6.15-6.16 - Electromagnetic induction and generators

When voltage is induced

A conductor is a material that allows charge to move. Moving a straight conductor across magnetic field lines can induce a voltage across its ends. Motion along the field lines does not. For a coil, the field passing through its area must change: merely sliding a whole coil through a uniform field without changing that field through it does not induce a net voltage around the coil. A voltage can also be induced when the magnetic field through a coil changes, even if the coil itself is not moving.

The key word is change. A magnet pushed into a coil changes the magnetic field through the coil, so a sensitive voltmeter can show a deflection. A magnet held still inside the coil does not keep changing the field through the coil, so the induced voltage falls to zero.

If the conductor or coil is part of a complete circuit, the induced voltage can drive an induced current. If the circuit is not complete, you can still talk about the induced voltage, but there is no continuous current around the circuit.

[DIAGRAM: asset_name: Electromagnetic induction and generators - diagram 01; asset_slug: p53_electromagnetic_induction_and_generators__diagram_01; recommended_method: image_gen; description: White 16:9 NovaLearn pale-palette diagram with grey outlines and dark-grey labels. Moving rose-N/blue-S magnet beside a continuous clay coil connected by two leads to a sensitive voltmeter, with sage flux paths and a deflected needle.]

Size of induced voltage

The size of the induced voltage depends on how quickly the magnetic field through the conductor or coil changes. A faster change gives a larger induced voltage.

For International GCSE answers, the important practical factors are:

The wording matters. "A stronger magnet" or "a stronger magnetic field" is clearer than "a bigger magnet", because a physically larger magnet is not always stronger. "More turns on the coil" is clearer than "more coils".

Worked example: a student wants a larger voltage from a simple generator. Rotating the coil faster increases how quickly the magnetic field through the coil changes. Using a stronger magnet increases the field being changed. Adding more turns means the induced voltages in the turns add together, so the total voltage is larger.

Rotating magnet in a coil

One way to generate electricity is to rotate a magnet inside a fixed coil of wire. The magnet must turn so that its north and south poles alternately face the coil: simply spinning a bar magnet about its own north–south axis does not change the field through the coil. As its pole direction changes, the magnetic field through the coil changes again and again. This changing field induces a voltage across the coil.

The turning magnet must be supplied with energy, for example by a rotating shaft. The generator transfers energy from the rotating object to the electrical circuit. The magnet is not used up; it provides the changing magnetic field that makes induction possible.