5.2.4Electricity and Magnetism
The Earth’s Magnetic Field — Oxford AQA International GCSE Combined Science - Physics Revision Notes & Topical Questions
5 min read 6 parts Lesson 15 of 18
Part 1 of 4The Earth as a magnet
Shape of the field
- Shape of the field The Earth has a magnetic field like that of a giant bar magnet, with a field that extends far out into space.
- Outside the planet the field lines come out near the geographic south pole and go back in near the geographic north pole.
Origin
- Origin The field is produced by the movement of molten iron and nickel in the liquid outer core, not by a permanent magnet in the Earth.
Magnetic poles
- Magnetic poles The magnetic poles are near, but not exactly at, the geographic poles.
- They also move slowly over time.
Part 2 of 4How a compass works
1A small magnet
- A small magnet A compass contains a small bar magnet, the needle, which is free to turn.
2Field acts on it
- Field acts on it The Earth's magnetic field exerts forces on the poles of the needle.
3Aligns
- Aligns The needle turns until it lines up with the field.
4Points north
- Points north The end that points to geographic north is the north-seeking pole, attracted to a magnetic south pole in the northern hemisphere.
Part 3 of 4Magnetic poles and fields
| Rule | Detail |
|---|---|
| Like poles | Repel each other |
| Unlike poles | Attract each other |
| Field line direction | From the north pole to the south pole of a magnet |
| Field strength | Strongest where the field lines are closest together |
| Compass near a magnet | Points along the field line, away from a north pole and towards a south pole |
Part 4 of 4Investigating a field
1
Place a bar magnet in the middle of a sheet of paper.
2
Put a plotting compass near one pole and mark the positions of both ends of the needle with dots.
3
Move the compass so that its first end sits at the previous second dot, and mark again.
4
Keep going until you reach the other pole, then join the dots to form a smooth field line.
5
Repeat from other starting points to see the whole shape. Far from the magnet, the compass swings to point north because of the Earth's field.
Key terms
| Term | Meaning |
|---|---|
| Magnetic field | The region around a magnet where another magnet or magnetic material feels a force. |
| Compass | A small magnet on a pivot that shows the direction of a magnetic field. |
| Field line | A line showing the direction of the force on a north pole placed in the field. |
| Outer core | The liquid layer of molten iron and nickel in which the Earth's field is generated. |
| Magnetic pole | A place on a magnet where the magnetic force is strongest. |
Avoid these mistakes
1
Saying the Earth's north pole is a magnetic north pole. The geographic north pole is near a magnetic south pole of the Earth's field.
2
Thinking that the field comes from a solid magnet at the centre. It comes from moving liquid metal.
3
Drawing field lines which cross each other or have no arrows.
4
Saying that a compass points to a place, rather than lining up with the field.
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