Induced magnetism

What induced magnetism is. Iron, steel, cobalt and nickel are magnetic materials — each is built from countless tiny atomic regions that can align to act like miniature magnets. In an unmagnetised piece, these regions point every which way and cancel out, so the object shows no overall magnetism. Bring a strong magnet close, and its magnetic field nudges those tiny regions to swing into line with it, even without any contact or stroking. The object becomes a temporary magnet purely by being in the field — this is induced magnetism, or magnetic induction.

The polarity rule. The induced poles are never random — they follow one fixed pattern. The end of the magnetic material nearer the inducing pole always develops the opposite polarity to that pole, while the farther end develops the same polarity. So a magnet's north pole held near a nail induces a south pole at the near end of the nail and a north pole at the far end (a piece of iron, once magnetised, always has two poles, just like the magnet inducing it). Because the induced near pole is opposite to the magnet's facing pole, and unlike poles attract, the nail is pulled toward the magnet — the very attraction we set out to explain.

Induction before attraction. This is the syllabus's central point: the nail cannot be attracted until it is first magnetised. Induction is the cause; attraction is its consequence, not an independent, separate effect. This single cause-and-effect ordering also explains why a magnet only ever pulls in magnetic materials — wood, plastic, and copper have no aligned tiny magnetic regions to line up, so no induced poles form in them, and there is nothing for the magnet to attract.

Chains of induced magnets. Once the nail is magnetised, it has its own north and south poles and can, in turn, induce magnetism in a second nail touching it, which can induce a third, and so on. Each pair of touching ends ends up with opposite polarity, so consecutive pieces attract each other — this is exactly why a single bar magnet can hold up a short dangling chain of pins or nails, each one turned into a tiny induced magnet by the one above it.

Magnetic field lines. These are imaginary lines drawn to picture a magnetic field's direction and strength at every point in space around a magnet.

Their key properties, worth knowing by heart:

• They emerge from the north pole of a magnet and enter at the south pole, outside the magnet.
• Inside the magnet itself, the lines run from the south pole back to the north pole, so each line forms a closed loop overall.
• Two field lines never cross one another.
• Lines are drawn closer together where the field is strong, and farther apart where it is weak.
• The tangent to a field line at any point gives the direction of the magnetic field at that point.

The goal here is to reason through, in cause-and-effect order, exactly why a magnet's field turns an ordinary unmagnetised nail into a temporary magnet with a predictable pair of poles, and why that induced magnetism is always the first event, with attraction following as its direct and unavoidable consequence.

Step 1. Start with what is inside a piece of iron before it meets a magnet. It is made of enormous numbers of tiny regions that each behave like a miniature magnet with its own north and south pole. In an unmagnetised nail these tiny regions point in every possible direction, so their fields cancel each other out almost completely, and the nail as a whole shows no detectable magnetism.

Step 2. Bring the nail into the field of a bar magnet. The magnet's field does not touch the nail physically, but it exerts a turning influence on every one of those tiny internal regions, twisting them to line up with the direction of the field passing through the nail. No stroking or contact is needed — the field itself does the aligning.

Step 3. Because the field lines run from the magnet's north pole, out into the space around it, and curve back in at the south pole, the direction of the field at the nail's near end is such that it produces a pole there that is opposite to the pole facing it, while the alignment carries through the nail so that the far end ends up with the same polarity as the near facing pole of the magnet. This is not arbitrary — it follows directly from the geometry of the magnet's own field lines threading through the nail.

Step 4. With its tiny regions now aligned, the nail behaves as a genuine (if temporary) magnet, with two distinct poles of its own. Only now does the force of attraction switch on: the induced near pole, being opposite to the magnet's facing pole, is pulled toward it, because unlike poles attract. If the nail is pulled away from the magnet's field, the tiny regions lose their alignment and the induced poles fade — confirming that the magnetism was induced by the field's presence, not a permanent property the nail already had.

Step 5. This ordering, first induction then attraction, also explains everyday exceptions cleanly. Non-magnetic materials like wood or copper have no such alignable tiny regions, so step 2 fails for them: no induction occurs, so step 4 never happens, and they are never attracted, no matter how strong the magnet.

Question. A bar magnet is held with its south pole facing an unmagnetised iron nail, without touching it. A second identical nail is then placed so it touches the free end of the first nail, forming a two-nail chain. State the polarity induced at every end along the chain, and explain whether the two nails stay attached to each other.

Step 1. The end of the first nail nearer the magnet's south pole is induced with the opposite polarity, so it becomes a north pole. The far end of the first nail, correspondingly, becomes a south pole.

Step 2. This induced south pole at the free end of the first nail now acts like a magnet's south pole for the second nail touching it. By the same rule, the near end of the second nail (touching the first) is induced with the opposite polarity, becoming a north pole, and the far, free end of the second nail becomes a south pole.

Step 3. At the junction between the two nails, the touching ends are a south pole (bottom of nail 1) and a north pole (top of nail 2) — opposite polarities, so unlike poles attract, and the nails cling together.

Answer. Nail 1: north pole near the magnet, south pole at its free end. Nail 2: north pole at the end touching nail 1, south pole at its free bottom end. The two nails stay attached because induction gives every pair of touching ends opposite polarities, and unlike poles always attract.

• Assuming a magnet attracts any metal — only magnetic materials (iron, steel, cobalt, nickel) can be induced and attracted; copper, aluminium, and other non-magnetic metals are not.
• Getting the polarity rule backwards — the induced pole nearer the magnet is always opposite to the facing pole, never the same; it is the farther end that matches.
• Treating attraction as a separate event from induction — attraction can only happen after induction has occurred; the nail must become a magnet first.
• Mixing up field lines with real physical threads in space — they are a visual tool for direction and strength, not objects you could touch or that actually exist as lines.

Ask the tutor: Want to try predicting induced polarities in a longer chain of nails, or see more examples of the near-opposite, far-same rule in action?