Physics
Electromagnets & motors
Magnets at the push of a button: how current creates magnetic fields and makes motors spin.
What you need first
The door buzzer at the front door, the blender in the kitchen, the fan in your laptop, and the car outside: electromagnets and motors are everywhere. They all rest on a discovery that changed the world: can create magnetism, and exactly when you need it.
Current creates a magnetic field
When current flows through a wire, a forms around the wire. On its own it's weak, but there's a trick: wind the wire into a coil with many turns and the fields of all the turns add up. Put an iron core inside the coil and the field becomes many times stronger still. That's your electromagnet: more turns or more current means more force.
Like every magnet, the coil has two poles, a north pole and a south pole. Like poles push each other away, opposite poles attract. Which pole ends up on which side depends purely on the direction in which the current flows through the coil.
N facing S: they attract
The big advantage: switchable
An ordinary magnet is always on. An electromagnet, however, can be switched on and off. That's exactly what the crane at the scrapyard uses: current on, and tonnes of scrap iron stick to the magnet. Current off, and everything drops off right where it should. The door buzzer works the same way: when someone in the flat presses the button, an electromagnet releases the latch and the door can be pushed open.
From magnet to motor and generator
An electromagnet can do even more: its poles can be flipped by reversing the direction of the current. This is exactly what the electric motor is built on. Inside it, a rotatable coil sits between fixed magnets. Attraction and repulsion pull the coil around, and just before it would stop, the poles are flipped: the coil is pulled onward and keeps spinning. The generator is the reverse: move a coil and a magnet relative to each other, for example in a bicycle dynamo or on the shaft of a wind turbine, and a voltage builds up, so current flows once the is closed. Almost every power station works this way, whether it runs on wind, water, coal, or nuclear fuel. Only manage without a generator.
Exercises
0 of 6 solvedTime to try it yourself. You can't break anything, every attempt counts.
What forms around a current-carrying wire?
A motor spins 3000 times per minute. How many times does it spin per second?
Why does the scrapyard crane use an electromagnet instead of a permanent magnet?
Match each part or machine to what it does.
A coil with 200 turns creates a certain magnetic field. Roughly how many turns do you need for double the field strength, at the same current and the same coil length?
Order by strength of the magnetic field, from weakest to strongest.
- 1The coil with an iron core added
- 2A single straight wire carrying current
- 3The wire wound into a coil with many turns
Where this leads