# Electromotive Force in a Wire

How to induce an EMF (electromotive force) in a wire, also known as the generator effect.

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Up Learn – A Level physics (aqa)

## Magnetic Fields and Electromagnetic Induction

Last time, we saw that Michael Faraday got current to flow round a loop of wire by moving a magnet through the loop.

But why did the current flow?

Well, it’s all to do with electromotive force.

We’ve seen before, in the Electricity section, that a battery, or cell has an electromotive force, or emf, which drives electrons round a circuit.

And the emf is equal to the potential difference across the cell when there is no current flowing through it… which we can measure with a voltmeter.

Also for emf we use this symbol, which is the Greek letter epsilon.

Now, it turns out that when we move a wire through a magnetic field, we can create an emf between the ends of the wire.

For example, here’s a wire in a magnetic field

At the moment, there’s no emf between the ends of the wire.

But if we move the wire, like this then, while it’s moving, there is an emf between the ends of the wire.

But as soon as it stops moving, the emf drops to zero

And if we move the wire like this, we get an emf along the wire.

And if we move the wire like this,we also get a emf.

But if we move the wire parallel to the field lines…or parallel to its own length…we don’t get an emf between the ends of the wire.

So, when we move a wire through a magnetic field, we get an emf along the wire…so long as that movement isn’t parallel to the field lines, or parallel to the length of the wire.

And we say we’ve induced an emf in the wire

Now, another way to think about the movement of the wire is whether it cuts through the magnetic field lines.

For instance, here, it looks like the wire is cutting through the field lines..and so we know that we’ll induce an emf!

But here, when the wire’s moving parallel to the field lines, the wire isn’t cutting through the field lines, so we won’t induce an emf.

And here, the wire’s moving parallel to its own length, so it isn’t cutting through field lines either, and we won’t induce an emf.

So in general, when a wire cuts through magnetic field lines, we get an emf along the wire.

So now, in which of these wires would we induce an emf along the wire?

We would induce an emf along these wires, because they cut through magnetic field lines.

And when a wire cuts through magnetic field lines, inducing an emf along the wire, we call this electromagnetic induction.

And if the wire is connected to a circuit, the emf in the wire causes a current to flow round the circuit… Just like a battery.

And it actually doesn’t matter whether we move the wire, or the magnetic field.
As long as there is relative motion between the wire and the magnetic field, such that the wire cuts through magnetic field lines, we induce an emf along the wire.

So, to sum up…

If we move a wire through a magnetic field, such that it cuts through magnetic field lines, we induce an emf in the wire.

We call this process electromagnetic induction.