What is an electromagnet?

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 plane coil

A plane coil is a single turn of wire made into a circle. When an electric current flows, the magnetic field forms into circles around opposite sides of the circle. The field is strongest inside the coil.

Current flows clockwise round the loop. These circle anti-clockwise around the loop at right-angles to the current flow. Field generated within loop flows north to south directly through the screen.Magnetic field lines around a plane circular coil

Solenoids

A 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 fields 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

Electromagnets

A solenoid with an iron core is called an electromagnet. The iron core increases the solenoid's magnetic field strength. A simple electromagnet is made by coiling wire around an iron nail.

A coil of insulated wire is wrapped around an iron nail. One end of the coil of wire is connected to a battery. The other is connected to a switch in the circuit.

Electromagnets are used in devices such as electric bells, and door locks that can be controlled remotely.

Magnetic field strength

Magnetic field strength is often called magnetic flux density and is given the symbol B.

Magnetic field strength is defined as the force acting per unit current in a wire of unit length, which is perpendicular to the field, and it is measured in tesla (T).

A magnetic field has a strength of one tesla. This is if a wire with a current of one amp and is one metre long experiences a force of one newton.