Generating a current

If an electrical conductor moves relative to a magnetic field, or if there is a change in the magnetic field around a conductor, a potential difference is induced across the ends of the conductor. If the conductor is part of a complete circuit, a current is induced in the conductor. This is called the generator effect.

Note the two separate ways of producing it: move the conductor, or change the field. Either will do, and both amount to the same thing — the conductor experiencing a changing magnetic field.

An induced current generates a magnetic field that opposes the original change, either the movement of the conductor or the change in magnetic field. That opposition is why a generator becomes harder to turn when more current is drawn from it.

You should be able to recall the factors that affect the size of the induced potential difference or induced current:

• the speed of the movement — moving the conductor or magnet faster gives a larger induced potential difference • the strength of the magnetic field — a stronger magnet gives a larger induced potential difference • the number of turns on the coil — more turns give a larger induced potential difference.

You should also be able to recall the factors that affect the direction of the induced potential difference or induced current:

• the direction of movement — reversing it reverses the induced potential difference • the orientation, or polarity, of the magnetic field — reversing the poles reverses it too.

Reversing both at once leaves the direction unchanged.

You should be able to apply the principles of the generator effect in a given context. The reasoning is always the same: identify whether the conductor is experiencing a changing field, say what would make the change faster or larger, and remember that no relative movement means no induced potential difference at all. A magnet held still inside a coil induces nothing.

The generator effect is used in an alternator to generate ac, and in a dynamo to generate dc.

You should be able to explain how the generator effect is used in each, and draw or interpret graphs of potential difference generated in the coil against time.

Both work the same way: a coil is rotated in a magnetic field, so the coil continually cuts through the field lines and a potential difference is induced. The difference between them is entirely in how the coil is connected to the external circuit.

Reading the graphs: for both, the potential difference is zero when the coil is parallel to the field lines, because at that instant it is not cutting through any, and it is greatest when the coil is at right angles to them and cutting through them fastest. Turning the coil faster increases both the frequency and the peak potential difference, so the humps get taller and closer together.

Microphones use the generator effect to convert the pressure variations in sound waves into variations in current in electrical circuits.

You should be able to explain how a moving-coil microphone works:

1. Sound waves reach a flexible diaphragm, and the pressure variations make it vibrate. 2. A coil of wire attached to the diaphragm vibrates with it, moving within the field of a permanent magnet. 3. The coil is moving relative to a magnetic field, so by the generator effect a potential difference is induced across it. 4. Because the movement reverses as the diaphragm goes back and forth, the induced potential difference alternates. 5. The frequency of the induced current matches the frequency of the sound wave, and a louder sound moves the diaphragm further, giving a larger current.

This is a loudspeaker running backwards. A loudspeaker uses the motor effect to turn a current into movement and then sound; a microphone uses the generator effect to turn sound and movement back into a current.