The Generator Effect

The generator effect is the opposite of the…

motor effect. Instead of electricity being used to create motion, motion is being used to create electricity.

What is the Generator Effect?

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.

How can you induce a potential difference?

• By moving a conductor relative (wire) to a magnetic field - cutting through the field lines and therefore inducing a potential difference in the wire.

• Or moving the magnet in a coil of wire - as the magnet moves through the coil, the field lines cut through the turns on the coil - inducing a potential difference in the coil

What is the requirement for an induced potential difference to cause a current to flow?

The conductor must form a closed loop or be part of a complete circuit.

What happens when you move the magnet or conductor in the opposite direction?

Then the potential difference or current will be reversed.

Factors affecting the induced potential difference:

• speed at which the wire, coil or magnet is moved - increasing the speed increases the rate at which field lines are cut - increases p.d.

• number of turns on the coils in the wire - more coils = more potential difference induced because each coil cuts through the magnetic field lines and the total p.d. induced is the result of all the coils cutting the field lines

• size of the coil - increasing area of coils increases p.d. - more wire to cut through magnetic field lines

• strength of magnetic field - increasing strength increases p.d. induced

What does Lenz’s law state?

The direction of an induced potential difference always opposes the change that produces it.

What happens when a magnet is pushed north end first into a coil of wire?

The end of the coil closest to the magnet becomes a north pole. This is because, due to the generator effect, a potential difference will be induced in the coil and this potential difference always opposes the change that produces it. This means the coil will apply a force to oppose the magnet being pushed into the coil. Therefore, the end of the coil closest to the magnet will become a north pole so it will repel the north pole of the magnet.

What happens when a magnet is pulled away from the coil of wire?

The end of the coil closest to the magnet becomes a south pole. This is because, due to the generator effect, a potential difference will be induced in the coil and this induced potential difference always opposes the change that produces it. So the coil will apply a force to oppose the magnet being pulled away from the coil. This means the end of the coil closest to the magnet becomes a south pole so it will attract the north pole of the magnet.

Describe how rotation of a magnet can cause the Generator Effect:

As you turn the magnet, the magnetic field through coil changes, this change in the magnetic field induces a p.d., which can make current flow in the wire.

Every time the magnet moves through half a turn, the direction of the magnetic field through the coil reverses, when this happens the p.d. reverses so the current flows in the opposite direction around the coil of wire. If you keep turning the magnet in the same direction - like always clockwise - then the potential difference will keep reversing every half turn and you’ll get an alternating current.

Applications of a the generator effect:

• To generate a.c. in an alternator

• To generate d.c. in a dynamo

What is a simple alternator?

A type of generator that turns mechanical energy to electrical energy in the form of alternating current.

Describe how alternators generate alternating current:

Alternators rotate a coil in a magnetic field, or rotate a magnet in a coil. Construction is similar to a motor. As the coil/magnet spins, a current is induced in the coil, this current changes direction every half turn. Alternators have slip rings & brushes instead of a split-ring commutator, so the contacts don’t swap every half turn. This means an alternating potential difference is produced & therefore an alternating current.

What do the slip rings and brushes provide between the coil and the voltmeter?

A continuous connection.

Describe how Dynamos generate direct current:

They work in the same way as an alternator but have a split-ring commutator instead of 2 separate slip rings. This swaps the connection every half turn to keep the current flowing in the same direction. So the induced pd does not reverse its direction and instead varies from zero to a maximum value twice each cycle of rotation & never changes polarity - current is always positive or negative.

Alternator a.c output:

Dynamo d.c. output:

What device is used to show how potential difference generated in a coil changes over time?

An oscilloscope.

How do microphones use the generator effect?

They convert sound waves into electrical current.

When sound waves reach the microphone, pressure variations cause the diaphragm to vibrate - causing the coil to move back & forth through the magnetic field produced by the magnet. As it does this, the coil cuts through the field lines and induces a potential difference in the coil - the pd will be alternating as the coil is continually changing direction due to the diaphragms vibrations.