GCSE Science/The motor effect

GCSE Science/Electricity

The motor effect is the term used when a current-carrying wire in the presence of a magnetic field experiences a force. A simple experimental demonstration will show you that this is true.

Place a wire that is connected to a power pack in between the poles of a horseshoe magnet. Turn on the power and the wire moves. Often the movement is only very slight because a typical horseshoe magnet is not very strong.

The force depends on a number of things:

How strong the magnetic field is inside the loop of the magnetic coil as it is usually iron and iron is a good conductor of magnetic attraction.(B)
How much current is flowing through the wire. (I)
The angle formed between the wire and the direction of the magnetic field.(θ)
The length of the wire, carrying the current, in the magnetic field.(L)

The force obeys the formula: $F=IBL\sin(\theta )$

The first two points are pretty much obvious, so let's look at the third point in a little more detail. The magnetic field of a horseshoe magnet points pretty much in a straight line from the north pole to the south pole. If the wire cuts this field at right angles the resulting force will be a maximum. If the wire runs parallel to the field, from the north the south pole or vice versa, the wire will still experience the motor effect. However, the net result of this force is along the wire, not perpendicular to it, thus the wire does not turn.

Having said that, at GCSE you only really need to think about situations where the field and the wire cut each other at right angles. The force is always at right angles to both the field and the current flowing in the wire. This means that if you draw the direction of the magnetic field and the wire on a piece of paper the force will be out of the plane of the paper pointing straight up or down {More about how you work out which way later).

Look at the diagram above. For simplicity, only the two ends of the horseshoe magnet have been drawn. Also the power pack and connecting wires are not shown either. The magnetic field is going into the screen. The current is traveling from right to left. The black line represents the force, and therefore the direction that the wire moves.

Q1) Name one way that the force on the wire could be increased.
Q2) Name as many ways as you can to reduce the force on the wire.
Q3) Name two ways of reversing the force on the wire, so that it pushes the wire up rather than down.

Fleming's Left Hand Rule

This rule allows you to work out which direction the force will point in. Arrange your left hand with your thumb, first finger, and second fingers all pointing at right angles to one another.

Point your First finger in the direction of the magnetic field.
Point your SeCond finger in the direction of the current.
The Thumb will then give you the direction of the force (thrust).

Remember you have to use your left hand for this! Although you may use your right hand so long as you swap the direction of current with the force

Q4) A student uses her right hand instead of her left. What effect will that have on the force she works out?

You will recall that a current carrying wire is surrounded by its own magnetic field. The diagram below shows the wire end on in a magnetic field of two magnets that are NS facing. The field due to the magnets is shown in blue, and the field due to the current in the wire is shown in black.

Notice the direction of the two fields as shown by the arrows. On top of the wire the fields are both going in the same direction. They add up making an overall strong field.

Underneath the wire, they go in opposite directions. They cancel each other out to some extent making an overall weaker field. The new field is shown in the diagram below.

See how the field above the wire is stronger. The lines are closer together. Below the wire the field is weaker (due to partial canceling out) the field lines are further apart. The force pushes the wire downwards, away from the strong field into the weak field. It's as if the field lines try to repel each other. They don't like being squashed together and try to straighten out. They also act as if they are made of elastic bands, they don't like being stretched out of shape. (This is just a model of what's going on. The lines aren't real, they don't actually try to push each other away, but I find it a way of helping me understand what's going on. If it doesn't help you, don't use it)

Q5) Use the idea of field lines not liking being squashed together to explain why two magnets with like poles facing each other repel.
(Q6) Use the idea of field lines being made of elastic to explain why two magnets with opposite poles facing attract one another.

A simple electric motor

Ok, so far we have been looking at the force that results when we put a current carrying wire in a magnetic field. In this section we will look at a practical use for this force.As you have probably already guessed from the name of this page, the practical use is going to be an electric motor.

Look at the diagram above. A rectangular loop of wire is sitting inside a magnetic field. We can consider the current in the four sections of the loop and work out which way the force acts.

On the left hand side of the loop the current is flowing into the page or screen. The magnetic field will be going from left to right so from Flemming's left hand rule the force will be downwards.
On the back and front of the loops the current is parallel to the magnetic field so there is zero force.
On the right hand side of the loop the current is coming out of the page or screen. The field is still going from left to right so the force will be upwards.

The net result of these different forces is that there will be a turning moment that makes the coil rotate by 90°. At that point the upwards and downwards forces will be acting along the same line and the coil will stop turning. Another way to think about it is to consider the loop as a tiny little one turn solenoid. The solenoid will have a little north pole and a little south pole and will therefore move until its north pole lines up with the south pole of the magnet on the right, and its south pole lines up with the north pole of magnet on the left.

This is all very interesting but not much use as a motor. We want something that keeps turning all the time the current flows. They way this is achieved is by the use of the commutator – a circular metal ring that is split into two halves. The ends of the wire loop turn around inside the commutator. They are in electrical contact with it. One side of the commutator is connected to the positive output of a power pack or battery . the other half of the commutator is connected to the negative.

Let's look at what happens as the coil turns inside the commutator:

The coil turns clockwise because of the forces on the two halves as explained above.
As the coil turns the wires slip around on the inside of the commutator.
When the coil is in a vertical position there is no force on it, but its momentum carries it forward a little.
The wires inside the commutator make contact with the other half of the ring. I.e. the wire that was in contact with the positive half of the commutator now touches the negative half and vice versa.
This causes the current to flow the other way around the coil, which reverses the little solenoid magnet.
The coil therefore tries to line itself up with the magnetic field by continuing to turn so that it ultimately points in the other direction.
Once the coil is vertical, and facing the other way, the wires touch their original halves of the commutator. and the whole cycle repeats over and over

Q7)A student sets up an electric motor and turns it on. The coil turns clockwise. List two ways she could reverse the direction.

Summary

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