Electricity and magnets: two sides of one idea
A wire carrying current makes a magnet, and a moving magnet makes current. Build an electromagnet and see how motors and generators work.
For a long time, electricity and magnetism looked like two separate mysteries. Lightning and static on one side, compasses and lodestones on the other. Then, in 1820, a Danish teacher named Hans Christian Ørsted noticed something odd during a lecture: a compass needle twitched when he switched a current on near it.
Electricity and magnetism are two sides of the same thing.
Current makes a magnet
A wire carrying current is surrounded by a magnetic field, an invisible region where a magnet or compass feels a push. It's very weak around a single straight wire. But wind the wire into a coil, and the fields from each loop add up. The coil acts like a bar magnet, with a north end and a south end.
That is an electromagnet. And it has a superpower that an ordinary magnet lacks: you can switch it on and off.
Build one
Change the number of turns, the current and the iron core. Watch the paperclips.
Strength 160 (arbitrary units).More turns, more current or an iron core all make it stronger. Flipping the battery swaps north and south.
You should see:
- More turns of wire give a stronger magnet.
- More current gives a stronger magnet.
- An iron core makes it far stronger, because the iron becomes a magnet too and adds its own field.
- Flipping the battery swaps north and south.
- Zero current means no magnet at all.
Electromagnets are everywhere
- Scrapyard cranes use huge electromagnets to lift cars, then drop them when the current is switched off.
- Doorbells and locks use small ones to pull a latch.
- Speakers use one to push a cone back and forth and make sound.
- Electric motors use electromagnets that push and pull on other magnets, making a shaft spin. That's how fans, washing machines and electric cars turn electricity into motion.
Magnets make current
Eleven years after Ørsted, an English scientist named Michael Faraday found the reverse: move a magnet near a coil of wire, and a current flows in the wire. Hold the magnet still and there is nothing. It's the change that does it.
This is how a generator works. Something spins magnets inside coils (steam, flowing water, wind or a bicycle wheel), and the changing magnetic field pushes current round the wire. Nearly all of the world's electricity is made this way.
A motor turns electricity into spinning. A generator turns spinning into electricity. They are the same idea, run in opposite directions.
Words to know
- Magnetic field
- The invisible region around a magnet or current where magnetic forces act.
- Electromagnet
- A coil of wire that acts as a magnet when current flows through it.
- Motor
- A machine that uses electromagnets to turn electrical energy into motion.
- Generator
- A machine that uses moving magnets and coils to turn motion into electrical energy.
Check yourself
Name three ways to make an electromagnet stronger.
Show the answer
More turns of wire, more current, and an iron core.
What is the big advantage of an electromagnet over a normal magnet?
Show the answer
It can be switched on and off, and its strength can be changed.
What happens in a coil if you hold a magnet completely still next to it?
Show the answer
Nothing. Current is only made when the magnetic field changes, for example when the magnet moves.
How are a motor and a generator related?
Show the answer
They are almost the same machine used in opposite directions: a motor turns electricity into motion, a generator turns motion into electricity.