P7.2 Magnetic compasses and the Earth's field

The Earth as a magnet

The Earth behaves like a giant magnet with a magnetic south pole near the geographic North Pole and a magnetic north pole near the geographic South Pole. This sounds backwards — but consider: the north end of a compass needle points towards geographic north because it is attracted to the magnetic south pole located there.

The Earth's field is thought to be caused by the movement of molten iron in the outer core, acting as a natural dynamo. This mechanism is still an area of active geophysical research.

The magnetic compass

A compass contains a small permanent magnet — the needle — that is free to rotate. The N-pole of the needle aligns with the local magnetic field direction, pointing approximately towards geographic north. This makes it useful for navigation.

Field mapping with a compass: Place a bar magnet on paper; use the plotting compass to map field lines (the N-end of the compass always shows you the direction of the local field). The compass needle aligns along the resultant of the Earth's field and the bar magnet's field.

Evidence that the Earth's core is magnetic

• Compass needles align consistently around the globe, pointing towards geographic north.
• The pattern of field lines around the Earth matches that of a dipole magnet.
• Paleomagnetism: the direction of fossil magnetic minerals in ancient rocks records past reversals of the Earth's field — evidence for a dynamic, changing core field.

Exam technique

• If asked "why does a compass point north?" — say: the compass N-pole is attracted to the magnetic south pole near geographic north.
• Distinguish between geographic north (rotation axis) and magnetic north (where compass points).