In 1820 Hans Christian Ørsted noticed a compass needle twitch when he switched on a current nearby. Electricity and magnetism turned out to be linked: every current makes a magnetic field.
Key idea
An electric current makes a magnetic field around it. Wind the wire into a coil and the fields add up, giving a field like a bar magnet's. Put an iron core inside and the field becomes much stronger: an electromagnet, which you can switch on and off.
Why it works
Around a straight wire, the field forms circles. In a coil, the circles from every loop point the same way through the middle, so they add: more turns or more current make a stronger magnet. The iron core's own tiny magnets line up with the coil's field and add their much larger field. Switch off the current and the field disappears, which is why a scrapyard crane can drop what it picked up.
Worked example: Make it stronger
Name three ways to make an electromagnet stronger.
- Increase the current through the coil.
- Wind more turns of wire.
- Use an iron core.
Answer: More current, more turns, an iron core
An electric motor puts this to work: a magnetic field pushes on a current-carrying coil, making it spin. That turns electrical energy into motion in fans, drills and electric cars.
Common mistake
Thinking only permanent magnets make magnetic fields. Any electric current does, which is how an electromagnet works with no magnet inside it at all. Check: reverse the current and the electromagnet's poles swap.