Poles of an electromagnet

Every current-carrying coil has two poles, just like a bar magnet. When current flows through a coil of wire, the coil starts behaving as a magnet with two distinct ends. One end acts as a north pole, the other as a south pole. You test for these poles exactly the way you would test an ordinary bar magnet: hold a compass near each end and watch which way the needle settles, or bring a second magnet close and see whether the ends attract or repel.

The identity of each pole follows the direction of current. Which end becomes north and which becomes south is decided purely by which way the current is flowing around the coil — it is not fixed by how the coil is built or wound. If the current direction through the coil is reversed (for instance, by swapping the two wires where they connect to the cell), the pole at each end flips too: the end that was north becomes south, and the end that was south becomes north.

Poles obey the same attract/repel rule as any magnet. Once you know the pole at each end of two coils (or a coil and a bar magnet), the outcome follows the one rule every student already knows:

where: "like poles" means two north poles facing each other, or two south poles facing each other; "unlike poles" means a north pole facing a south pole. This single rule, combined with knowing how current direction sets pole identity, is enough to predict the behaviour of any electromagnet setup in this chapter — no formula or numeric calculation is needed at Class 8 level.

This reversibility is the key contrast with a permanent magnet. A bar magnet's north and south poles are fixed the moment it is made and cannot be swapped by any ordinary means. An electromagnet's poles, by contrast, exist only because current is flowing, and they can be flipped at will, as often as you like, just by reversing the current — a property permanent magnets simply do not have.

The goal here is to explain, in words a Class 8 student can reason through without any formula, why an electromagnet has two poles at all and why those specific poles are tied to current direction rather than to the coil itself — this is the chapter's key qualitative link between the magnetic effect of current and ordinary bar-magnet behaviour, and it is what every exam question in this topic is really testing.

Step 1. Recall the magnetic effect of current, already established earlier in the chapter: a wire carrying current has a magnetic effect around it, strong enough to deflect a compass needle placed nearby, and reversing the current's direction reverses which way the needle deflects. This single fact — current direction controls the direction of the magnetic effect — is the seed of everything that follows.

Step 2. A coil is many loops of the same current-carrying wire wound together, so the magnetic effect of every loop adds up at the two ends of the coil, making one end behave like a concentrated north pole and the other like a concentrated south pole — the coil as a whole starts acting exactly like a bar magnet, even though it is only wire and (usually) an iron core with current flowing through it.

Step 3. Because the pole at each end came from the current's direction in Step 1, reversing that current reverses the magnetic effect at both ends simultaneously — the end that behaved as north now behaves as south, and vice versa. Nothing about the coil's shape or winding changed; only the direction electrons are flowing changed, and that alone decides which end is which pole.

Step 4. This is the essential difference from a bar magnet. A bar magnet's poles come from the fixed alignment of tiny magnetic domains set permanently when the magnet was made — there is no current to reverse, so the poles cannot be swapped by any everyday means. An electromagnet's poles exist only while current flows and are entirely re-assignable: switch off the current and the poles vanish (the iron core loses most of its magnetism); reverse the current and the poles trade places. This on-demand, reversible pole behaviour is precisely why electromagnets are useful in devices like electric bells, cranes and buzzers, where a magnet needs to be switched on, off, or flipped instantly.

Question: A student winds a coil around an iron nail, connects it to a cell through a switch, and tests both ends with a compass. The left end repels the north-seeking end of the compass needle, and the right end attracts it. The student then disconnects the two wires from the cell and reconnects them the opposite way round, closing the switch again. State the pole at each end of the coil now, and explain your reasoning.

Step 1. Interpret the first test. The left end repels the north-seeking end of the compass needle. Since like poles repel, the left end must itself be a north pole. The right end attracts the north-seeking end, and since unlike poles attract, the right end must be a south pole. So initially: left = north, right = south.

Step 2. Identify what swapping the wires does. Reconnecting the two wires the opposite way round on the cell reverses the direction of current flowing through the entire coil — the same current now travels around every loop in the reverse sense compared to before.

Step 3. Apply the reasoning from "Why it works." Reversing the current direction reverses the magnetic effect at both ends of the coil together, so the end that was previously a north pole becomes a south pole, and the end that was previously a south pole becomes a north pole.

Step 4. State the answer for each end. The left end, previously north, is now a south pole; the right end, previously south, is now a north pole. If the student tested again with the compass, the left end would now attract the needle's north-seeking end and the right end would repel it — the complete opposite of the first test.

Competitive stretch

Question: Two identical coils, P and Q, sit side by side with their nearby ends facing each other, each on its own cell and switch. With both switches closed, the nearby ends attract each other strongly. The teacher then reverses the current direction in coil P only, leaving coil Q untouched, and closes both switches again. What happens to the nearby ends now?

Step 1. In the original setup the nearby ends attract, so — since unlike poles attract — the nearby end of P and the nearby end of Q must be different poles (say P's nearby end is north and Q's is south).

Step 2. Reversing the current in P only flips both of P's poles, including its nearby end, which becomes south; Q's current is unchanged, so Q's nearby end stays south.

Step 3. Now both nearby ends are south — the same pole facing the same pole. Since like poles repel, the two coils will repel each other instead of attracting, the opposite of the original behaviour.

• Thinking an electromagnet has only one pole, or that its poles are somehow "different" from a bar magnet's — a current-carrying coil always has exactly two poles and follows the identical attract/repel rule as any magnet.
• Believing the poles are fixed by how the coil is wound rather than by the current's direction — the same coil can be north-left/south-right one moment and south-left/north-right the next, with nothing physically rebuilt.
• Reversing only one wire's role in your head while forgetting that both poles flip together — the north pole doesn't wander to a new spot; the identity of each end swaps at once.
• Mixing up pole reversal with a change in magnet strength — reversing current direction changes which end is north and which is south, but strength depends separately on the current's size and the number of turns in the coil, not on its direction.

Ask the tutor: Want to try predicting the poles and the attract-or-repel outcome for a few more coil arrangements?