Electromagnetic induction and generatorsIB MYP Physics: Revision notes

Electromagnetic induction is the production of a voltage, called an induced e.m.f. (electromotive force), in a conductor when it is in a changing magnetic field. This happens when:

• a magnet moves into or out of a coil, or
• a wire moves across a magnetic field so that it cuts the field lines.

If the conductor is part of a complete circuit, the e.m.f. makes a current flow. There is no e.m.f. if the magnet and coil are still, or if the wire moves parallel to the field lines.

Reversing the direction of movement, or swapping the magnet's poles, reverses the e.m.f.

The induced e.m.f. is larger when:

• the magnet or wire moves faster
• the magnet is stronger
• the coil has more turns

These all increase the rate at which field lines are cut.

Lenz's law says that the direction of an induced current is always such that its own magnetic field opposes the change that produced it.

Example: push the north pole of a magnet towards a coil. The end of the coil nearest the magnet becomes a north pole and repels the magnet. You must do work to push the magnet in, and this work is transferred to electrical energy. If the induced pole attracted the magnet instead, energy would be created from nothing, which is impossible.

An a.c. generator (alternator) has a coil that is turned in a magnetic field by a turbine or engine. As the coil rotates its sides cut the field lines, inducing an e.m.f. The sides move in opposite directions through the field every half turn, so the e.m.f. reverses every half turn.

The coil connects to the outside circuit through slip rings and carbon brushes, which allow the coil to turn without twisting the wires.

The output is a repeating wave. It is zero when the coil is perpendicular to the field (the sides move along the field lines) and at its maximum when the coil is parallel to the field (the sides cut the lines fastest).

Turning the coil faster, or using stronger magnets or more turns, gives a larger e.m.f.

d.c. (from a cell) is a steady voltage that always has the same direction. a.c. (from a generator) alternates, so the voltage rises, falls, reverses and repeats.

For an a.c. wave:

• the frequency is the number of complete cycles per second, measured in hertz (Hz)
• the amplitude is the maximum value of the voltage, measured from zero

If the coil of a generator turns twice as fast, the frequency doubles and the amplitude increases (it doubles).

Mains electricity is a.c. with a frequency of 50 Hz.