Physics
Double Award
Double Award
Cambridge IGCSE
TOPIC 4B: MAGNETISM
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.
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.
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'):
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:
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.
Test your understanding on 4.5a and 4.5b using this quick quiz on the motor effect:
