7.1.2a Magnetic fields and field lines

Magnetic fields

1. A magnetic field cannot be seen, but its effects can, for example a magnet attracting a piece of iron without touching it because the iron is inside the field.
2. The magnetic materials you need to know are iron, steel, cobalt and nickel.
3. A magnetic field can exert a force on another magnet or on any of these magnetic materials.

Forces on magnetic materials

1. The force between a magnet and a magnetic material is always attraction, so both the north pole and the south pole attract an unmagnetised iron nail.
2. This is different from two magnets, which can attract or repel depending on the poles, while a magnet and a magnetic material always attract.

Magnetic field strength

1. The strength of a magnetic field depends on the distance from the magnet.
2. The field is stronger closer to the magnet and weaker further away.
3. The field is strongest at the poles, which are the two ends of the magnet.
4. The force on another magnet or magnetic material is therefore greatest near the poles and weaker further away.
5. On a field-line diagram, the lines are closest together where the field is strongest, usually near the poles.

Magnetic field direction and field lines

1. Magnetic field lines show the shape and direction of a magnetic field, with arrows to show the direction.
2. Outside a magnet, field lines point away from the north pole and towards the south pole.
3. The field direction is therefore from the north pole to the south pole.
4. The direction is always defined using a north pole, because a south pole at the same point would feel a force in the opposite direction.

7.1.2b Compasses and the Earth's magnetic field

A compass contains a small bar magnet

1. A magnetic compass contains a small bar magnet, called the compass needle, that is free to turn.
2. Like any bar magnet, the needle has a north-seeking pole and a south-seeking pole.
3. The needle turns because it feels a force from a magnetic field, and its north-seeking end points in the direction of the field at that point.

The Earth has a magnetic field

1. The Earth has a magnetic field around it, so a compass needle lines up with it even when there are no magnets nearby.
2. A compass needle usually points approximately north-south, with its north-seeking end pointing towards the Earth's magnetic north.
3. This is evidence that the Earth produces a magnetic field, and because the field comes from inside the Earth, it is evidence that the core must be magnetic.

Plotting a magnetic field pattern using a compass

1. Place the bar magnet on a sheet of paper and draw around it to mark its position.
2. Place a plotting compass near one pole of the magnet.
3. Mark the direction shown by the needle with a small dot or arrow.
4. Move the compass so the tail of the needle starts at the point just marked, then mark the new direction.
5. Repeat several times to trace out one field line.
6. Start at different points around the magnet and repeat to build up the full field pattern.
7. Add arrows to the field lines to show the direction of the field.
8. The needle changes direction from place to place because the direction of the field changes around the magnet.

Drawing the field pattern of a bar magnet

1. Outside the magnet, the field lines go from the north pole to the south pole, with arrows showing the direction.
2. The field is strongest near the poles, shown by field lines being closer together there.
3. The field is weaker further away, where the field lines are more spread out.
4. Field lines never cross, because the field can only have one direction at each point.

Compasses as evidence for the Earth's magnetic core

1. A compass needle is a small magnet, so it only settles in a fixed direction if a magnetic field acts on it.
2. It lines up in a consistent north-south direction almost anywhere on Earth, even when no magnet is nearby.
3. This shows the Earth produces a magnetic field, which is evidence that the Earth's core must be magnetic.