Electricity and magnetism12: The generator effect and applications

e.g. dc dynamo & ac alternator generators - generating electricity & microphone

Doc Brown's Physics Revision Notes

Suitable for GCSE/IGCSE Physics/Science courses or their equivalent

What do we mean by electromagnetic induction?

How does an alternator work?

How does a dynamo work?

How can you increase the power output from a generator?

(1) What is the 'generator effect'? What is electromagnetic induction?

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.

eg 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

(2) Demonstrations of electromagnetic induction - the generator effect

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

Demonstration 1 Stationary coil and moving magnet.

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 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 ans 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

(3) Two other things to consider before looking at examples of generators ...

(a) An induced current opposes the change that caused the induction!

We have seen that a change in magnetic field induces a current in a conductor e.g. a copper wire.

BUT, when a current flows through a wire, a magnetic field is created around the wire.

So, we are now dealing with two magnetic fields.

The magnetic field produced by the induced current in the wire always acts against the change that made it,

AND, it doesn't matter whether the induction is due to the movement of the wire or the movement of the magnetic field.

It's as if the 'system' is trying to change back to where it started i.e. the induced current opposes the change that made it.

At first you might think - how can we continuously extract electrical energy from the induction system?

BUT, remember, a generator requires a constant input of kinetic energy from e.g. a diesel engine or steam/water turbine.

You are building up an electrical energy store from another energy store!

You can't get energy for nothing!

(b) How can be change i.e. increase, the size of the induced potential difference

It is important to know how change the size of the induced p.d. or current flow.

To change the size of the induced pd you must change the rate at which the magnetic field changes.

You are usually interested in increasing the pd or current or both at the same time e.g.

Increasing the speed of rotation (motion)

Increased rotation of the coil or magnet means more magnetic lines of force are cut per unit time.

For a given strength of magnetic field the density of the lines of force are constant, but with increased motion you move through them faster.

Increasing the strength of the magnetic field

The greater magnetic flux density means more magnetic lines of force are cut per unit time.

Remember - the greater the strength of the magnetic field from a stronger magnet, the closer together are the lines of force.

Increasing the number of turns of wire per unit length

The greater the density of the coils, the more of the conductor the magnetic lines of force cut through.

A generator is a practical way of producing a continuous supply of electricity.

There are two main types - (a) the alternator and (b) the dynamo.

Both types make use of the generator effect to induce current, and the two simple generators described below both involve rotating coils of wire in a fixed magnetic field cutting through lines of force.

They both involve a similar circuit construction BUT there are some differences, so take care!

Both types described here involve rotating the coil in a fixed magnetic field, so the coil is continuously cutting through the magnetic flux lines of force.

The generator effect

All generators must have a source of power to rotate the coil of wire.

As the coil spins, it cuts through the magnetic field and a current is induced in the coil.

Dynamos are d.c. generators and alternators generate an a.c. current.

Electromagnetic induction - inducing a current in a rotating coil cutting through a magnetic field.

The direction of rotation can be predicted from Fleming's right-hand rule (NOT on the GCSE specification)

Consider the right side of the coil for clockwise motion (could be part of simple dc dynamo generator)

The thuMb represents the direction the force acts - direction of motion - downwards (emphasise the M).

The First Finger represents the direction of the magnetic field N => S (phonetically emphasise the F).

The SeCond finger predicts the direction of the induced convention current (emphasise the 'hard' C).

Repeat for the left side of the coil, moving upwards, and you should predict the current will be flowing in the opposite direction.

(a) The alternator ac generator - producing an alternating current/p.d.

A simple ALTERNATOR

Reminders: All generators must have a source of power to rotate the coil of wire.

As the coil spins, it cuts through the magnetic field and a current is induced in the coil.

Dynamos are d.c. generators and alternators generate an a.c. current.

Explaining how a simple ac alternator generator works

The construction is very similar to that of a simple electric motor.

The coil is rotated through the magnetic field by some external power source of kinetic energy.

As the coil rotates, cutting through the magnetic field, a current is induced in the coil.

The current will change direction after every half-turn.

To 'extract' the electrical current i.e. connect with the external circuit, ac generators use a system of slip rings and brushes.

This means the contacts don't swap every half-turn and so an alternating current (ac, alternating pd) is produced

This is different from the split-ring commutator used in the and the dynamo generator described next.

Brush contacts allow continuous electrical connection without inhibiting the movement of the commutator.

CRO oscilloscope traces from generators

An oscilloscope can show how the p.d. across the coil of a generator varies with time.

Three examples of oscilloscope traces from generators are shown above (x axis = time, y axis = pd).

1. This trace shows an alternating current i.e. the p.d. is changing from +ve to 0 to -ve values in a continuous cycle.

You can tell its an ac trace because it goes up and down of the horizontal axis of p.d. 0 V.

2. This is also a trace from an alternator generator, but the rotation of the coil is greater than for 1.

The height of the trace above 0 V at any point tells you the p.d. generated at that point.

The higher the peak from the 0 V horizontal axis, the greater the potential difference generated.

3. This is a trace from a dc dynamo generator

You can tell it is not an alternating current because the trace consists of a succession of half-cycles.

The generator describe above will produce traces 1. and 2.

(b) The dynamo dc generator - producing a direct current/constant p.d

A simple DYNAMO

Reminders: All generators must have a source of power to rotate the coil of wire.

As the coil spins, it cuts through the magnetic field and a current is induced in the coil.

Dynamos are d.c. generators and alternators generate an a.c. current.

Explaining how a simple dc dynamo generator works

A dynamo works like an alternator in that the coils are rotated through the magnetic field by some external source of kinetic energy.

However, unlike the alternator, but like the electric motor, it uses a split-ring commutator and NOT slip rings, to connect with the external circuit.

The split ring commutator swaps the connections every half-turn so the current keeps flowing in the same direction - direct current generation (dc).

Brush contacts allow continuous electrical connection without inhibiting the movement of the commutator.

CRO oscilloscope traces from generators

An oscilloscope can show how the p.d. across the coil of a generator varies with time.

Three examples of oscilloscope traces from generators are shown above (x axis = time, y axis = pd).

1. This trace shows an alternating current i.e. the p.d. is changing from +ve to 0 to -ve values in a continuous cycle.

You can tell its an ac trace because it goes up and down of the horizontal axis of p.d. 0 V.

2. This is also a trace from an alternator generator, but the rotation of the coil is greater than for 1.

Therefore trace 2. shows a greater a.c. frequency than 1.

The height of the trace above 0 V at any point tells you the p.d. generated at that point.

The higher the peak from the 0 V horizontal axis, the greater the potential difference generated.

3. This is a trace from a dc dynamo generator

You can tell it is not an alternating current because the trace consists of a succession of half-cycles.

The generator describe above will only produce trace 3.

How can you increase the p.d. and power output from a dynamo?

As already mentioned, you need to increase the rate of cutting through the magnetic flux - the lines of force.

There are four ways to do this based on the simple ac and dc generators described above ...

(i) Increasing the number of turns of wire in the coil.

The magnetic lines of force 'cut' through more wire per unit time.

(ii) By winding the coil on a soft-iron armature to increase the magnetic flux through the coil.

Iron concentrates the lines of force, so more lines of force are 'cut' through per unit time.

(iii) Increasing the rate of rotation of the coil or magnet.

More magnetic lines of force are 'cut' through per unit time.

(iv) By making the field magnet as strong as possible.

The magnetic flux is more dense, the lines of force are closer together, so more lines of force are 'cut' per time.

(v) Making the distance between the coil and the magnet as small as possible.

These factors apply to any electric generator design.

How does a microphone work? - a microphone works due to electromagnetic induction

A microphone works in the opposite way to a .

The diagram above illustrate the principle of a microphone e.g. for a vocalist or a telephone mouthpiece.

A microphone converts the energy of the pressure variation of sound waves into an electrical energy signal.

A microphone behaves like a loudspeaker in reverse -

The right-hand side of the diagram was borrowed from my loudspeaker diagram!

The coil of wire surrounds one pole of a permanent magnet, which is itself surrounded by the other pole of the magnet - look for the N and S on the diagram above.

The coil, in which the current is induced, is connected to the flexible diaphragm cone cover made of thin plastic or metal.

If the coil moves in the magnetic field, a p.d. is induced in the coil.

This is no different in principle to the working of a generator - a coil moves through a magnetic field,

but there is no rotation, its a 'to and fro' vibration effect.

The initial movement comes from the sound waves hitting the flexible diaphragm.

The sound wave vibrations cause the diaphragm to vibrate and move in resonance with the sound wave.

In turn, the coil automatically moves at the same frequencies as the sound wave inducing a varying current in the coil.

Also, the louder the sound, the bigger displacement of the coil.

Therefore the movement of the coil generates (induces) an electrical current signal that can be used to reproduce both the frequencies and relative volumes (loudness) of the sound waves.

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Electricity and magnetism revision notes index

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