Magnetism and induction

How it works

A magnetic flux Φ passes through the coil and depends on where the magnet is. A voltage is induced only while the flux changes: by Faraday’s law it is proportional to the number of turns N and to the rate of change of the flux. A magnet at rest, however strong, induces nothing. Lenz’s law sets the sign – the induced current opposes the change that caused it – so reversing the motion reverses the voltage. In a closed circuit a current I = ε / R flows.

ε = −N · dΦ/dtI = ε / Rwire field: B = μ₀I / (2πr)

Try it yourself

1. In “Induction” mode click “Slow and fast”, or drag the magnet towards the coil first slowly and then quickly. Faster motion moves the ammeter further; when the magnet stops, the voltage drops to zero.
2. Start “Oscillate magnet” and raise “Motion frequency” from 0.25 to 0.5 Hz. The magnet covers the same path twice as fast, so the peak voltage doubles. Doubling the number of turns does the same.
3. Click “Open circuit”. A voltage is still induced across the coil, but no current flows because the circuit is not closed.
4. Click “Reverse the direction”. Around a wire carrying 5 A the field 18 cm away is about 5.6 µT. When the current reverses, the circular field lines and the compass needle turn the other way.

Model limitations

The magnet is modelled as a point dipole and the coil as an ideal circular loop with a 6 cm radius; when the magnet passes through the centre of the coil, the demo uses a smoothed teaching model of the flux. The coil’s self-inductance and the effect of the current back on the magnet are ignored. The field lines do not include the field of the induced current.

Related experiments

• Electric circuits
• RLC circuits
• Fourier series