Electromagnets

A magnet you can switch off? It sounds like a contradiction — but it is exactly how a scrapyard crane lifts a whole car with the flick of a switch and then, with a second flick, lets it drop into the crusher. The secret is one of the quiet marvels of physics: an electric current makes a magnetic field. Send a current down a wire and a magnetic field springs up around it; cut the current and the field vanishes on the spot.

You have met a magnetic field around an ordinary bar magnet, and you have met an electric current as the flow of charge round a circuit. This page ties the two together — the link that powers motors, loudspeakers, doorbells, MRI scanners and the fastest trains on Earth. We build it up in three steps: a straight wire, then a coil, then a coil with an iron heart.

In 1820 a Danish teacher, Hans Christian Ørsted, was giving a lecture. He happened to switch on a current in a wire that lay near a compass — and the compass needle twitched sideways, as if a magnet had walked past. Nobody had ever seen electricity and magnetism touch before. That accidental twitch is the seed of every electric motor, generator and transformer built since. Whenever charge moves, a magnetic field is born around it — always.

Step 1 — a straight wire: circular field lines

Take a single straight wire and push a current through it. The magnetic field that appears does not point along the wire and it does not point straight out from it. Instead the field lines wrap around the wire in circles, like the rings of a target seen end on. Below, the wire points straight out of the page towards you (shown as a dot), and the current flows out with it; the field circles run round and round it.

Which way round do the circles go? Use the right-hand grip rule: imagine gripping the wire in your right hand with your thumb pointing the way the (conventional) current flows. Your curled fingers then point the way the field circles round. The field is also strongest close to the wire and fades as you move away — that is why the inner circles are drawn packed tightly and the outer ones more loosely.

Step 2 — wind it into a coil: the solenoid

A single wire gives a weak field spread thinly around it. But now bend the wire into a loop: on the inside of the loop, the circular fields from every part of the wire all point the same way, so they add up and reinforce. Wind the wire round and round into a long tube of many loops — a coil called a solenoid — and every turn piles its field onto the next.

The result is beautiful. Inside the solenoid the field lines gather into a strong, straight, even (uniform) stream running down the middle of the tube. Outside, they loop round from one end to the other. The whole coil behaves just like a bar magnet: it has a north pole at one end and a south pole at the other, and its field has exactly the bar magnet's shape. The difference is that this bar magnet only exists while the current flows.

Step 3 — the electromagnet lab

Here is the coil to play with. The vertical strokes are the turns of wire; straight arrows down the middle are the field inside, and the arrows above and below are the field looping back outside from N to S. Three things change the strength — try each one and watch the field lines multiply and the strength bar grow:

the current — a bigger current makes a stronger field;
the number of turns — more loops piled up means more field;
the core — slip a bar of soft iron down the middle and the field leaps up.

Now turn the current all the way down to zero. Every field line disappears, the poles vanish, and the strength bar empties. That is the headline of the whole page: no current, no magnet. A permanent bar magnet could never do this — and being able to switch the magnetism on and off, and dial it up and down, is exactly what makes electromagnets so useful.

The soft-iron core: a huge boost

Sliding a rod of soft iron into the coil can make the electromagnet many times stronger without changing the current at all. Iron is a magnetic material: the coil's field lines up the countless tiny magnets inside the iron, and they add their own magnetism to the coil's. The iron concentrates and amplifies the field.

Why soft iron, and not steel? Because soft iron only stays magnetised while the current is on. The instant you cut the current, the iron loses almost all of its magnetism and lets go. Steel would stubbornly stay magnetised — turning your switchable magnet into a permanent one that can never let go of the car it just picked up. For an electromagnet you want a core that forgets as fast as it remembers.

What an electromagnet does — in four lines

An electric current always produces a magnetic field around it (Ørsted).
Winding the wire into a solenoid concentrates that field into a strong, uniform, bar-magnet-shaped field with a north and a south pole.
The field is made stronger by more current, more turns, or a soft-iron core.
Switch the current off and the field disappears; reverse the current and the north and south poles swap over.

Why the on/off trick matters — electromagnets everywhere

A permanent magnet is stuck being a magnet forever, at one fixed strength. An electromagnet is under your command: on or off, strong or gentle, north or south. That single advantage puts them inside a huge share of the machines around you:

Scrapyard cranes — a giant coil lifts tonnes of scrap, then drops it by simply switching off the current.
Electric bells — an electromagnet yanks a hammer against a gong over and over (see below).
Relays — a small current works an electromagnet that switches on a much larger current, safely and remotely.
Loudspeakers and headphones — a changing current in a coil makes a changing magnetic force that pushes a cone to and fro, making sound.
MRI scanners — colossal electromagnets image the inside of your body.
Maglev trains — electromagnets lift the train clear of the track and pull it along, so it floats with almost no friction.

An electric bell is a wonderful little trick: a circuit that keeps breaking itself. Press the button and current flows through an electromagnet, which pulls an iron arm across — swinging a hammer to strike the gong. Ding! But as the arm moves, it pulls a contact apart and breaks its own circuit. The electromagnet switches off, the springy arm flies back, which closes the contact again — so the current flows, the magnet pulls, the hammer strikes, the circuit breaks… dozens of times a second. That frantic make-and-break is the brrrrring you hear, and it only works because an electromagnet can turn on and off in an instant.

These are the classic electromagnet traps — get them straight:

The magnetism comes from the current, not the metal. Switch the current off and the electromagnet is gone — that is the whole point of it, and the whole difference from a permanent magnet. If it stayed magnetic with the current off, it would be useless as a switchable magnet.
A soft-iron core boosts the field but does not become a permanent magnet. It is strongly magnetic only while the current flows, and forgets its magnetism the moment you switch off. That is precisely why we use soft iron and not steel.
Reversing the current does not turn the magnet off — it swaps the poles. The end that was north becomes south, and vice versa. Off means no current at all.
Adding more turns is not the same as using more wire length carelessly. It is the number of loops around the core that stacks up the field — winding the same length into more, tighter turns strengthens it.