Page 422 - Physics Coursebook 2015 (A level)

BOX 26.1: Using Fleming’s left-hand rule
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Cambridge International A Level Physics
Look at Figure 26.9. 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 26.9). 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.
First finger – Field seCond finger – Current
thuMb – Motion
Figure 26.9 Fleming’s left-hand (motor) rule.
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 26.10 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
If you think of the magnetic field lines as elastic bands then you can see why the wire is pushed out in the direction shown.
The production of this force is known as the motor effect, because this force is used in electric motors. In
a simple motor, a current in a coil produces a magnetic field; this field interacts with a second field produced by a permanent magnet.
magnetic field of magnets
magnetic field
+ of current-carrying =
combined magnetic fields
conductor
force
current into the plane of the paper
Figure 26.10 In the field of a permanent magnet, a current- carrying conductor experiences a force in accordance with Fleming’s left-hand rule. The fields due to the permanent magnet and the current (left and centre) combine as shown on the right.
QUESTION
Figure 26.11 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?
magnetic field
current
a b c
Figure 26.11 Three conductors in a magnetic field.
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the magnets.