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.