Electromagnets and the motor effect · GCSE Physics

Magnetic fields around wires

When a current flows through a conducting wire, a magnetic field is produced around the wire.
The field is made of circles around the wire. Its strength increases with the current and decreases with distance from the wire.
The right-hand grip rule gives the direction: point your thumb along the current, and your fingers curl in the direction of the field.

Shaping a wire into a coil (a solenoid) increases the strength of the magnetic field.
Inside a solenoid the field is strong and uniform. Outside, the field has a similar shape to that of a bar magnet.
Adding an iron core increases the strength of the field. A solenoid with an iron core is an electromagnet.
Electromagnets can be switched on and off, and their strength can be changed. They are used in cranes for scrap iron, relays and electric bells.

When a conductor carrying a current is placed in a magnetic field, the magnet and the conductor exert a force on each other. This is the motor effect.
Fleming's left-hand rule: first finger = field, second finger = current, thumb = motion (force). The fingers are held at right angles.
F = B I l: force (N) = magnetic flux density (tesla, T) × current (A) × length of conductor in the field (m).
The force is biggest when the wire is at right angles to the field, and zero when it is parallel.

A coil of wire carrying a current in a magnetic field tends to rotate. This is the basis of an electric motor. A split-ring commutator reverses the current every half turn to keep the coil turning.
(Separate sciences) Loudspeakers and headphones use the motor effect: a changing current in a coil in a magnetic field makes a cone vibrate, turning variations in current into pressure variations in the air (sound waves).

Electromagnet: A solenoid with an iron core, which is magnetic only when current flows.
Solenoid: A coil of wire that produces a magnetic field when current flows through it.
Right-hand grip rule: A rule for the direction of the magnetic field around a current-carrying wire.
Motor effect: The force on a current-carrying conductor in a magnetic field.
Fleming's left-hand rule: A rule for the direction of the force in the motor effect.
Magnetic flux density: The strength of a magnetic field, measured in tesla (T).
Split-ring commutator: A device in a motor that reverses the current every half turn.