Magnetism and electromagnetism · Model

Electromagnets

A current makes a magnetic field. Wind the wire into a coil, drop a piece of iron down the middle, and you have a magnet with a switch.

Start here

The crane lifts a car. Then it lets go.

A scrapyard crane swings a flat disc over a wrecked car, lowers it until it touches, and lifts the whole car clear of the ground. It swings across to a skip — and the car falls off. Nobody unhooked anything and nothing moved on the disc.

How does it let go?

• AThe disc is turned over so its south pole faces the car instead
• BThe disc is an electromagnet, and the operator switches the current off
• CThe car is shaken loose, because a magnet cannot be switched
• DThe magnet is worn out by the heavy lift and has to recover between cars

Any wire with a current in it has a magnetic field around it. Around a single straight wire that field is a set of rings, and it is weak — you need a sensitive compass right against the wire to see it at all.

Winding the wire into a coil changes that. Every turn adds its field to the turns beside it, and inside the coil they all point the same way, so the fields stack up. A coil like this is called a solenoid, and the field it makes outside itself is the same shape as a bar magnet's: it has a north end, a south end and lines that loop from one to the other.

Putting an iron core down the middle makes it dramatically stronger. The iron is magnetised by the coil's field and adds its own, many times over. Soft iron is chosen because it lets go again the moment the current stops, which steel does not.

Three things make an electromagnet stronger: more turns on the coil, more current through it, and an iron core. And one thing makes it different from every permanent magnet: switch the current off and the magnetism goes. Reverse the current and the north and south ends swap over.

At the bench · a coil, a supply and a pile of paper clips

Build one and see what it lifts.

Change a control to begin

Commit first. An electromagnet is holding a chain of paper clips. The switch is opened. What happens to the clips?

• AThey stay put, because the iron core is now a permanent magnet
• BThey hold on for a few seconds while the field drains out of the core
• CThey all fall at once, because the field goes when the current does
• DThe bottom ones fall and the top ones stay, because the top ones are touching the iron

The figure

Four jobs only a switchable magnet can do

Scrapyard crane

Lifts a car, then drops it on command. A permanent magnet would never let go.

Door lock

Holds a fire door shut while current flows. Cut the power and every door in the building is free — which is the point.

Relay

A small current pulls an iron arm across and closes a second, much bigger circuit. That is how a car starter is worked by a key.

Loudspeaker

A coil in a permanent magnet's field, driven by a current that changes thousands of times a second, pushing a cone in and out.

Stronger

more turns · more current · an iron core

Different

switch it off and it stops · reverse the current and the poles swap

!

Think again

“The iron core is what makes the magnetism. The coil just holds it.”

The other way round. Take the core out and the coil still works — weakly, but it works, and a compass held at either end shows a definite north and a definite south. Leave the core in and switch off, and a soft iron core does essentially nothing. The current makes the field; the iron responds to it and multiplies it. You can prove which is which on the bench in one move: set an air core and turn the current up, and clips lift. Set an iron core and open the switch, and they fall.

“Adding more turns makes it stronger because there is more wire, so more current.”

More wire is more resistance, not more current — if anything a longer coil on the same supply carries slightly less. What extra turns give you is more contributions of field in the same place. Each turn produces its own field, all the turns are wrapped round the same middle, and inside the coil they all point the same way, so twenty turns produce roughly twenty times what one does at the same current. That is why the bench keeps turns and current as separate controls: they are two different reasons for the same result.

Mastery ladder

Not started yet.

Rungs 3 and 4 you mark yourself.

Rung 1 · Choose the change

An electromagnet holds four paper clips. You want it to hold more, and you may change one thing. Which of these will not help at all?

• ASwapping the soft iron core for a plastic one of the same size
• BWinding twice as many turns onto the coil
• CTurning the supply up so twice as much current flows
• DPutting a soft iron core in where there was only air

Rung 2 · Predict the switch

An electromagnet with a soft iron core is holding a chain of clips. The switch is opened. What happens?

• AThe clips stay, because the iron core has become a permanent magnet
• BThe clips stay for a while and then fall as the field slowly drains away
• CThe clips fall, because the iron core has become a south pole
• DThe clips fall, because the field goes when the current does

Rung 3 · Explain

Explain why a scrapyard crane uses an electromagnet rather than a very strong permanent magnet, and say what the iron core in it is for.

Your explanationComplete

Rung 4 · Take it somewhere new

A fire door in a school is held open by an electromagnet on the wall, and closes by itself when the fire alarm sounds. Explain how the alarm makes the door close, and explain why this design is safer than a catch that has to be released by someone.

Your answerComplete

Key note

A current in a wire produces a magnetic field around it. Winding the wire into a coil puts many turns round the same space so their fields add, and the result has the same shape as a bar magnet's field, with a north end and a south end. A soft iron core is magnetised by that field and multiplies it many times over, and lets go again the instant the current stops. More turns, more current and an iron core each make the magnet stronger. Switching off removes the magnetism completely, and reversing the current swaps the ends over — neither of which a permanent magnet can do.

Going further

Soft iron and hard steel are the same element doing two different jobs. Soft iron takes on magnetism easily and loses it just as easily, which is exactly what a crane or a relay needs. Steel is harder to magnetise and hangs on to it, which is what a permanent magnet needs. Wrap a coil round a steel bar, run a current, switch off, and you have made a permanent magnet — one of the standard ways of making one.

Electromagnets are why an MRI scanner exists. Its main magnet is a coil of superconducting wire kept at a few degrees above absolute zero, where the wire has no resistance at all, so an enormous current runs round it for years without a supply and without heating up. The field it makes is tens of thousands of times the Earth's, which is why nothing steel goes into the room: an oxygen cylinder brought too close is pulled in hard enough to kill.

Eye protection on. The coil gets hot within a minute, so switch off between tries and let it cool. Keep fingers, hair and loose sleeves clear of the spinning part. Use only the low-voltage supply your teacher gives you.

The bench is a teaching model. The number of paper clips is worked out from a simple rule in which the field goes up in proportion to the turns and to the current, and an iron core multiplies it by a fixed factor of twenty-five; a real core's multiplication depends on the iron and falls away sharply once the iron saturates, so the largest numbers here are optimistic. The plastic former is treated as having exactly the same effect as no core at all, which is right to well within anything a school bench could measure. The supply is treated as delivering whatever current is set whatever the coil, and the coil is treated as not heating up — a real coil at the top settings would get hot enough to matter within a minute. The coil is drawn as eight loops at every setting, because the drawing is a symbol and the turn count is the readout beside it, and the hanging chain is drawn to ten clips however many are held. Field strength is a relative figure with the strongest setting on this bench set to 100, and no value in tesla is given because the unit is beyond this stage.

Lesson content © MrBadmusAI.