Magnetic field due to current-carrying coil

When a current flows in a wire, it creates a circular magnetic field around the wire.

This magnetic field can deflect the needle of a magnetic compass.

The strength of the magnetic field is greater:

  • closer to the wire;
  • if the current is increased.
Wire runs vertically, current flowing bottom to top. Magnetic field rotates anticlockwise. Second wire runs vertically, current flowing top to bottom. Magnetic field rotates in a clockwise direction.Magnetic fields around a wire carrying an electric current

A coil of wire (solenoid)

A coil of wire, or solenoid, consists of a wire coiled up into a spiral shape.

When an electric current flows, the shape of the magnetic field is very similar to the field of a bar magnet.

The field inside a solenoid is strong and uniform.

The small magnetic field caused by the current in each coil add together to make a stronger overall magnetic field.

Current flows towards you in the top five wires, away from you in bottom five. Magnetic fields flow anti-clockwise around top five wires, clockwise around bottom. North pole right. South pole left.A solenoid, shown here in cross section, has a stronger electromagnetic field than a single straight wire

The north pole of the electromagnet can also be found by using your right hand.

Point the fingers of your right hand in the same direction as the current is flowing in the coil.

Your thumb points to the north pole of the electromagnet.

Key point

The magnetic field produced by the current in a coil of wire:

  • is very similar to the field of a bar magnet;
  • reverses when the direction of the current in the coil is reversed.