Induced Potential and the Generator Effect

(Higher Tier only) (Separate Physics only)

The whole of this lesson is Higher Tier and Separate Physics content. It is not part of the Combined Science course.

This lesson is the mirror image of the motor effect. In the motor effect, a current in a magnetic field produces movement. In the generator effect, movement in a magnetic field produces electricity. The same physics runs both ways, which is why understanding one helps you understand the other.

Everything here follows from one idea: whenever the magnetic field passing through a conductor changes, a potential difference is induced across that conductor.

(Higher Tier only) (Separate Physics only)

A potential difference (pd) is induced across a conductor in two situations:

• the conductor moves relative to a magnetic field (for example, a wire cutting through field lines);
• the magnetic field around the conductor changes (for example, a magnet pushed into or out of a coil).

If the conductor forms part of a complete circuit, this induced pd drives an induced current. This is the generator effect.

The pd is only induced while there is movement or a changing field. Hold the magnet still inside the coil and the induced pd falls to zero, because nothing is changing.

You can demonstrate this by pushing a bar magnet into a coil connected to a sensitive ammeter: the needle deflects one way as the magnet enters, returns to zero when it stops, and deflects the other way as the magnet is pulled out.

(Higher Tier only) (Separate Physics only)

The direction of the induced current is not random. An induced current always flows in the direction that opposes the change that produced it.

The induced current creates its own magnetic field, and this field always acts to oppose the original change in the field.

If you push a north pole into a coil, the coil's induced current makes the near end a north pole too, pushing back against the incoming magnet. Pull the magnet out and the induced current reverses, making the near end a south pole to try to hold the magnet in.

This is why a generator is hard to turn when it is supplying a large current: you must do work against this opposing force, and that work is the source of the electrical energy generated. Energy is conserved, so the electricity does not come for free.

(Higher Tier only) (Separate Physics only)

The size of the induced pd can be increased by:

• moving the magnet or conductor faster;
• using a stronger magnet (stronger magnetic field);
• adding more turns to the coil.

The direction of the induced pd is reversed by:

• reversing the direction of movement (pushing in versus pulling out);
• reversing the magnetic field (swapping the poles of the magnet).

Change made	Effect on induced pd
Move faster	Larger pd
Stronger magnet	Larger pd
More turns on the coil	Larger pd
Reverse the movement	pd reverses direction
Reverse the magnet's poles	pd reverses direction
No movement (held still)	No induced pd

(Higher Tier only) (Separate Physics only)

A generator spins a coil in a magnetic field (or a magnet inside a coil). As the coil rotates, the field through it changes continuously, inducing a pd. Two designs differ only in how they connect the coil to the circuit.

• An alternator uses two slip rings, one on each end of the coil. The connections never swap, so the output reverses direction every half turn: it produces alternating current (ac).
• A dynamo uses a split-ring commutator, the same part as a motor. It swaps the connections every half turn, so the output always flows the same way: it produces direct current (dc).

The output graphs of pd against time show the difference clearly. Both peak when the coil is horizontal (cutting field lines fastest) and are zero when the coil is vertical (moving along the field lines, not cutting them).

Feature	Alternator	Dynamo
Connection type	Slip rings	Split-ring commutator
Output current	Alternating (ac)	Direct (dc)
pd–time graph	Full sine wave, going positive then negative	All humps on the same side (dc), never going negative

Spinning the coil faster increases both the frequency and the peak pd of the output, because the field is changing more quickly.

(Higher Tier only) (Separate Physics only)

A microphone is the reverse of a loudspeaker. A loudspeaker uses the motor effect to turn a current into sound; a microphone uses the generator effect to turn sound into a current.

Inside a moving-coil microphone, a coil is attached to a flexible diaphragm and sits in the field of a permanent magnet. The steps are:

1. Sound waves are pressure variations in the air; they make the diaphragm vibrate.
2. The vibrating diaphragm moves the coil back and forth in the magnetic field.
3. This changing field through the coil induces a pd, and so a current, by the generator effect.
4. The induced current varies in step with the sound: its size and direction copy the pressure variations of the sound wave.

A louder sound moves the diaphragm further and faster, inducing a larger current; a higher-pitched sound vibrates the coil at a higher frequency, so the current alternates faster.

The result is an electrical signal that is an accurate copy of the original sound, ready to be recorded or amplified.

1. Forgetting that something must be changing

A pd is only induced while the conductor is moving or the field is changing. If the magnet is held still inside the coil, there is no induced pd, even though the field is strong.

2. Using the motor effect by mistake

The generator effect is movement in, electricity out. The motor effect is the opposite: current in, movement out. A microphone uses the generator effect; a loudspeaker uses the motor effect.

3. Mixing up alternator and dynamo

Slip rings give alternating current (alternator). A split-ring commutator gives direct current (dynamo). It is easy to swap these, so learn the pairing carefully.

4. Wrong shape for the pd–time graphs

The alternator's graph is a full sine wave that goes both positive and negative. The dynamo's dc graph never goes negative: every hump is on the same side of the axis.

5. Ignoring the opposing force

The induced current opposes the change that caused it, so work must be done to keep the generator turning. This is why a bicycle dynamo makes pedalling harder, and it is a direct result of conservation of energy.