A current-carrying wire is surrounded by a magnetic field. This magnetic field will interact with an external magnetic field, giving rise to a force on the conductor, just like the fields of two interacting magnets. A simple situation is shown in Figure 24.8.

The magnets create a fairly uniform magnetic field. As soon as the current in the copper rod is switched on, the rod starts to roll, showing that a force is acting on it. We use Fleming’s left-hand (motor) rule to predict the direction of the force on the current-carrying conductor, as explained in Practical Activity 24.1.

PRACTICAL ACTIVITY 24.1

 

Using Fleming’s left-hand rule

Look at Figure 24.9. There are three things here, all of which are mutually at right angles to each other – the magnetic field, the current in the rod and the force on the rod. These can be represented by holding the thumb and the first two fingers of your left hand so that they are mutually at right angles (Figure 24.9). Your thumb and fingers then represent:

- thuMb – direction of Motion
- First finger – direction of external magnetic Field

- seCond finger – direction of conventional Current.
If the thumb and first two fingers of the left hand are held at right angles to one another, with the First finger pointing in the direction of the Field and seCond finger in the direction of the Current, then the thuMb points in the direction of the Motion or force.
You should practise using your left hand to check that the rule correctly predicts these directions.

Explaining the magnetic force

We can explain this force by thinking about the magnetic fields of the magnets and the current-carrying conductor. These fields combine or interact to produce the force on the rod.
Figure 24.10 shows:
- the external magnetic field of the magnets
- the magnetic field of the current-carrying conductor
- the combined fields of the current-carrying conductor and the magnets.

Question

 

4) Figure 24.11 shows three examples of current-carrying conductors in magnetic fields. For each example, decide whether there will be a magnetic force on the conductor. If there is a force, in what direction will it act? Note the cross in the circle shows the current is into the plane of the paper, as in Figure 24.4.

Figure 24.11: Three conductors in a magnetic field.