Magnets: Polarity, Attraction and Repulsion

Hold a bar magnet and you cannot see anything happening, but the ends are not the same. Every magnet has two poles: a north pole (marked N) and a south pole (marked S). The force a magnet makes is strongest at its poles.

Poles always come in pairs. You can never get a magnet with only a north pole or only a south pole. If you cut a bar magnet in half, you do not end up with a separate N piece and a separate S piece. Instead, each smaller piece becomes a whole new magnet with its own north pole and its own south pole.

The behaviour of two magnets depends only on which poles face each other. This is the rule of polarity:

• Like poles repel. Two north poles (N and N) push apart. Two south poles (S and S) push apart.
• Unlike poles attract. A north pole and a south pole (N and S) pull together.

Because magnetism works across a gap, it is a non-contact force. You can feel two magnets push or pull before they ever touch, and the closer the poles get, the stronger the push or pull becomes.

Do not confuse poles with charges. The rule you have just read, like repels, unlike attracts, is word for word the rule for electric charge from the electrostatic and gravitational forces lesson, and that is exactly why the two get mixed up. They are different forces with different causes. The electrostatic force is caused by charge, which an object gains or loses when electrons move onto it or off it, and any material at all can be charged by rubbing, including a plastic balloon. Magnetism is caused by the arrangement inside a few particular metals, mainly iron, nickel and cobalt, and rubbing a balloon will never make it magnetic no matter how long you try.

Two quick tests separate them in practice. Rub a plastic ruler on your sleeve: it will pick up small pieces of paper, which is electrostatic, and it will do nothing whatever to a steel paper clip. Hold a bar magnet near that same clip and the clip jumps, which is magnetic, and the magnet will not lift paper. If a force acts across a gap you have narrowed it to the non-contact family, and it is what the objects are made of and what has been done to them that tells you which member of the family you are looking at.

Magnets do not attract everything, and the list of what they do attract is far shorter than most people expect. The magnetic materials are essentially iron, nickel and cobalt, plus alloys that contain them. Steel is on the list only because steel is mostly iron, which is worth noticing because it explains a lot of everyday cases at once: a paper clip, a fridge door and a tin can all grip a magnet for the same single reason.

Everything else is not magnetic at all. Plastic, wood, glass, copper and aluminium are not attracted even slightly, and the two metals in that list are the ones that catch people out. Copper and aluminium look like the sort of metal that ought to stick, and they do not, which tells you something important: being a metal is not what makes a material magnetic. Only those few particular elements are. That gives you a quick test with no equipment at all, and it is more informative than it looks. Touch an unknown object with a magnet. If it grips, the object contains iron, nickel or cobalt. If it does not, it may still be metal, so the test identifies the material rather than sorting metals from non-metals.

A compass is a very useful magnet. The needle inside a compass is a tiny bar magnet that can spin freely. Because Earth itself acts like a giant magnet, it has a magnetic field all around it. The compass needle lines up with Earth's magnetic field, so its north-seeking pole points roughly toward Earth's north. This is how explorers and bushwalkers have found their way for centuries.

Australian context: the magnetic compass on a four-wheel drive in the outback still works the same way as one used by early explorers crossing the continent. The needle is just a small magnet swinging to line up with Earth's magnetic field.

The magnetic force is not the same at every distance. The closer an object is to a magnet, the stronger the pull, and as you move it away the force gets weaker very quickly. Hold a magnet a few centimetres from a paper clip and nothing happens; slide it closer and at some point the clip suddenly jumps across the gap to meet it.

The word to hold on to there is quickly. This is the same behaviour you met with the electrostatic and gravitational forces, and it happens for the same reason: the magnet's influence spreads outward in every direction at once, so at a greater distance it is spread far thinner and much less of it lands on the clip. The reason a magnet seems to have a sharp "range" is not that it switches off at a certain point. It is that the force is falling away so steeply that it drops below what is needed to overcome friction and lift the clip, and everything after that is too weak to see.

That is also why the clip's jump looks so sudden. As the clip starts to move it gets closer, closer means stronger, stronger means it moves faster, which brings it closer still. Once the pull wins at all, it wins increasingly, so the last centimetre happens almost instantly. Nothing new switched on; the same rule simply fed back on itself.

You can investigate this yourself with a simple, safe fair test. One easy method is to count how many paper clips a magnet can hold through an increasing number of sheets of paper or cards. Each extra sheet pushes the paper clip a little further from the magnet, so the magnet should hold fewer clips. Another method is to slowly slide a magnet toward a single paper clip and measure the distance at which the clip first moves.

To make it a fair test, you change only ONE thing (the variable):

• Change: the distance between the magnet and the paper clip (for example, the number of paper sheets in the gap)
• Keep the same: the same magnet, the same paper clips, the same surface, the same way of measuring
• Measure: how many clips are held, or the distance at which the clip first moves

For each pair of magnet ends below, decide whether they will attract (pull together) or repel (push apart). Then say whether each pair is "like poles" or "unlike poles".

For row 5, remember a paper clip is not a magnet but it is a magnetic material. Will it be pulled toward the magnet or pushed away?

This is the practical investigation for the syllabus: test the effect of distance on the action of a magnet. Read the safety note above first, then plan your fair test.

What you need: one bar magnet (a larger classroom magnet, not tiny magnet balls), a pile of paper clips, and several sheets of paper or thin card.

Method:

• Place 1 sheet of paper over a pile of paper clips. Lower the magnet onto the sheet and count how many clips it lifts.
• Add another sheet so there are 2 sheets, then 3, then 4, then 5, increasing the distance each time.
• Each time, record how many paper clips the magnet can lift.

Fair test: the variable you change is the distance (number of sheets). Keep the magnet, the paper clips and the way you measure exactly the same every time.

Fill in a results table like this in your book, then answer the questions below.