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Electromagnetic induction What is induction?
Inducing a voltage
Interactive graphic of induction in wire
Picture 4.4 Animation of wire in field.

We can induce a voltage in a wire using a magnetic field. We need to make the wire move through the field. We call the voltage the induced EMF (electromotive force).

The faster the conductor moves through the field, the bigger the induced EMF. This is Faraday's law.

If we move the wire the other way, then the direction of the EMF will be reversed.

The EMF will drop to zero if the wire

The wire needs to be cutting through the lines of flux in order to induce an EMF.

We get the biggest induced voltage when these three quantities are at right angles to each other:

  • the motion of the conductor
  • the magnetic field B
  • the wire (and hence induced EMF).

Interactive graphic of induction in wire
Picture 4.5 Lenz's Law
Why do we get a voltage?

Imagine some free electrons (or beam of electrons) being fired into the magnetic field. There will be a force on the electrons. The electrons have a negative charge. This means that although the electrons are moving from left to right, they are like a current flowing from right to left.

We can use Fleming's Left hand Motor Rule to find out the direction of the force. It is downwards. So the electrons are pushed downwards.

A piece of copper wire also contains free electrons. So when the wire moves into the field, the electrons are pushed downwards. This leaves behind a net positive charge at the top of the wire. Hence, the charge is separated in the wire setting up a voltage. The top has become more positive and the bottom has become more negative.

Interactive graphic of Lenz Law
Picture 4.6 Lenz's law
Which way is the force?
This EMF is like the EMF of a cell. It is available to drive a current around a circuit. If we attach a load to the wire, then a current will flow. We call this an induced current. However, as soon as we take a current from the wire, the wire will feel a force (a wire carrying a current in a magnetic field feels a force).

We can use Fleming's Left Hand Motor Rule to work out the direction of the force. In this case it is to the left.

In other words, the force will push against the motion of the wire. The wire will slow down. If we want to keep it moving, we'll have to push it.

If we take a bigger current from the wire, we'll have to push it harder. The bigger the current we get from the induced EMF, the more work we have to put in.

This makes sense: we don't get something for nothing. When we take a bigger current, we are getting the induced EMF to do more electrical work for us. Therefore, we have to put in more mechanical work. This is the conservation of energy.

Lenz's Law
When we start taking a current from the induced voltage, there is a force on the wire. We have already seen that the force will tend to slow the wire down - or make it harder to keep it going. This is expressed in Lenz's Law:

The induced current flows in such a way as to oppose the motion that produced it.

Lenz's Law is based on the idea of the conservation of energy. If the induced current did not flow in this way, then we would be able to get something for nothing.

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Question 12
a) In picture 4.5, the electrons are moving from left to right. This is like a current flowing from right to left. Why is the current in the opposite direction to the electrons' movement?

b) Explain why there is a force on a wire from which we take an induced current.

c) The direction of the force opposes the motion that led to the induced current.
i. Describe what would happen if this were not the case.

ii. Whose law is this?