Magnetism and electromagnetism lesson
Earth's Magnetic Field and Compasses: Explore the relationship between a magnetic compass, Earth's magnetic field, and evidence of a magnetic core.
Earth's Magnetic Field and Compasses is part of Permanent and induced magnetism, magnetic forces and fields in GCSE AQA Higher Physics.
5 min
Reading time
19
Lessons in this topic
GCSE AQA Higher
Pathway
Expert reviewed
Content review
Earth's Magnetic Field and Compasses: Explore the relationship between a magnetic compass, Earth's magnetic field, and evidence of a magnetic core.
Use this lesson to stay inside Permanent and induced magnetism, magnetic forces and fields while connecting the detail back to the wider Magnetism and electromagnetism topic.
Read the core explanations from Earth's Magnetic Field and Compasses before testing yourself from memory.
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.
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.
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.
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.
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.
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.
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-20.
Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Explore the relationship between a magnetic compass, Earth's magnetic field, and evidence of a magnetic core.
By the end of this lesson, you should be able to Explain how the behaviour of a magnetic compass provides evidence that Earth's core is magnetic; Describe the structure and function of a magnetic compass; Apply knowledge of magnetic fields to explain compass needle alignment; Evaluate the relationship between Earth's magnetic field and compass navigation.
Read each section, then close the notes and explain the idea from memory. Check your explanation against the page and finish with an exam question from Magnetism and electromagnetism.