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.

12.2.1 Magnetic field around a current-carrying conductor

The magnetic effect of a current

1. A wire with no current in it has no magnetic field around it and has no effect at all on a nearby compass.
2. Switch the current on and a magnetic field appears in the space around the wire. Switch it off and the field vanishes at once.
3. The field is produced by the moving charges in the wire. Any moving charge is surrounded by a magnetic field, and a current is simply a great many charges moving together.
4. The conductor itself is not made of a magnetic material. Copper is not attracted by a magnet, yet a copper wire carrying a current still produces a field.
5. The current is not used up in making the field. The same current arrives back at the supply, and the field lasts exactly as long as the charge keeps flowing.
6. This single fact is the root of the whole of electromagnetism. Every electromagnet, relay, loudspeaker and electric motor works because a current makes a magnetic field.

Shape and direction of the field

1. The magnetic field around a long straight conductor is a set of concentric circles lying in planes at right angles to the wire, with the wire itself at the centre of every circle.
2. Concentric means the circles share the same centre. They do not overlap and they do not cross.
3. The field goes around the wire, not along it. The current runs along the wire while the field wraps round it, and the two directions are at right angles.
4. This field has no north pole and no south pole. The lines are closed loops that never begin or end anywhere, which is why a straight wire cannot be labelled N and S.
5. The way round the circles run is decided by the direction of the conventional current, and it is found with the right-hand grip rule.
6. To use the rule, grip the wire in your right hand as though holding a pen:
   1. point the thumb along the wire in the direction of the conventional current
   2. the curled fingers then sweep round the wire the same way as the magnetic field lines
7. The rule uses the right hand and it uses conventional current, which flows from the positive terminal of the supply round to the negative terminal. Electrons in the metal drift the opposite way.
8. Reversing the current reverses the field. Every circle keeps its shape and its size, but every arrow on it turns the other way round.
9. In a diagram of a vertical wire seen from above, a current flowing upwards out of the page gives anticlockwise circles, and a current flowing downwards into the page gives clockwise circles.

How strong the field is

1. Two things decide the strength of the field around a long straight conductor: the size of the current and the distance from the conductor.
2. A larger current gives a stronger field, because more charge is flowing past each point every second.
3. A greater distance from the wire gives a weaker field, so the field is strongest right at the surface of the conductor and fades away as you move outwards.
4. On a field-line diagram this is drawn by spacing the circles unevenly: they are packed tightly close to the wire and drawn further and further apart towards the outside.
5. There is no equation to learn or use for this field. What is assessed is the qualitative link: more current means a stronger field, more distance means a weaker one.
6. The field around a single straight wire is weak. A wire carrying a few amperes barely disturbs a compass held a hand's width away, which is why bigger effects need the current concentrated into a coil.

12.2.2 Solenoids and electromagnets

Solenoids and electromagnets

1. A solenoid is made by winding a single length of insulated wire into many closely packed loops, each loop being called a turn.
2. The wire is insulated so that the current is forced to travel all the way round every turn instead of taking a short cut where the turns touch.
3. The same current passes through every turn in the same direction, one after another, because the coil is a single unbroken wire in series with the supply.
4. Each turn on its own is a short piece of current-carrying conductor, and so each one produces its own circular magnetic field.
5. The field of the whole solenoid is the combination of all these separate fields, added together point by point through the space in and around the coil.
6. The result looks like the field of a bar magnet. The field lines emerge from one end of the solenoid, loop round the outside and return at the other end, so a solenoid has a north end and a south end.
7. The important difference from a bar magnet is that a solenoid can be switched off. Break the circuit and the field disappears instantly, which is what makes an electromagnet so useful.
8. Reversing the current swaps the two ends over, so the end that was a north end becomes a south end. A permanent magnet cannot be reversed this way.

The field inside the solenoid

1. Along the axis running through the middle of the coil, the field contributed by every single turn points the same way, straight down the centre of the solenoid.
2. Because the contributions all point the same way, they add together. Two hundred turns each adding their share build a field far stronger than any one turn could produce alone.
3. The field inside is also almost uniform, meaning it has nearly the same strength and the same direction everywhere in the central region of the coil.
4. It works out this way because every turn is identical and evenly spaced, so wherever you stand inside the coil the arrangement of turns around you looks much the same.
5. On a diagram the inside field is drawn as lines that are:
   1. straight and parallel to the axis of the coil
   2. evenly spaced across the width of the coil
   3. packed much more closely than the lines drawn outside, showing the much stronger field
6. The word almost matters. Right at the two open ends the lines begin to fan out, so the field is only truly even well inside a long coil.

The field outside the solenoid

1. Outside the coil the geometry is different. A point out there sits on the far side of some turns and near the near side of others, so the fields reaching it point in a range of different directions.
2. Because those directions oppose one another, the contributions partly cancel instead of adding, so what is left over is small.
3. The field outside is therefore much weaker than the field inside, and it is drawn with widely spaced lines that spread out around the coil.
4. Weaker does not mean zero. The outside field is real, it is the part that reaches out to attract a nail, and it is what a plotting compass held beside the coil responds to.
5. The same field lines that run down the inside of the coil come back round the outside, so the loops are continuous. Crowding them into the narrow inside and spreading them over the wide outside is exactly what makes one region strong and the other weak.

Making an electromagnet stronger

1. Increasing the current strengthens the field, because every turn then contributes a larger share.
2. Increasing the number of turns strengthens the field, because there are more contributions to add together in the same length of coil.
3. Adding an iron core down the middle strengthens the field greatly. The coil's field magnetises the iron, and the magnetised iron then produces a field of its own that adds to the coil's.
4. The core must be iron rather than steel, so that it loses its magnetism the moment the current is switched off and the electromagnet truly releases its load.
5. Winding the turns closer together also helps, because packing more turns into each centimetre of coil raises the field along the axis.
6. These design choices show up in everyday devices: a scrapyard crane magnet uses a very large current and a heavy iron core, while a doorbell uses a small current and many fine turns.