12.1.1 Magnetic poles

Magnets and magnetic poles

1. A magnet produces a magnetic field in the space around it, and any other magnet placed in that space feels a push or a pull.
2. Every magnet has exactly two poles, and they always come as a pair: one north-seeking pole, labelled N, and one south-seeking pole, labelled S.
3. The full names come from what a freely suspended magnet does. Left to hang on a thread, it turns until one particular end points towards the Earth's geographic north, and that end is the north-seeking pole.
4. In diagrams and written answers the shortened names north pole and south pole are used, and both spellings of the idea earn credit.
5. The poles are the two regions where the magnetic field is most concentrated, so the magnetic force produced by a bar magnet is strongest at its ends and weakest around its middle.
6. A bar magnet is usually drawn with the north pole shaded blue and the south pole shaded red, and the letters N and S printed on the correct ends.
7. The forces between magnets are non-contact forces. Two magnets do not need to touch, and the push or pull acts straight across the air gap between them.
8. Because magnetic force is a non-contact force, it still acts through thin non-magnetic barriers such as paper, a plastic ruler or a glass sheet.
9. A single pole on its own does not exist. Snapping a bar magnet in half gives two shorter bar magnets, each with a full north pole and a full south pole at its ends.
10. Magnetism is a property of the object, not something stored up and used. A magnet keeps exerting forces indefinitely without running down like a battery.

Attraction and repulsion

1. Whether two magnets pull together or push apart is decided by which pair of poles is facing across the gap.
2. Unlike poles attract. A north pole facing a south pole gives a pull, so each magnet is pushed towards the other.
3. Like poles repel. A north pole facing a north pole gives a push apart, and so does a south pole facing a south pole.
4. There are only four possible pairings, and the rule sorts all of them:
   1. north facing south gives attraction
   2. south facing north gives attraction
   3. north facing north gives repulsion
   4. south facing south gives repulsion
5. The two magnets always experience forces of the same size acting in opposite directions, so when they attract, each one is pulled towards the other rather than one magnet doing all the work.
6. Turning one magnet through $180^{\circ }$ swaps which of its poles faces the other magnet, so attraction becomes repulsion and repulsion becomes attraction without moving the second magnet at all.
7. The size of the magnetic force grows sharply as the gap between the poles is reduced, and falls away quickly as the magnets are separated. Doubling the separation reduces the force by much more than half.
8. Repulsion is the only certain test that an object is a magnet. Attraction alone is not enough, because a magnet also attracts an unmagnetised piece of iron or steel.
9. Two identical bar magnets pushed north pole to north pole will spring apart if one is released, because the repulsive force acts along the line joining the poles.

12.1.2 Magnetic materials; permanent and induced magnets

Magnetic materials

1. Only a small number of materials are magnetic. The four you are expected to name are iron, steel, nickel and cobalt.
2. Steel is an alloy of iron with a small amount of carbon, so it behaves magnetically because of the iron it contains.
3. Being a metal is not the same as being magnetic. Copper, aluminium, brass, gold and silver are all metals, yet a magnet does not attract any of them.
4. Non-metals such as wood, plastic, glass, rubber and card are never magnetic.
5. This difference is put to work in recycling. A rotating magnetic drum lifts steel food tins out of a moving stream of waste while aluminium drinks cans travel past untouched.
6. A magnetic material can do two separate things: it is attracted by a magnet, and it can itself be magnetised so that it becomes a magnet.
7. A piece of magnetic material can be magnetised by stroking it repeatedly in one direction with one pole of a permanent magnet, or by placing it inside a coil carrying a current.
8. A magnet can be demagnetised by heating it strongly, by hammering it, or by placing it inside a coil carrying an alternating current that is then slowly withdrawn.

Permanent and induced magnets

1. A permanent magnet needs nothing done to it. It has its own north pole and south pole, it produces a magnetic field at all times, and that field is still there when every other magnet is taken away.
2. An induced magnet borrows its magnetism. A piece of magnetic material becomes a magnet only while it sits inside the field of another magnet, and it loses most or all of that magnetism when it is taken out of the field.
3. The chain of reasoning for induced magnetism runs in four steps:
   1. The magnetic material is placed inside the magnetic field of a permanent magnet.
   2. The field magnetises the material, so the material becomes an induced magnet with its own poles.
   3. The end of the material nearest the permanent magnet always becomes the opposite pole to the pole it is facing.
   4. Unlike poles attract, so the material is pulled towards the permanent magnet.
4. Because the nearest end always takes the opposite pole, induction always produces attraction. A permanent magnet cannot repel an unmagnetised piece of iron whichever pole is offered to it.
5. This is why a chain of paper clips can hang from a single magnet. The first clip becomes an induced magnet, its far end becomes a free pole, and that pole induces magnetism in the next clip down the chain.
6. Take the permanent magnet away and the chain falls apart, because each clip loses the induced magnetism that was holding the next one on.
7. How much magnetism is kept afterwards depends on the material. Iron keeps almost none, while steel can keep a great deal and may end up as a weak permanent magnet.

Choosing the right magnetic material

1. Magnetic materials are sorted by how well they hold on to magnetism once the magnetising field has gone.
2. Permanent magnetic materials are harder to magnetise in the first place but keep their magnetism well. Steel is the standard school example, and alloys containing nickel and cobalt make the strongest permanent magnets used in industry.
3. Temporary magnetic materials magnetise very easily but lose that magnetism just as easily. Iron is the standard example.
4. Use a permanent magnetic material whenever the object has to stay magnetic without any power supply:
   1. compass needles, which must go on pointing along the Earth's field for years
   2. fridge magnets and the catches that hold cupboard doors shut
   3. the fixed magnets inside loudspeakers, headphones and electric motors
   4. magnetic tool holders and screwdriver tips that keep hold of steel screws
5. Use a temporary magnetic material whenever the magnetism has to be switched on and off:
   1. the iron core of a scrapyard crane magnet, so a car body can be lifted and then dropped
   2. the iron core inside an electric bell, which must release the hammer between strikes
   3. the iron core in a relay, so a small current can open and close a separate switch
   4. magnetic door locks that release the moment the current is cut, which is what makes them safe in a fire
6. Putting a steel core in a crane magnet would be a design fault. The steel would stay magnetised after the current was switched off, so the load would not drop.

12.1.3 Magnetic fields

Magnetic fields and field lines

1. A magnetic field fills the space around a magnet. It cannot be seen, so it is drawn using magnetic field lines, which are a model rather than real threads in the air.
2. A field-line diagram carries three separate pieces of information at once:
   1. the shape of the field, given by the paths the lines follow
   2. the direction of the field, given by the arrowheads drawn on the lines
   3. the relative strength of the field, given by how closely the lines are packed together
3. The arrow on a field line points the way a free north pole would be pushed if it were placed at that point.
4. Field strength is read from the spacing of the lines and nothing else. Closely packed lines mean a strong field, widely spaced lines mean a weak field.
5. Two field lines never cross. If they did, the field at the crossing point would have two directions at once, which is impossible.
6. The number of lines drawn is a choice made by whoever draws the diagram. Only the way the spacing changes from place to place carries any meaning.

The field around a bar magnet

1. Outside a bar magnet the field lines are curved and they always run from the north pole to the south pole.
2. The lines leave the magnet at the north pole, sweep round through the space beside the magnet, and return to it at the south pole, forming closed loops.
3. The pattern is symmetrical about the long axis of the magnet, so the loops on one side mirror the loops on the other.
4. The lines are packed most tightly at the two poles, which is why the field is strongest there and why a bar magnet picks up the most paper clips at its ends.
5. Moving away from the magnet the lines spread apart, so the field gets weaker with distance. At the midpoint of the long side the field is at its weakest.
6. When you are asked to draw the field around a bar magnet, four things earn the marks: curved lines, arrows pointing from north to south, lines closest together at the poles, and no lines crossing.
7. A horseshoe magnet is a bar magnet bent round so that its two poles face each other, which crowds the field lines into the narrow gap and gives a strong field in a small space.

Fields between two magnets

1. When two magnets are placed near each other, their fields combine into a single pattern, and the shape of that pattern depends on which poles face each other.
2. Unlike poles facing: the field lines run straight across the gap from the north pole of one magnet into the south pole of the other, and the two magnets are pulled together.
3. Like poles facing: the field lines curve sharply away from each other and bulge outwards, because the two fields push in opposite directions in the gap.
4. Directly between two equal like poles there is a neutral point. The two fields cancel exactly, so no field lines pass through it and a plotting compass placed there would not settle in any particular direction.
5. The narrow region between two unlike poles held a short distance apart is close to a uniform magnetic field, which is why this arrangement is used whenever a steady, even field is needed.
6. A uniform field is drawn with lines that are:
   1. straight rather than curved
   2. parallel to one another, because the direction is the same everywhere
   3. equally spaced, because the strength is the same everywhere
   4. all arrowed in the same direction, from the north pole towards the south pole
7. The field near the open ends of the gap bends outwards and stops being uniform, so the uniform region is only the central part between the pole faces.
8. Every field that is not uniform is called non-uniform. The field around a single bar magnet is non-uniform, because both the direction and the spacing change from point to point.

12.1.4 Plotting compasses and the Earth's magnetic field

Plotting compasses

1. A plotting compass is simply a very small bar magnet mounted on a low-friction pivot inside a clear case, so it is free to swing round in the horizontal plane.
2. Placed in a magnetic field, the needle turns until it lies along the field line passing through that point, and then it stops.
3. It turns because the field pulls the needle's north pole one way and its south pole the opposite way. These two forces act at opposite ends of the needle, so they twist it rather than dragging it sideways.
4. Once the needle is lined up with the field the two forces are directly opposed, the twist disappears, and the needle settles.
5. The north-seeking end of the needle, usually the coloured or arrow-shaped end, points the way the field is directed at that point.
6. Near a bar magnet the compass therefore points away from the magnet's north pole and towards its south pole, matching the arrows drawn on a field-line diagram.
7. A compass measures direction only. It gives no reading for how strong the field is, so the shape of a whole field has to be built up by moving the compass from point to point.
8. Ring several compasses round a magnet at once and each needle points a slightly different way, because each one is responding to the field at its own position.
9. This is the advantage the compass has over iron filings. The filings line up with the field but give no arrows, while the compass needle carries its own arrowhead.

The Earth's magnetic field

1. A magnetic compass carried outdoors, far from any magnet, still swings round and settles with its north-seeking end pointing roughly towards geographic north.
2. It does this anywhere on the planet, at any time of day, and it returns to the same direction every time it is disturbed.
3. The Earth's field has a shape much like the field of a bar magnet buried inside the planet and tilted slightly from the axis the Earth spins about.
4. That imagined magnet has to be the other way round from what people expect. A compass north-seeking pole is attracted towards the far north, so the magnetic south pole of the Earth's field lies near geographic north.
5. The reasoning that links this to the Earth's core runs as a chain, and each link is worth a mark:
   1. A compass needle is itself a small magnet that is free to rotate.
   2. A freely pivoted magnet only turns and settles like this when it is sitting in a magnetic field.
   3. The compass behaves this way with no other magnet anywhere near it, so the field must be coming from the Earth.
   4. The Earth's surface rocks are not magnetic enough to produce a field of this size and steadiness, so it must be generated deeper down.
   5. This is evidence that the core of the Earth must be magnetic.
6. This is indirect evidence. Nobody has ever reached the core, so a conclusion about it is drawn from an observation made at the surface.
7. The core is far too hot for a solid permanent magnet to survive, so the field is instead produced by the movement of molten iron and nickel in the outer core.
8. The Earth's field is weak compared with a laboratory bar magnet, which is why a plotting compass sitting close to a bar magnet ignores the Earth almost entirely and lines up with the magnet instead.
9. That weakness matters in the laboratory. Move the compass a long way from the bar magnet and the Earth's field starts to win, which is why traced field lines go astray at the edges of the paper.