OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Explain how the behaviour of a magnetic (dipping) compass is related to…
Full specification wording
Magnets and magnetic fields: explain how the behaviour of a magnetic (dipping) compass is related to evidence that the core of the Earth must be magnetic
A magnetic compass needle is a small magnet that turns freely.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 4 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A magnetic compass needle is a small magnet that turns freely. A plotting compass held horizontally aligns with the local field, so its needle points roughly north–south, which shows the Earth behaves as though it has a magnetic field. A dipping compass is balanced so the needle can tilt in the vertical plane; away from the equator the needle dips downward, and the size of the dip changes with latitude. A freely suspended magnet therefore does not stay horizontal: it tilts because the Earth's field has a vertical component as well as a horizontal one. The consistent alignment and dip of compass needles at many places on the Earth's surface are evidence that the Earth has its own magnetic field, which is explained by a magnetic core. The field behaves as if produced by a large magnet within the Earth, with the magnetic poles near the geographic poles.
What earns the marks
- States that a compass needle is a small magnet that aligns with the local magnetic field.
- Explains that the needle dipping downward shows the Earth's field has a vertical component.
- Concludes that consistent alignment and dip of compass needles provide evidence that the Earth has a magnetic field and hence a magnetic core.
- Describes how the dip changes with position, for example being different at the equator and near the poles.
Common mistakes and how to put them right
Claims a compass needle points to the geographic North Pole exactly.
Put it right: State that it aligns with the Earth's magnetic field, which is close to but not identical with geographic north.
Assumes the needle always stays horizontal.
Put it right: Explain that a dipping compass tilts because of the vertical component of the Earth's field.
Treats the dip as random.
Put it right: Link the consistent dip pattern to evidence for a magnetic core.
Exam tips
- Link each observation of the compass (alignment, dip) to the conclusion about the Earth's field.
- Use the word 'evidence' explicitly when explaining why the core is inferred to be magnetic.
- Distinguish the horizontal alignment from the vertical dip when both are relevant.
Try the lesson
The start of the Magnets and magnetic fields: explain how the behaviour of a magnetic (dipping) compass is related to evidence that the core of the Earth must be magnetic lesson, made from checked teaching notes for this specification point.
Have a go first
A dipping compass is free to rotate in a vertical plane. What does it show about the Earth's magnetic field?
Listen
A magnetic compass needle is a small magnet that turns freely. A plotting compass held horizontally aligns with the local field, so its needle points roughly north–south, which shows the Earth behaves as though it has a magnetic field. A dipping compass is balanced so the needle can tilt in the vertical plane; away from the equator the needle dips downward, and the size of the dip changes with latitude. A freely suspended magnet therefore does not stay horizontal: it tilts because the Earth's field has a vertical component as well as a horizontal one. The consistent alignment and dip of compass needles at many places on the Earth's surface are evidence that the Earth has its own magnetic field, which is explained by a magnetic core. The field behaves as if produced by a large magnet within the Earth, with the magnetic poles near the geographic poles. A strong answer earns marks for these points. States that a compass needle is a small magnet that aligns with the local magnetic field. Explains that the needle dipping downward shows the Earth's field has a vertical component. Concludes that consistent alignment and dip of compass needles provide evidence that the Earth has a magnetic field and hence a magnetic core. Describes how the dip changes with position, for example being different at the equator and near the poles. Exam tip: Link each observation of the compass (alignment, dip) to the conclusion about the Earth's field.
Key words
Say what each word means before you turn the card.
Compass needle as a small magnet
Try answering aloud or in your head before turning the card.
Earth's magnetic field
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Dipping compass and vertical field component
Try answering aloud or in your head before turning the card.
Evidence for a magnetic core
Try answering aloud or in your head before turning the card.
See every definition
- Compass needle as a small magnet
- A needle that is a small magnet and turns freely, aligning with the local magnetic field.
- Earth's magnetic field
- The field the Earth behaves as though it has, which a plotting compass held horizontally aligns with, so its needle points roughly north–south.
- Dipping compass and vertical field component
- A compass balanced so the needle can tilt in the vertical plane; away from the equator the needle dips downward because the Earth's field has a vertical component as well as a horizontal one.
- Evidence for a magnetic core
- The consistent alignment and dip of compass needles at many places on the Earth's surface are evidence that the Earth has its own magnetic field, which is explained by a magnetic core.
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