Electromagnets
Can you build a magnet you can switch off? Wind a coil, add an iron core and switch on the current, then see how many paper clips it can lift and which way the compass points.
Class 7 · Class 10 Magnetic Effects of CurrentTry this
0 of 5 donePick up your first paper clip.Hint: switch on, with the iron core in
Lift 10 paper clips at once.Hint: more turns and more current
Drop all the clips without touching them.Hint: what happens when the current stops?
Make the compass needle swing the other way.Hint: reverse the current
Take out the iron core and try at full power. Can it lift anything?Hint: switch off Iron core, then turn everything up
What's happening
Electric current makes magnetism
A wire carrying current has a magnetic field around it: a compass near it swings. Wind the wire into a coil and the fields of all the loops add up, so the coil behaves like a bar magnet with a north and a south pole. Switch off the current and the magnetism disappears.
Three ways to make it stronger
More turns of wire, more current, and an iron core. The iron's tiny magnetic regions line up with the coil's field and add their own, making the magnet hundreds of times stronger. At very high currents the iron 'fills up' (saturates) and stops helping as much.
Which end is north?
Use the right-hand thumb rule: curl the fingers of your right hand round the coil in the direction the current flows, and your thumb points to the north pole. Reverse the current and the poles swap. That's how electric motors and loudspeakers push and pull.
Who figured this out?
Hans Christian Ørsted
1820, DenmarkNoticed a compass needle swing when he switched on a current in a nearby wire: the first proof that electricity and magnetism are linked.
William Sturgeon
1824, EnglandMade the first electromagnet by winding wire round a horseshoe of iron. It could lift many times its own weight.
Joseph Henry
1830s, USAWound many turns of insulated wire to build far more powerful electromagnets, and used them in early electric bells and telegraphs.
Learn more
- Electromagnet · Wikipedia
- Right-hand rule · Wikipedia
Check yourself
1. What happens to an electromagnet when you switch off the current?
2. Which of these does NOT make an electromagnet stronger?
3. You reverse the direction of the current in the coil. What happens?
4. Who discovered that an electric current can move a compass needle?
Common questions
How do you make an electromagnet?
Wrap insulated copper wire many times around an iron nail, leaving the two ends free. Connect the ends to a cell or battery through a switch. When current flows, the nail becomes a magnet and can pick up pins or paper clips. Switch off and they drop.
What is the magnetic effect of electric current?
An electric current produces a magnetic field around the wire carrying it. This was discovered by Hans Christian Ørsted in 1820. Coiling the wire concentrates the field, which is the basis of electromagnets, electric motors, generators and loudspeakers.
What is the right-hand thumb rule?
For a straight wire, hold it in your right hand with your thumb pointing along the current: your curled fingers show the direction of the magnetic field lines around it. For a coil, curl your fingers in the direction of the current and your thumb points to the north pole.
How can you make an electromagnet stronger?
Increase the number of turns of wire, increase the current flowing through the coil, and use a soft iron core. Packing the turns closer together also helps.
What is the difference between a permanent magnet and an electromagnet?
A permanent magnet is always magnetic and its strength is fixed. An electromagnet is magnetic only while current flows, its strength can be changed by changing the current or turns, and its poles can be swapped by reversing the current.
Where are electromagnets used?
In electric bells, cranes that lift scrap iron, electric motors and fans, loudspeakers and earphones, relays and circuit breakers, MRI scanners in hospitals and maglev trains.
A teaching model: strength grows with turns × current and is boosted by iron until the iron saturates. Real magnets also depend on coil length and shape.
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