Electromagnets

A coil, not a bar, is the starting point. A single straight wire carrying current already has a weak magnetic field circling it — that is the ordinary magnetic effect of current. An electromagnet is built by taking that same wire and winding it into many turns, one after another, to form a coil, then passing current through the whole coil at once. Because every turn's field is oriented the same way through the middle of the coil, the fields of all the turns add up rather than cancel, so a coil of, say, fifty turns produces a far stronger combined field down its axis than one straight length of wire ever could carry on its own.

The core multiplies the effect. Slide a piece of soft iron into the hollow middle of the coil — this is the core — and the electromagnet gets stronger still. The core itself becomes magnetized by the coil's field while current flows, and the core's own magnetism adds on top of the coil's, so an iron-core coil outperforms an identical air-core coil carrying the same current. Soft iron is chosen deliberately: it magnetizes readily when a field is applied but does not "remember" that magnetism once the field is removed, which is exactly the behaviour an on/off magnet needs.

It behaves exactly like a bar magnet, with two poles. While current flows, the coil-and-core combination attracts magnetic materials (iron filings, pins, nails) and has a north pole at one end and a south pole at the other, just like a permanent bar magnet. Reverse the direction of the current — by flipping the cell's terminals — and the poles reverse too: whichever end was north becomes south, and south becomes north. The pole pattern is not a fixed property of the coil; it is set entirely by which way the current circulates through the turns.

The magnetism is temporary — and that is the whole point. The single feature that separates an electromagnet from a bought bar magnet is that its magnetism exists only while current flows. Open the switch, and the current stops, the coil's field collapses, and the soft-iron core — never having "remembered" its magnetism in the first place — loses almost all of it within an instant. A bar magnet cannot be switched off this way. This on/off control, available at the flick of a switch, is what makes electromagnets so useful: a crane can grip a car body and then release it simply by cutting the current, something a permanent magnet could never do.

The goal of this reasoning is to see, without any formula, why a coil-plus-core arrangement is a stronger and more controllable magnet than either a bare wire or a bought permanent magnet, using only what is already known about the magnetic effect of a current-carrying wire.

Step 1. Recall that a single straight current-carrying wire produces a magnetic field circling the wire — the magnetic effect of current, established before electromagnets are introduced. That field is real but weak, since it comes from just one length of wire.

Step 2. Bend that wire into a loop and its field lines all thread through the loop's centre in the same direction, instead of merely circling a straight line. Wind many such loops into a coil, and the fields of every loop line up and reinforce each other down the coil's axis — many small pushes in the same direction adding to one large push. This is why a coil is dramatically stronger than the straight wire it was made from.

Step 3. Slip a soft-iron core inside the coil. The coil's field magnetizes the core itself, and the core's own magnetism adds on top of the coil's field rather than opposing it, so the total field with the core in place beats the coil alone with air inside it.

Step 4. Because soft iron does not hold on to its magnetism once the magnetizing field is removed, switching the current off collapses the coil's field, and the core — never having stored magnetism it could keep — drops back to unmagnetized almost immediately. "Coil field adds up" plus "core does not remember" is exactly what produces a magnet that is strong while switched on and essentially inert while switched off — something a permanent bar magnet, magnetized once and kept that way, can never offer.

Question: A scrapyard wants an electromagnet, built from a coil of wire, a switch and a cell, that can (a) lift heavy iron scrap off a pile and carry it across the yard, and (b) drop the scrap precisely over a sorting bin, without anyone touching the load. A worker suggests using a strong bar magnet instead, since it is simpler to make. Explain, step by step, why the electromagnet is the right choice, and how it should be built and operated to work well.

Step 1. Identify what the job needs: strong attraction to pick the scrap up, and attraction that switches off on command to release it. A bar magnet gives the first but not the second — its magnetism cannot be turned off, so the scrap could never be released without prying it away.

Step 2. Build the lifter as a coil of insulated wire wound many times around a soft-iron core, connected through a switch to a strong power source. Many turns, rather than a single loop, ensure the fields of all the turns add up, and the iron core adds its own magnetism on top.

Step 3. To pick up the scrap, the operator closes the switch: current flows, the coil-and-core magnetizes, and the strong combined field attracts and holds the iron pieces, stronger than a bare wire because of the added turns and the core.

Step 4. To release the scrap, the operator opens the switch: current stops, the coil's field disappears, and the soft-iron core — holding no permanent magnetism of its own — loses its attraction almost at once, so the scrap drops cleanly at the chosen spot. Answer: the electromagnet is the correct choice because its magnetism can be switched on to lift and off to release, a control a permanent bar magnet cannot offer; it should be a many-turn coil around a soft-iron core, operated with a switch.

• Thinking an electromagnet stays magnetic once made, like a shop-bought bar magnet — its magnetism exists only while current is actually flowing through the coil.
• Assuming any core material works as well as any other — a soft-iron core adds real strength, while non-magnetic materials such as wood or plastic inside the coil add nothing at all.
• Ignoring current direction — reversing which way the current flows through the coil reverses which end becomes the north pole and which becomes the south pole; the poles are not fixed.
• Mixing up "soft iron" with "steel" or other materials that stay magnetized — a core that keeps its magnetism after the current is switched off would defeat the entire purpose of an electromagnet.

Ask the tutor: Want to see how changing the number of turns or the current strength changes an electromagnet's pulling power?