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School Physics: Electricity-magnetism 9.5 Earth's magnetic field and a compass

GCSE level physics notes: Magnetism: 9.5 Origin of Earth's magnetic field & how a compass works

[Author © Dr WP Brown PhD: Doc Brown's physics exam revision notes suitable for students of UK IGCSE & GCSE level physics courses, ~ US grades 9-10 physics, updated Feb 20th 2026 *]

[KEY POINTS and learning objectives for this page, after initial notes]

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INDEX for physics notes on magnetism

9.5 Earth's magnetic field and compass

Swirling currents of heated and cooling molten (mainly) iron metal in the Earth's outer core create an electric current, which, due to the rotation of the Earth, create a magnetic field.

diagram of the Earth's magnetic field and the Earth's north and south poles The Earth's magnetic field

A compass works by the alignment of its needle with the Earth's magnetic field.

The geographic north pole is actually the south pole of the Earth's magnetic field - this is due to the convention established from electromagnetic science - it was an arbitrary choice, rather like the convention for the direction of flow of electricity - which is opposite to the flow of electrons!

Therefore the Earth's magnetic north pole is at the geographical south pole.

Therefore the magnetic south pole of the compass needle is attracted and points to the geographic north pole of the Earth.

The magnetic north is a few degrees away from the true geographic north pole based on the Earth's spin axis.

The fact that a magnetised-magnetic compass needle always aligns itself in a particular direction on the Earth's surface, is proof that the Earth has a permanent magnetic field.

However, it has been deduced from geological studies of ancient rocks that the Earth's magnetic poles flip every so often, perhaps as often as every 200 000 to 300 000 years now.

This can be detected in the geological make-up of rocks and provides evidence for plate tectonic movements.

Tectonic Plates - Wegener's theory, evidence for plate movement

 

a compass for navigating using an OS map

The trusty compass to help you navigate the countryside with your OS map.

However, these days, many walkers are using GPS systems for navigation, but I still like a 'real map' and a 'real compass'!

 

INDEX for physics notes on magnetism

Key points about magnetism - The Earth's magnetic field and how a compass works

Information sources for Doc Brown's key points: IGCSE-GCSE physics are based on textbooks & syllabus-specifications for students taking the UK AQA, Edexcel, OCR 21st Century Science, OCR Gateway science suite, WJEC, CCEA and CIE GCSE physics 9-1 level science examinations

Summary of the Earth's magnetic field and how a compass works, tailored for GCSE/IGCSE Physics students across WJEC, CCEA, CIE, AQA, Edexcel, and OCR.

The Earth's Magnetic Field: Origin and Nature

Origin of the Earth's Magnetic Field

  • The Earth behaves like a giant bar magnet with a magnetic field surrounding it.
  • This field is generated by the movement of molten iron and nickel in the Earth’s outer core.
  • The process is called the geodynamo effect:
    • Convection currents in the liquid outer core
    • Combined with Earth’s rotation
    • Create circulating electric currents → generate a magnetic field

Nature of the Magnetic Field

  • The field lines:
    • Emerge from the magnetic south pole (near the geographic North Pole)
    • Enter at the magnetic north pole (near the geographic South Pole)
  • The field is strongest at the poles and weakest at the equator.
  • The magnetic poles drift over time due to changes in the core.

How Does a Compass Work?

Feature Explanation
Compass needle A small permanent magnet that is free to rotate.
Alignment The needle aligns with the Earth’s magnetic field lines.
North-seeking pole Points towards the Earth’s magnetic south pole, which is near the geographic North Pole.
Why it works Because opposite poles attract, the north pole of the compass needle is attracted to the Earth's magnetic south pole.

Remember: The magnetic north pole of the Earth is actually a magnetic south pole!

Typical Exam Board Requirements

Key Focus Areas
Earth's magnetic field, compass use, field line direction, bar magnet comparison.
Magnetic field of Earth, compass alignment, magnetic versus geographic poles.
Compass as a magnetic field detector, Earth's field as evidence of a magnetic core.
Earth's magnetic field origin, compass use, magnetic pole reversal.
Compass needle alignment, magnetic field generation, field line diagrams.
Earth's field as a bar magnet, compass use, field line direction, pole reversal.

Student Tips for Exams

  • Use correct terminology: magnetic field, geodynamo, convection currents, magnetic poles.
  • Draw field lines: Show direction from magnetic north to south (outside the Earth).
  • Explain compass behaviour: Always link to attraction between opposite poles.
  • Compare to bar magnets: Earth’s field is similar in shape to that of a bar magnet.
  • Know the difference: Between magnetic and geographic poles.

Keywords, phrases and learning objectives on magnetism

Be able to describe the origin of Earth's magnetic field and explain how a compass works.

Know the Earth's iron core creates a permanent magnetic field.

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Based on the syllabus-specifications for students taking the IGCSE/GCSE level physics examinations summary revision notes and key points on explaining the origin of the Earth's magnetic field for students taking the AQA igcse/gcse physics notes on explaining the origin of the Earth's magnetic field, Edexcel gcse physics notes on explaining the origin of the Earth's magnetic field,  OCR 21st century GCSE physics notes on explaining the origin of the Earth's magnetic field, OCR gateway GCSE physics notes on explaining the origin of the Earth's magnetic field, WJEC gcse physics notes on explaining the origin of the Earth's magnetic field, CCEA gcse physics notes on explaining the origin of the Earth's magnetic field for students taking CIE Cambridge igcse physics, or any other GCSE or IGCSE level physics exams notes on explaining the origin of the Earth's magnetic field, useful for US grade 9-10 physics courses, Explaining the importance of explaining how a magnetic compass works & usefulness in GCSE level physics, What you need to know about explaining how a magnetic compass works & usefulness for GCSE level physics, Explaining what is the use of explaining how a magnetic compass works & usefulness knowledge in GCSE level physics, Examples of explaining how a magnetic compass works & usefulness explained when studying GCSE level physics, What is the significance of explaining how a magnetic compass works & usefulness in GCSE level physics, Describing and explaining the theory of explaining how a magnetic compass works & usefulness when studying GCSE level physics, revision notes for explaining how a magnetic compass works & usefulness in exams, online exam help for explaining how a magnetic compass works & usefulness, revision notes for explaining how a magnetic compass works & usefulness, what do I need to learn about explaining how a magnetic compass works & usefulness for by GCSE physics exam? revision summary for explaining how a magnetic compass works & usefulness, help in teaching explaining how a magnetic compass works & usefulness, learning notes for explaining how a magnetic compass works & usefulness, help to understand the explaining how a magnetic compass works & usefulness topic in preparation GCSE physics exam question, how to prepare for questions involving explaining how a magnetic compass works & usefulness in a GCSE physics examination?

INDEX for physics notes on magnetism

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