A wire carrying a current has a magnetic field round it. For a long straight wire the field lines are circles centred on the wire.

• A bigger current gives a stronger field.
• The field gets weaker further from the wire.
• Reverse the current and the field reverses.

Right-hand grip rule: grip the wire with your right hand, thumb pointing along the current. Your fingers curl the way the field lines go. If the current comes towards you (out of the page), the circles go anticlockwise; if it goes away from you, clockwise.

Wind the wire into a long coil, a solenoid, and the fields of all the turns add up.

• Inside: a strong, uniform field (straight, evenly spaced lines).
• Outside: the same shape as a bar magnet's field.

To find the north end, curl the fingers of your right hand the way the current flows round the turns: your thumb points to N. Seen from the north end, the current flows anticlockwise.

A solenoid with an iron core is an electromagnet. It is stronger when you:

• add more turns,
• increase the current,
• use an iron core (iron is magnetised easily and loses it as soon as the current stops).

Its big advantage over a permanent magnet: you can switch it off.

• Scrapyard crane: switch off to drop the car.
• Electric door lock: the entry button sends a current through a coil, which pulls the iron bolt back.
• Relay: a small current in one circuit magnetises a coil, which pulls an iron contact and closes a second circuit carrying a large current (a car's starter motor).

Reversing the current does not make an electromagnet weaker or stronger. It only swaps which end is north.