Magnetic force

Magnetic force is the force a magnet exerts on another magnet, or on a magnetic material — a material such as iron, nickel, or cobalt that a magnet can attract. Wood, plastic, glass, and aluminium are non-magnetic; a magnet has no effect on them at all. Crucially, magnetic force needs no touching. A magnet can pull a paperclip across a table, or two magnets can shove each other apart while still separated by a visible gap — this is exactly what makes it a non-contact force, in the same family as gravitational force and electrostatic force, and different from contact forces like muscular force or friction. Every magnet, no matter its shape, has two ends called poles: a north pole and a south pole. You cannot have a magnet with only one pole — cut a bar magnet in half and each piece grows its own north and south pole. The way two magnets behave when brought together depends entirely on which poles are facing each other, and the rule is short enough to memorise in one line:

• Like poles repel. North facing north pushes apart; south facing south pushes apart.
• Unlike poles attract. North facing south pulls together, and so does south facing north.

This single rule is the whole of Grade 8 pole behaviour — there are no field-strength numbers or formulas at this stage, only the qualitative fact of attraction or repulsion. It also marks an important difference from gravity: gravitational force between two masses is always attractive, but magnetic force can be either attractive or repulsive, and switching from one to the other is as easy as turning a magnet around. A magnet's pull is not limited to other magnets. Bring a magnet near an iron nail, and the nail is attracted — even though the nail itself is not a magnet and cannot attract or repel anything else. This is the key distinction between a magnet (has its own poles, can attract or repel) and a magnetic material (can only ever be attracted, never repelled, by a magnet). Everyday magnetic force shows up constantly: a compass needle is a tiny magnet that turns to align with Earth's own magnetic influence and settle roughly north-south; fridge magnets hold notes against a steel door using attraction; and a stack of ring magnets can be arranged so that like poles face each other, making an upper ring float above the one beneath it on a cushion of repulsion.

There is no formula to derive here — Grade 8 treats magnetic force qualitatively — but the rule "like repel, unlike attract" is not just something to memorise; it follows from a consistent picture of how magnets act on the space around them.

Step 1. Every magnet exerts its influence outward from both poles, and this influence is what lets it act on another magnet without touching it — the basis of the force being a non-contact one.

Step 2. When two similar poles are brought close (north near north, or south near south), their outward pushes oppose each other, and the net effect observed is that the magnets are driven apart — repulsion.

Step 3. When two opposite poles are brought close (north near south), their influences work with each other rather than against each other, and the net effect observed is that the magnets are drawn together — attraction.

Step 4. A magnetic material like iron behaves differently from a magnet: it has no fixed poles of its own, so it cannot repel a magnet under any arrangement — but it becomes attracted whenever any pole of a magnet is brought near it. This asymmetry (only ever attracted, never repelled) is precisely what lets you tell a plain magnetic material apart from a genuine magnet using nothing but a single labelled magnet, as the worked example below shows.

Question. A student has three identical-looking metal rods: an iron rod, an aluminium rod, and an unmarked bar magnet. She has one clearly labelled bar magnet. Describe a procedure using only the labelled magnet to identify each of the three rods, and predict what happens when its north pole is brought near each one in turn.

Step 1. Bring the labelled magnet's north pole near the aluminium rod. Aluminium is not a magnetic material, so there is no attraction and no repulsion — the rod does not respond at all. This identifies the aluminium rod.

Step 2. Bring the labelled magnet's north pole near the iron rod. Iron is a magnetic material, so the rod is attracted toward the magnet, regardless of which pole is used — a magnet always attracts an unmagnetised magnetic material, never repels it. This identifies the iron rod.

Step 3. Bring the labelled magnet's north pole near one end of the unmarked bar magnet. Two outcomes are possible: if that end is attracted, it must be the unmarked magnet's south pole, since unlike poles attract; if that end is instead repelled, it must be the unmarked magnet's north pole, since like poles repel.

Step 4. The decisive test is repulsion itself: only a true magnet can ever push back against the labelled magnet's like pole. The iron rod, being merely a magnetic material, can only ever be pulled in, never pushed away. Answer: the rod that shows no response at all is aluminium, the rod that is only ever attracted (never repelled) is the iron rod, and the rod that can be made to repel the labelled magnet's north pole is the unmarked bar magnet — and whichever end repels is itself a north pole.

Competitive stretch

If the same student now turns the unmarked magnet around completely, so that the end she had identified as its south pole now faces the labelled magnet's north pole, the two north-facing-south arrangement is restored and the pair will pull together strongly again — turning a magnet around does not change which end is which pole, only which pole is presented to the other magnet.

• Assuming a magnet only ever attracts — it can just as easily repel another magnet when like poles face each other.
• Confusing a "magnetic material" (like iron) with a "magnet" — iron is attracted by a magnet, but an ordinary iron nail has no poles of its own and can never repel anything.
• Forgetting that magnetic force needs no contact — it acts across a visible gap of empty space, unlike friction or muscular force.
• Remembering the rule backwards, as "like poles attract, unlike poles repel," instead of the correct "like repel, unlike attract."
• Thinking a non-magnetic material like aluminium or plastic will show a weak attraction — it shows none at all, however close the magnet is brought.

Ask the tutor: Want to see how magnetic force is put to work in real devices, like compasses, magnetic latches, or MRI machines?