Physics
Cambridge IGCSE

TOPIC 4B: MAGNETISM

# 4.5b Magnetic Devices

In the last section, you will have learned that a current in a conductor inside a magnetic field gives rise to a force. This effect is used in a range of devices, allowing electrical energy to be converted into kinetic energy. The force is due to what is called the 'motor effect', as it is primarily used in motors. There are motors in microwave ovens, hair dryers and fans to name a few common household appliances, as well as used in all electric vehicles.

In this section, we will look at how motors, loudspeakers and relays use electromagnetism to make them function.

## Motors

The diagram shown in figure 1 consists of a coil placed in a magnetic field. There is a current flowing around the coil as shown by the arrows.

Figure 1: A coil in a magnetic field

The motor effect discussed in section 4.5a says that one side of the coil will be pushed upwards, whilst the other side will be pushed downwards as the direction of current is reversed. If the coil is free to twist, then it will start to rotate. This is the basis for all motors.

Using the left-hand rule, can we predict the direction of rotation?
The magnetic field is from north to south - right to left in this diagram. Sides A and C are parallel to this field, and so there is no 'motor effect' force. Using the left-hand rule shows that side B will be pushed upwards, and side D will be pushed downwards.

However, there is a problem with this simple setup. Side B will move to the top, side D to the bottom, and then the coil should stop there. There needs to be a way of then making the current flow in the opposite direction, forcing side B back down the other side of the rotation. This is illustrated in the video here, which shows really clearly how the changing direction of current results in a changing force on each side of the coil:

YouTube: How does an Electric Motor work? (Jared Owen)

As shown in this video, the most common method of reversing the current is with a device called a split ring commutator fastened to the end of the coil, as shown here in figure 3:

Figure 2: A split ring commutator

Current flows through some blocks called contact brushes that touch the sides of the metal semicircular rings. Current flows through the ring and then onto the coil as shown in figure 1. However, as the coil rotates, so does the ring (green in this diagram). Eventually the other half of the ring touches the left-hand brush, and the coil current is reversed.

The end result is the coil is always forced to rotate in the same direction.

### Changing the turning effect of a motor

There are several changes that can be made to the design above to make a stronger force and hence a larger moment (see section 1.5c on 'moments'):

• 1. Increase the current in the coil
• 2. Increase the strength of the magnetic field across the coil
• 3. Use many turns on the coil, as shown in the YouTube video above.

## Loudspeakers

A loudspeaker ('speaker') is a very common device that uses the motor effect, covered in section 4.5a. Speakers can be found in music systems, headphones, TV sets, laptops and phones. They are all around us.

To understand how they work, have a look at figure 3:

Figure 3: A loudspeaker

The coil is wrapped around a tube that can slide along the centre section of an 'E' shaped magnet. If a current is passed through the wire coil, it produces a magnetic field. The motor effect then produces a force on the coil. The details of the construction are not required here, but you should be able to describe how the force produced by the motor effect makes the coil slide sideways along the magnet. This pushes a paper cone to the left or right, depending on the direction of current flow.

If a high frequency alternating current (a.c.) is supplied to the coil, the paper cone moves backwards and forwards, vibrating quickly. This rapidly increases and decreases the air pressure in front of the cone, producing a sound wave of the same frequency as the a.c. current.

The end result is that electrical energy is transferred by sound waves.

Headphones are simply very small loudspeakers and generally work in exactly the same way. Each ear has its own loudspeaker. There are some clever alternative designs, but larger sized headphones as shown here use magnetism as described in this section.

Motors and loudspeakers utilise the motor effect (section 4.5a), and rely on the force produced. Consequently there are several ways of improving the speaker or motor by increasing the force, as discussed in 4.5a and also in the questions below:

Questions:

1. A simple motor rotates due to the 'motor effect' forces acting on the coil. Suggest two ways of increasing the force on the coil, and hence the speed of rotation.

Any 2 of these:

• Increase the current (or supply voltage).
• Increase the magnetic field strength produced by the magnets.
• Increase the length of the wire in the field, by having more turns on the coil.

2. A loudspeaker as shown in figure 3 is connected to an a.c. electrical supply and produces a sound wave. Suggest one way the speaker or supply could be modified or changed to produce:

• a) A louder sound.
• b) A higher frequency sound.

a) To make the sound louder, you can increase the current (or voltage) from the supply, increase the strength of the magnet / field used in the speaker, or use more turns on the coil. (You could also increase the area of the paper cone, depending on the strength of the force produced by the speaker).

b) To produce a higher frequency sound, the frequency of the a.c. supply must also be increased. No other change will work.

Extension:

The best design of motor has a long coil width and length in the field, so that the area of the coil is larger. Why would increasing the width of the coil have any effect? (Increasing the length of sides A and C in figure 1).

In section 1.9 you will have learned about moments. A turning force depends on the force applied and the distance from the pivot. Therefore, a wider coil has a larger moment on the pivot.

## Relays

A relay is a simple electromagnetic switch. Instead of pushing a contact down with your finger, a small electromagnet is used to pull on an iron contact. A small current through the electromagnet coil will (usually) close the switch. This can be used to close the switch on a very high power/voltage circuit. For this reason, relays are often used when a small, low power electronic circuit is required to switch on a high power device such as a motor.

Figure 5: Animation of a relay at work, with circuit diagram

Tier

## Please choose a tier of entry

 Extended Tier (Core and Supplementary content, Grades A* to G) Core Tier (Core content only, grades C to G) Remember my choice