disc edge with alternating poles facing out, using a small compass to check the polarity.
Mount the disc on the motor shaft and fix a coil holder so the coil sits 3 to 5 mm from the passing magnets.
Connect the 400-turn coil to the multimeter on AC volts, or to the sound-card input through a 10 kΩ resistor in series for protection.
Spin the disc at five different speeds from 200 to 1000 rpm, and record rpm and the peak or RMS voltage.
Repeat for the 100 and 200-turn coils at the same speeds.
Plot peak voltage against rpm for each coil and compare the slopes.
Plot voltage against number of turns at a fixed speed.
Measure the frequency from the scope and compare with the formula rpm x 2 pairs divided by 60.
Write the report on Faraday’s law, results and the effect of air gap.
What you should see
The graphs are straight lines through the origin: doubling the speed or the turns roughly doubles the voltage. The measured frequency matches the formula, and reducing the air gap raises voltage.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
01Why does voltage rise with speed?
The flux through the coil changes faster, and induced EMF equals the rate of change of flux linkage.
02What happens if all magnets face the same way?
Each magnet still gives one positive and one negative pulse, but the waveform becomes spiky with gaps, and there are four cycles per turn instead of two, so the sine-wave formula no longer fits.
03What is Lenz’s law?
The induced current opposes the change that produces it, so the generator is harder to turn when loaded.
04Why does a smaller air gap increase the output?
The magnetic field is stronger closer to the magnet, so more flux links the coil.
Take it further
Add a bridge rectifier and a 1000 µF capacitor (stripe to the negative side) to get DC and light an LED through a 330 Ω resistor at the highest speed.
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
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