KS3 · Physics

Using electromagnets (motors, bells)

Press an electric bell's switch once and hold it. Why does it ring again and again, not just once? Inside is a magnet that keeps switching itself off.

The electric bell

One press of the switch. Why does it ring again and again?

3 stages are missing. Use the rest of the loop to work out which goes where.

  1. Stage 1: Switch pressed: current flows in the coil

    The switch is held down and the contact point is closed, so the circuit is complete.

  2. Stage 2: The coil and soft-iron core become magnetised

    With a current in it, the coil wrapped round its iron core is now an electromagnet.

  3. Stage 3: The electromagnet pulls the soft-iron armature

    The armature is a springy piece of soft iron, and the electromagnet attracts it.

  4. Stage 4: The clapper hits the bell

    As the armature moves, it moves the clapper (the striker), and the clapper hits the bell.

  8. Stage 8: The contact point closes again

    The circuit is complete again, so it is back to stage 1. The loop repeats for as long as the switch is held down.

…and stage 8 leads back to stage 1.

Temporary or permanent?

ElectromagnetvsPermanent magnet

The bell only works because its magnet can be switched off. Here is what that difference means.

Focus

Can you switch it off?

Electromagnet

Yes. Stop the current and it stops being a magnet.

Permanent magnet

No. It stays a magnet.

The insight

This is what makes electromagnets useful: you decide when the magnet is on and when it lets go.

When is it a magnet?

Electromagnet

Only while there is a current in its coil

Permanent magnet

All the time

While the current flows

Electromagnet

The coil behaves like a bar magnet

Permanent magnet

No current needed: it is already a magnet

Where you meet it in this lesson

Electromagnet

The crane, the door lock, the bell and the spinning coil of a motor

Permanent magnet

The small magnet in a home-made motor

Making an electromagnet

Bare wire or insulated wire?

To make an electromagnet, you wrap a coil of wire around an iron core and pass a current through the wire. While the current flows, the coil behaves like a bar magnet.

What kind of wire should the coil be made from?
How sure are you?

Scrapyard crane

What will the crane pick up?

The crane's electromagnet is switched on above this pile of scrap. Which items will it lift?

Still to sort

Lifted by the crane (0)

Magnetic materials, such as iron and steel

Where the line is: It is the material that decides, not the size or shape of the item.

Left behind (0)

Non-magnetic materials, such as copper and aluminium

Where the line is: Being a metal is not enough. Copper and aluminium are not magnetic materials.

6 of 6 still to sort.

Once the load is over the right spot, the operator switches the current off. The electromagnet stops being a magnet and the scrap drops.

Watch out: Don't sort by 'metal or not'. Every item here is a metal, but only the iron and steel are magnetic.

Electromagnetic door lock

Springs hold the iron bolt locked. Put the rest in order.

When each event happens, from pressing the switch to the door locking again

1 · Happens first7 · Happens last

  1. The bolt is pulled back and the door unlocks

  2. The switch is released

  3. The switch is pressed

  4. The springs pull the bolt back into the locked position

  5. The electromagnet is magnetised and attracts the iron bolt

  6. The electromagnet is demagnetised

  7. A current flows in the electromagnet's coil

Electric motor · Build one on a desk

A motor you could make yourself
batterymagnetsafety pinsafety pincoil of insulated wirebare endbare end

Showing 1 layer: Simple motor

Explore

Tap a part of the motor to see what it does.

Motors are in anything that moves when it is plugged in or runs on batteries: an electric screwdriver, a vacuum cleaner, a washing machine, an electric fan, an electric toothbrush.

Tap each part to see its job. Every electric motor has at least one electromagnet, working with either a permanent magnet or another electromagnet.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

A magnet you can switch off, and why that one trick runs cranes, door locks, bells and motors.

What you need to know

  • An electromagnet is a coil of insulated wire wrapped round an iron core, with a current passing through the wire.
  • It is a temporary magnet: it is a magnet only while there is a current in its coil, so it can be switched on and off.
  • Cranes, electromagnetic door locks, electric bells and motors all use that on/off switching.
  • The core and armature of an electric bell are soft iron, which is easily demagnetised.

The big picture

An electromagnet is a coil of insulated wire round an iron core, with a current in the wire. While the current flows, the coil behaves like a bar magnet; switch the current off and it stops. That makes it a temporary magnet, and being able to switch it on and off is what makes it useful. A crane drops its iron and steel when the current goes off. A door lock pulls its iron bolt back only while the switch is pressed. A bell keeps ringing because each strike breaks its own circuit. And every motor's spinning coil is an electromagnet.

Key points

1With a current, the coil behaves like a bar magnet. Switch the current off and it stops behaving like a magnet.
2The wire is insulated so the current goes round every turn, and the current in each turn adds to the electromagnet's strength.
3A scrapyard crane lifts many pieces of iron and steel at once, leaves copper and aluminium behind, and drops its load when the current is switched off.
4In an electromagnetic door lock, pressing the switch magnetises the electromagnet, which pulls the iron bolt back to unlock the door. Releasing the switch lets the springs pull the bolt back to locked.
5An electric bell keeps ringing because the armature's movement opens the contact point and breaks the circuit. The magnetism goes, a spring pulls the armature back, the contact closes, and the cycle repeats while the switch is pressed.
6An electric motor's spinning coil is an electromagnet. With a current in it, its magnetic field and the field of the magnets around it interact and push it round.

Worked example

Problem

Explain why a scrapyard crane uses an electromagnet rather than a permanent magnet.

⚠ Watch out

Thinking a bell's electromagnet stays switched on for the whole time you hold the switch down. It doesn't: every strike opens the contact point and switches the electromagnet off, and that is exactly what lets the armature spring back and strike again.

🧠

Memory hook

On: grab. Off: let go. Ask every device the same two questions (what happens when the current is on, and what happens when it goes off?) and you can explain how it works.

✓

Check yourself

Without looking back: what makes an electromagnetic door lock unlock when you press the switch, and what locks it again when you let go?

Flashcards

(13)
What is an electromagnet made from?
A coil of insulated wire wrapped round an iron core, with a current passing through the wire.
What happens to an electromagnet when the current is switched off?
It stops behaving like a magnet. While the current flows, the coil behaves like a bar magnet.
Why is an electromagnet called a temporary magnet?
It is a magnet only while there is a current in its coil, so it can be switched on and off.
Why is the coil made of insulated wire?
Insulation makes the current go round every turn, and each turn adds to the electromagnet's strength.
Which materials will a scrapyard crane's electromagnet pick up?
Magnetic materials such as iron and steel. It leaves non-magnetic copper and aluminium behind.
How does a scrapyard crane drop its load in the right place?
The operator switches the current off, so the electromagnet is no longer a magnet and the load falls.
In an electromagnetic door lock, what holds the iron bolt in the locked position?
Springs.
What happens inside an electromagnetic door lock when the switch is pressed?
A current flows in the coil, the electromagnet is magnetised and attracts the iron bolt, pulling it back so the door unlocks.
In an electric bell, what does the electromagnet attract, and what does that do?
A springy soft-iron armature. It moves the clapper (striker) so that it hits the bell.
What breaks the circuit in an electric bell each time it strikes?
The armature's movement opens the contact point.
Why are the core and armature of an electric bell made of soft iron?
Soft iron is easily demagnetised, so they stop attracting each other when the electromagnet is switched off.
What makes the coil of an electric motor spin?
With a current in it, the coil is an electromagnet. Its field and the field of the magnets around it interact and push it round.
Name some everyday devices that contain an electric motor.
For example: an electric screwdriver, vacuum cleaner, washing machine, electric fan or electric toothbrush.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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