Experiments · High school

Faraday’s magnet and coil experiment

Move the magnet in the coil: the galvanometer swings while the flux changes, and the current stops when the magnet stops.

In the learning section below the figure, predict, try, complete the tasks and check yourself with short questions.

Fig. 1Faraday’s magnet and coil experiment

Drag the magnet up and down, or pick a mode

Flux Φ (one turn): 0 mWb
Rate of change dΦ/dt: 0 mWb/s
Induced voltage ε: 0 mV
Current direction: no current
Galvanometer scale: ± 300 mV

ε against time (last 5 s)

−5 s± 300 mV0 s

How the magnet moves

Depth into the coil

A negative value: the magnet is above the mouth of the coil.

Back-and-forth speed (peak)

Number of turns N

Magnet strength Br

Coil cross-section A

Learn

Progress is kept on this page only; no points are given.

1Predict
2Try
3Explain

The magnet sits perfectly still right inside the coil. The flux through the coil is close to its largest value. What does the galvanometer show?

Purple lines: the magnet’s field lines (from N to S)Arrows: direction of the induced current (in the coil and the wires)All the numbers come from one model: the field on the magnet’s axis uses the axial formula for a uniformly magnetised bar (far away it falls off like a dipole field); across each turn the field is taken to equal its value on the axis (Φ = B·A) and is averaged along the coil. ε = −N·dΦ/dt; the minus sign is Lenz’s law. Flux is counted positive downwards through the coil. This model overstates the flux: in reality the field is this strong only inside the magnet, and outside it the field turns back the other way; at these sizes the real flux and voltage come out at roughly a third of these values, though the order of magnitude is the same. In spring mode, friction is added as a small damping.

Faraday's induction experiment: how does a magnet make a current in a coil?Read the article