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School Physics: Electricity-magnetism Section 12.1 The generator effect

Electromagnetic effects: 12.1 What is the 'generator effect'? What is electromagnetic induction? Demonstrations of electromagnetic induction

Doc Brown's Physics exam study revision notes

12.1A What is the 'generator effect'? What is electromagnetic induction?

Electricity is movement of electrically charged particles - but a stream of charge creates its own magnetic field.

Magnetism is to do with the field of magnetic flux associated with a magnet or magnetic materials.

This means electricity and magnetism are strongly interrelated.

You can generate electricity from the generator effect which is an example of electromagnetic induction.

You can induce a potential difference (p.d.) in an electrical conductor ...

(i) in a wire moving relative to a magnetic field.

eg a wire or coil moving between the poles of a stationary permanent magnet.

OR (ii) a wire experiences a changing magnetic field.

e.g. the wire may be stationary and the magnet rotated by it, of more effectively, a magnet rotated in a coil of wire.

These are simple examples of the electromagnetic induction effect.

The 'wire' can be any conductor and if it is part of a complete circuit a current will flow.

What these examples have in common is a conductor moving through a magnetic field or a magnetic field moving through a conductor - this induces the electromagnetic effect.

The electromagnetic induction effect is used in generators, loudspeakers and transformers

12.1B Demonstrations of electromagnetic induction - the generator effect

You can demonstrate this electromagnetic induction in the following ways ...

Demonstration 1 Stationary coil and moving magnet.

Moving a magnet in and out of a coil.

Moving a wire through a magnetic field.

Both simple demonstrations induce a tiny current in the wire - electromagnetic induction.

There are several ways you can increase the induced potential difference (and hence the current flow)

1. Moving the wire or the magnet faster - increase rate of the magnetic field passing through the wire in which the current is induced.
2. Using a more powerful magnet - increase in the magnetic flux passing through the wire.
3. Using a multi-coil of wire - the same magnetic field is interacting with a greater length of wire.

These factors are considered when designing electric motors or electrical generators.

You set up a circuit consisting of an insulated copper wire coil connected to a very sensitive ammeter.

You can use a 'modern' digital mA ammeter or a galvanometer - a 'pointer' dial version of a very sensitive ammeter from before the days of digital instruments! You also need a permanent magnet.

If you bring the magnet near the coil, but stationary, nothing appears to happen.

BUT, if you move the permanent magnet 'in and out' of the coil, a p.d. and current flow are induced in the coil.

It is the change in the magnetic field the wire experiences that induces the pd and current flow in it.

So, to start with, keeping the poles of the magnet pointing in the same direction ...

when the magnet 'goes in' you should get a 'blip' of an ammeter reading above 0.0 A in one direction and when you pull the magnet out, you get a 'blip' of an ammeter reading of less than 0.0 A.

The reason for these two opposite, but numerically equal ammeter readings, is that the coil and magnet are constants BUT if you change the direction of motion you change the direction of the p.d. and induced current.

Similarly, if you swap the poles of the magnet around, the two readings are reversed - the +ve ammeter reading becomes -ve and the -ve reading becomes +ve.

In other words, if you change the direction of the magnetic field you change the direction of the induced p.d. and current.

So, reversing one thing reverses another and you only get an induced p.d. with movement!

Note: If you keep on moving the magnet in and out of the coil you produce a continuous alternating current (a.c.) - this is the principle by which an alternator works, but you keep the magnet stationary and move the wire.

See the second demonstration of electromagnetic induction described below.

Demonstration 2 A moving coil and stationary magnet.

You need a U shaped permanent magnet or two permanent magnets.

The coil of wire is connected to a sensitive ammeter.

While the coil is stationary the ammeter reading stays at zero (0.0 A)

However, as you move the coil in and out of at 90o to the magnetic field you induce a p.d. and current.

It is the change in the magnetic field the wire experiences that induces the pd and current flow in it.

So, again, reversing one thing reverses another and you only get an induced p.d. with movement!

When you move the coil in one direction the induced current flows one way (e.g. ammeter reads >0.0 A) and if you move the coil in the other direction, the induced p.d. and current are also reversed (e.g. ammeter reads <0.0 A).

Here the magnetic field has a constant direction but the motion is continually reversed, once again producing an alternating current as the induced p.d. is also reversed.

Demonstration 3. Other ways of moving the magnetic field and coil relative to each other

There are all sorts of other demonstrations to show electromagnetic induction e.g.

(i) You can rotate a magnet inside a larger coil

The coil is stationary, but the magnetic field is constantly changing and 'cutting' through the wire.

For every half-turn of the magnet, the direction of the magnetic field reverses and so the p.d. reverses too and the current flows in the opposite direction.

Therefore, with continuous rotation, you produce an alternating current.

(ii) You can rotate a coil in a stationary magnetic field

Here the wire is continually 'cutting' through a magnetic field.

This is a more controlled variation of demonstration 2 and also see the

REMEMBER: It is the change in the magnetic field the wire experiences that induces the pd and current flow in it.

So, both these demonstration amount to the same thing - conducting wire and magnetic field moving relative to each other, and ....

... the rotation of the magnet or the coil is actually how generators work to produce either ...

(i) an alternating current (ac), in which the current direction periodically reverses,

or (ii) a direct current (dc) which only flows in one direction.

The electromagnetic induction effect is used in generators, loudspeakers and transformers

Keywords, phrases and learning objectives on the generator effect of an electric current

Be able to explain what we mean by is the 'generator effect'.

Be able to explain what is electromagnetic induction.

Be able to understand simple demonstrations of electromagnetic induction - investigation methods.

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