Faraday’s law. An EMF is induced when the magnetic flux through the coil changes: EMF = −N dΦ/dt. The minus sign is Lenz’s law: the induced current is in the direction that opposes the change.

Why two peaks? As the magnet falls towards the coil, the flux through it increases, which gives an EMF in one direction. When the magnet has passed through and is moving away, the flux through the coil decreases, which gives an EMF in the opposite direction. In the middle, when the middle of the magnet is at the coil, the flux is at its largest and is not changing, so the EMF is zero.

Why is the second peak bigger and narrower? The magnet is falling faster, so the flux changes faster: a bigger EMF for a shorter time.

Why are the two areas equal? The area under the EMF graph is −N ΔΦ. The flux through the coil is zero before the magnet arrives and zero again after it has passed, so the total change is zero: the area above the axis equals the area below. (From a low release height the magnet starts with a little flux already through the coil, so the areas are only nearly equal.)

What the controls do. More turns give a proportionally bigger EMF. A higher release height means a faster magnet, so bigger and narrower peaks. Flipping the magnet reverses both peaks. It makes no difference to their size.

The EMF is in relative units. The fall, in centimetres and milliseconds, is real free fall (g = 9.8 m/s²), so the animation is slowed down compared with the graph’s time axis. The magnet is modelled as two magnetic poles 4.5 cm apart, and the coil has a radius of 2.5 cm.