Earth's Magnetic Field and Compasses Revision Notes | GCSE AQA Higher Physics

1. What Makes a Compass Work?

Hold a compass in your hand and watch the needle swing. It always points north, no matter where you are on Earth. But why does this happen?

The answer lies in magnetism. A compass contains a small bar magnet that can rotate freely. This tiny magnet responds to the largest magnetic field around it — Earth's own magnetic field.

Think about what this tells us. If a compass works everywhere on Earth, from London to Sydney to the North Pole, then Earth itself must be acting like a giant magnet. The compass needle is following the invisible magnetic field lines that surround our planet.

2. Inside a Magnetic Compass

A magnetic compass is surprisingly simple. The key component is a magnetised steel needle that can rotate freely on a pivot. One end of this needle is the north pole, the other is the south pole.

The needle sits inside a circular housing marked with directions — N, S, E, W and the degrees between them. Some compasses also contain a liquid to dampen the needle's movement, preventing it from swinging wildly.

The crucial point is that the needle can move freely. If you tried to force it to point east, it would swing back to north as soon as you let go. This tells us there's a constant force acting on it — the magnetic force from Earth's field.

**Trap**: Many students think the red end of a compass needle is attracted to the North Pole. Actually, the red end IS a north pole, and opposite poles attract. So the red north pole of the needle is attracted to Earth's magnetic south pole, which happens to be near the geographic North Pole.

3. Earth's Magnetic Field Revealed

The fact that compasses work everywhere on Earth reveals something remarkable about our planet. Earth must have its own magnetic field — and this field must be strong enough to affect compass needles thousands of kilometres away from its source.

Scientists have mapped Earth's magnetic field by studying compass readings from around the globe. The pattern that emerges looks just like the field around a giant bar magnet. Field lines emerge from near the South Pole, curve around the planet, and enter near the North Pole.

But here's the key insight: magnetic fields don't just appear from nowhere. They're created by moving electric charges — usually in the form of electric currents. For Earth to have such a strong, planet-wide magnetic field, there must be massive electric currents flowing inside our planet.

4. Evidence from Compass Behaviour

Compass behaviour provides several pieces of evidence that Earth's core must be magnetic. First, compasses work consistently across the entire planet. From the Arctic to Antarctica, from sea level to mountain tops, the needle always aligns in the same direction relative to Earth's magnetic field.

Second, the strength of the magnetic field varies predictably with location. Compasses respond more strongly near the magnetic poles and more weakly near the magnetic equator. This matches exactly what we'd expect from a magnetic field generated deep inside Earth.

Third, Earth's magnetic field isn't perfectly aligned with the geographic poles. The magnetic north pole is actually about 11 degrees away from the geographic North Pole. This offset tells us the magnetic field comes from processes in Earth's core, not from Earth's rotation.

Finally, Earth's magnetic field changes over time. The magnetic poles wander slowly, and the field strength varies. These changes can only be explained by dynamic processes in Earth's molten core — flowing liquid iron that creates and maintains the magnetic field.

5. The Source: Earth's Molten Core

The evidence from compasses points to one conclusion: Earth's core must contain moving electric charges that generate the magnetic field. But what could create such massive currents inside our planet?

The answer lies in Earth's structure. Deep beneath the solid crust and mantle lies the core — a sphere of molten iron and nickel at temperatures exceeding 5000°C. This liquid metal is constantly moving due to heat from radioactive decay and the cooling of the planet.

When liquid iron moves, it carries electric charge with it. These moving charges create electric currents, and electric currents always generate magnetic fields. The result is a self-sustaining system called the geodynamo — Earth's molten core acts like a giant electromagnet.

This explains why compass needles respond to Earth's field. The small bar magnet in the compass aligns with the much larger magnetic field generated by the churning liquid iron thousands of kilometres below our feet.

6. Exam Technique and Common Mistakes

Exam questions about Earth's magnetism and compasses often test your understanding of the connection between compass behaviour and Earth's internal structure. The key is to link observations about compasses to conclusions about what must be happening inside Earth.

When explaining how compass behaviour provides evidence for Earth's magnetic core, structure your answer in three parts: describe what compasses do, explain what this tells us about Earth's magnetic field, and conclude what this reveals about Earth's core.

Common mistakes include confusing magnetic north with geographic north, thinking compass needles are attracted to the North Star, or forgetting that magnetic fields require moving electric charges. Remember: no moving charges means no magnetic field.

Worked example: Worked Method: Earth's Magnetic Field and Compasses

Use the relevant formula, substitute values with units, calculate, then state the final answer clearly.

1. Write down the correct physics formula. — Write down the correct physics formula.
2. Substitute known values with units. — Substitute known values with units.
3. Rearrange if needed and calculate carefully. — Rearrange if needed and calculate carefully.
4. Give the final answer with correct unit and sensible precision. — Give the final answer with correct unit and sensible precision.

Answer: Final answer should include unit and appropriate significant figures.