ElectromagnetismAQA GCSE Physics: Revision notes

Whenever an electric current flows through a conducting wire, a magnetic field is produced in the space around it.

• The field forms concentric circles around the wire
• Field direction reverses if the current direction reverses
• Field strength depends on:
  • the size of the current (more current = stronger field)
  • the distance from the wire (further away = weaker field)

Shaping a wire into a coil, called a solenoid, makes the magnetic field much stronger than a straight wire, because the fields from each loop add together inside the coil.

• Adding an iron core inside the solenoid increases the field strength further, because iron is easily magnetised — this combination is called an electromagnet
• Unlike a permanent magnet, an electromagnet can be switched on and off (and its strength can be varied) simply by controlling the current
• The field pattern of a solenoid looks like that of a bar magnet, with clear north and south poles at its ends

Many everyday devices rely on the magnetic field of a current to operate — for example electric bells, relays and magnetic locks. In each case, current flowing through a coil creates a magnetic field which attracts a piece of iron or steel, causing a mechanical movement (such as a switch closing or a striker hitting a bell).

When interpreting how such a device works, always trace the chain: current flows → magnetic field created → iron/steel part attracted → mechanical action occurs.

The motor effect is what happens when a current-carrying conductor is placed in a magnetic field: the conductor experiences a force.

• This force exists because the conductor's own magnetic field interacts with the external magnetic field
• The direction of the force can be found using Fleming's left-hand rule:
  1. Point the First finger in the direction of the magnetic Field
  2. Point the seCond finger in the direction of the Current
  3. The THumb then points in the direction of the Thrust (force/motion)
• If the current or the field is reversed, the direction of the force reverses too

The size of the force on a current-carrying conductor in a magnetic field is calculated using:

$F = BIl$

• F = force, in newtons (N)
• B = magnetic flux density, in tesla (T)
• I = current, in amperes (A)
• l = length of conductor in the field, in metres (m)

The force is greatest when the conductor is at right angles to the magnetic field, and is zero when the conductor runs parallel to the field.

This force is what causes a current-carrying coil placed in a magnetic field to rotate — the basis of an electric motor. As the coil turns, forces on opposite sides of the coil act in opposite directions, creating a turning effect (moment) that spins the coil continuously (with a split-ring commutator reversing the current every half turn).

Loudspeakers and headphones convert variations in electric current into sound waves using the motor effect.

1. An alternating current flows through a coil of wire (attached to a paper or plastic cone) that sits inside a permanent magnetic field
2. As the current varies, the force on the coil (motor effect) varies in size and direction
3. This makes the coil — and the attached cone — vibrate back and forth
4. The vibrating cone pushes and pulls the air, creating sound waves at the same frequency as the alternating current