The motor effect

When a current flows through a conducting wire a magnetic field is produced around the wire. The strength of the magnetic field depends on the current through the wire and the distance from the wire.

So a larger current gives a stronger field, and the field is weaker further from the wire.

Shaping a wire to form a solenoid increases the strength of the magnetic field created by a current through the wire. The magnetic field inside a solenoid is strong and uniform.

The magnetic field around a solenoid has a similar shape to that of a bar magnet. Adding an iron core increases the strength of the magnetic field of a solenoid. An electromagnet is a solenoid with an iron core.

You should be able to draw the magnetic field pattern for a straight wire carrying a current and for a solenoid, showing the direction of the field.

For a straight wire the field lines are concentric circles around the wire, closer together near the wire where the field is stronger. For a solenoid, draw straight parallel lines inside pointing along the axis, curving round the outside from one end to the other exactly as they would for a bar magnet.

You should also be able to describe how the magnetic effect of a current can be demonstrated. Place a plotting compass near a wire; when the current is switched on, the needle deflects, and it deflects the other way if the current is reversed. Iron filings sprinkled on card around a wire will also settle into circles.

You should be able to explain how a solenoid arrangement can increase the magnetic effect of the current.

The explanation: each turn of the coil produces its own magnetic field, and in a solenoid the turns are arranged so that these fields point the same way inside the coil. They therefore add together, giving a field far stronger than a single wire would produce. Adding an iron core strengthens it further, because the iron becomes magnetised by the field and produces a field of its own on top of it.

Physics only students should be able to interpret diagrams of electromagnetic devices in order to explain how they work — such as an electric bell, a relay or a scrapyard crane. In each case the same reasoning applies: a current makes an electromagnet, the electromagnet attracts a piece of iron, and switching the current off releases it.

When a conductor carrying a current is placed in a magnetic field, the magnet producing the field and the conductor exert a force on each other. This is called the motor effect.

You should be able to show that Fleming's left-hand rule represents the relative orientation of the force, the current in the conductor and the magnetic field.

Hold the thumb, first finger and second finger of the left hand at right angles to one another:

• First finger — Field, pointing from north to south • seCond finger — Current, in the conventional direction, from positive to negative • thuMb — Motion, the direction of the force.

You should be able to recall the factors that affect the size of the force on the conductor: the magnetic flux density of the field, the current through the conductor, and the length of conductor in the field. Increasing any of the three increases the force.

The force is also greatest when the conductor is at right angles to the field, and there is no force at all when the conductor lies along the field.

For a conductor at right angles to a magnetic field and carrying a current:

force = magnetic flux density × current × length F = B I l

force, F, in newtons, N; magnetic flux density, B, in tesla, T; current, I, in amperes, A; length, l, in metres, m.

This equation is given on the Physics equation sheet.

A coil of wire carrying a current in a magnetic field tends to rotate. This is the basis of an electric motor.

You should be able to explain how the force on a conductor in a magnetic field causes the rotation of the coil in an electric motor.

The explanation, step by step:

1. Current flows through the coil, which sits in a magnetic field. 2. On one side of the coil the current flows one way, and on the other side it flows the opposite way. 3. By Fleming's left-hand rule, the force on one side is therefore upwards and the force on the other side is downwards. 4. Two equal forces in opposite directions on opposite sides of the coil produce a turning effect, so the coil rotates. 5. Every half turn, a split-ring commutator reverses the direction of the current in the coil. Without it, the coil would turn half a turn and then be pushed back the other way; with it, the forces keep pushing the same way round and the rotation continues.

The speed of rotation is increased by increasing the current, using a stronger magnetic field, or adding more turns to the coil — all of which follow from F = B I l. The direction of rotation is reversed by reversing the current, or by reversing the magnetic field. Reversing both leaves the direction unchanged.

Loudspeakers and headphones use the motor effect to convert variations in current in electrical circuits to the pressure variations in sound waves.

You should be able to explain how a moving-coil loudspeaker and headphones work.

The explanation:

1. A coil of wire is attached to a paper or plastic cone and sits in the field of a permanent magnet. 2. An alternating current from the amplifier flows through the coil. 3. The current in a magnetic field produces a force on the coil, by the motor effect. 4. Because the current alternates, its direction reverses continually, so by Fleming's left-hand rule the force reverses too — pushing the coil and cone backwards and forwards. 5. The moving cone pushes on the air in front of it, creating the compressions and rarefactions of a sound wave. 6. The frequency of the sound matches the frequency of the alternating current, and a larger current gives a larger movement and so a louder sound.