Physics
Cambridge IGCSE

TOPIC 4B: MAGNETISM

# 4.5c Induction

The motor effect is caused when a current passes through a wire that is in a magnetic field. A force is produced that makes the wire move. A current and a field produce movement.

But can this effect work backwards? Could movement in a field produce a current? In fact this can be done, and was first discovered by Michael Faraday in 1831. It is the way all of our electricity is generated. Just move a wire through a magnetic field, and a current is produced in the wire (if connected in a circuit), as shown in figure 1. This effect of inducing a voltage and hence a current is called the generator effect.

Figure 1: The generator effect - creating electricity

Technically, it is an e.m.f. that is produced, and this voltage makes a current flow in a circuit. The wire must cut through the field lines - it does not work if you move the wire parallel to the field. It also does not work if the wire is stationary, a common mistake!

### Standard Practical: Describe an experiment to demonstrate electromagnetic induction

Figure 2 shows a method of producing a higher induced voltage and therefore current, making it easier to demonstrate induction. Instead of a single wire being used, use a coil with many turns. This time the magnet (and magnetic field) is moving rather than the coil, but the field lines are still cutting through the wires and this produces a current as shown on the milliammeter.

Figure 2: The generator effect using a coil

In this simple demonstration, moving the magnet produces an induced current. As soon as it stops moving, there is no induction as no field lines are being cut. Moving the magnet in the other direction reverses the current. You could also measure the e.m.f. produced by use a voltmeter/millivoltmeter instead of the milliammeter shown in figure 2.

To make the induced voltage/current larger, you can:

• Increase the strength of the magnetic field.
• Cut through field lines at a higher speed.
• Have a longer wire in the field - done easily by using a coil of wire with many turns.

### Direction of induced current

As induction is basically the motor effect working backwards, the direction of induced current is in the opposite direction to that required for the motor effect to create motion. In section 4.5a, you should have learned that the motor effect uses Fleming's left-hand rule to predict the force. We can therefore use Fleming's right-hand rule to predict the direction of the induced current, caused by the induced e.m.f., as a right hand is an inverse of a left hand.

Figure 3: Fleming's right-hand rule

Note that If the magnetic field is reversed, it will reverse the current direction. If you move the magnet in the opposite direction, the current also reverses.

However, the induced current always follows a simple rule: The induced current will make its own magnetic field around the wire(s), and this field MUST oppose the magnetic field that created it. For example, the current that is produced in the coil shown in figure 2 above produces its own magnetic field. This second magnetic field must try to repel the magnet falling through the coil and stop it moving. This rule can be used to find the direction of the current flowing. (If you are interested, this rule is called Lenz's Law).

Questions:

1. Describe the conditions necessary for induction to occur.

• Movement..
• of a wire / coil / conductor..
• through a magnetic field /cutting through field lines / perpendicular to a magnetic field.

2. Using figure 1 above (repeated here), predict the direction of the induced current in the wire shown.

The direction of the induced current can be found using Fleming's right-hand rule (RHR):
In figure 1, the magnetic field is to the right, and this is shown by the first finger in Fleming's RHR. The motion is downwards as shown by the thumb. This leaves the second finger pointing 'out' of the diagram. The current is in the direction from X to Y, indicated in the diagram shown below:

## A.C. Generators

If we keep moving the magnet in figure 2 (above) in and out of the coil, we will continuously generate a voltage and hence a current, as long as field lines are being cut. The easiest way to make the movement of a magnet (or coil) continuous is to spin one relative to the other. This will produce a constantly changing voltage.

Figure 4 shows a standard way this is done:

Figure 4: A simple electrical generator

This is the basis of all electrical generators. Typically a coil is rotated at high speed inside a strong magnetic field. The coil will have hundreds of turns. As each side of the coil passes up then down through the field, it will produce an alternating voltage and current (A.C.). A device that generates an a.c. output is often called an alternator. Many alternators rotate the magnets inside a coil instead, but the effect is the same.

### Slip rings

In order for the a.c. output to be connected to a circuit, metal 'slip rings' are used that connect to contact brushes, as shown below in figure 5. This arrangement means that one side of the coil is always connected to the same side of the output terminals, producing a sine wave a.c. output.

Figure 5: A generator with output slip rings

### The A.C. Generator output

The graph in figure 6 shows the output of a simple alternator. Notice that the amplitude and the period of the a.c. output can be measured from this graph.

Figure 6: The a.c. output from an alternator

You should be able to explain how the graph here matches the coil rotation. The most important thing to remember is that the faster the wires in the coil cut through the magnetic field, the higher the induced e.m.f., and so the peak voltage corresponds to when the wires of the coil are moving vertically up/down through the magnetic field.

Watch this YouTube clip to see how the voltage varies as the coil rotates:

YouTube: ScienceWorld - How electricity is generated

Can you relate the position of the generator coil to the peaks, troughs and zeros of the e.m.f.? For example where will the induced e.m.f. be zero? In this case, there is no e.m.f. when the wires of the coil are moving parallel to the magnetic field (horizontally), and to do this the coil in the video (or in figure 5) must be aligned vertically.

What do you think would happen if the alternator coil is spun round at twice the speed? In this case, two things happen: The frequency is twice as high, as you may have guessed, but the output voltage is also twice as high. This is because the magnetic field (the magnetic 'flux') is being cut at twice the rate. The output is shown in figure 7.

Figure 7: An alternator being spun at twice the speed.

### Dynamos

Some generators have an arrangement of contacts to the coil such that the voltage produced acts in a single direction. This is a direct current generator (D.C.) and is called a dynamo. To do this, a similar split ring commutator is needed as shown in section 4.5a,replacing the slip rings shown in figure 5.

Figure 8: A split ring commutator

The output of a dynamo is typically identical to the graphs shown here, but no negative voltage is generated. The negative output is 'flipped' by the half rings of the commutator, so that a positive e.m.f. is produced at all times as shown in figure 9:

Figure 9: The d.c. output of a dynamo

This output is always positive, but still varies up and down, so additional components are usually added to ensure the output is smooth and the voltage is constant.

Questions:

3. An a.c. generator consists of a rotating coil in a magnetic field as shown in figure 5 above, with 2 slip rings fastened to the coil.

• a) State one way the output voltage from the generator can be increased.
• b) Explain why an alternating voltage is produced.
• c) Explain why there is a point in the rotation where no voltage is produced.

a) To make the voltage higher, you can either increase the strength of the magnets / magnetic field, rotate the coil faster, or use more turns on the coil.

b) An a.c. voltage is produced because any one side of the coil moves up then down through the field as it rotates, making the direction of the induced voltage change constantly.

c) At one point of the rotation, the sides of the coil are moving parallel to the field, so no field lines are cut. Therefore, no voltage is induced.

4. An a.c. generator output graph is shown in figure 6 above.

• a) State the amplitude (in volts) of the output.
• b) State the period of the output.
• c) The coil is now spun at half the frequency. Calculate:
• (i) the new period of the output.
• (ii) the new amplitude of the output.

a) The amplitude of a wave is the height of the wave from the centre line; in this case it is 10 volts.

b) The period is the time taken for one full wave. From the graph, this is 4 x 10-3 s (or 4 milliseconds).

c) (i) The new period will be twice as long if the frequency is half as fast.
Therefore the time period will be 8 x 10-3 s.

(i) The amplitude will be half the original as the coil is cutting the magnetic field at half the speed.
Therefore the amplitude will be 5 V.

.

There are additional quiz questions on induction after the next section on transformers

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