Electromagnetism

When a current flows through a wire, it makes a magnetic field around the wire. The field lines are circles around the wire.

• The field is stronger with a bigger current.
• The field is weaker further away from the wire.
• Reversing the current reverses the direction of the field.

The right-hand grip rule: grip the wire with your right hand, with your thumb pointing the way the current flows. Your fingers curl the way the field lines go.

Winding a wire into a coil called a solenoid makes its magnetic field stronger.

• Inside the solenoid, the field is strong and uniform: the field lines are straight, parallel and evenly spaced.
• Outside, the field has the same shape as a bar magnet's.

Putting an iron core inside the solenoid makes the field even stronger. A solenoid with an iron core is an electromagnet.

Electromagnets are useful because you can switch them on and off, and change their strength by changing the current. They are used in scrapyard cranes, electric door locks and relays.

A relay lets a small current in one circuit switch on a large current in another. A car's starter motor uses one.

1. Closing a switch lets a small current flow through an electromagnet.
2. The electromagnet attracts an iron armature, which pivots.
3. The other end of the armature pushes two contacts together.
4. This completes the second circuit, so a large current flows through the motor.

When the switch opens, the electromagnet turns off, a spring pulls the armature back, and the contacts separate.