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OCR GCSE Physics A · J249
OCR J249 · P4.1d Check the specification (PDF) (opens in a new tab)
A compass needle is a small magnet. When it is free to turn, it settles along the magnetic field at its position. An ordinary compass turns mainly in a horizontal plane, so it shows the horizontal direction of Earth's field. This is why it is useful for navigation, although magnetic north and the geographic North Pole are not in exactly the same place.
A dipping compass allows its needle to turn vertically too. It reveals something an ordinary compass hides: Earth's magnetic field is not horizontal everywhere. Around the magnetic equator the needle lies approximately level. Farther north, its north-pointing end tilts downwards; farther south, its south-pointing end tilts downwards. At a magnetic pole, the needle is approximately vertical. The angle of this tilt from the horizontal is called magnetic dip.
A dipping compass follows the local magnetic field. Its changing tilt shows that Earth's field is not horizontal everywhere.
The needle follows the local field direction, so its downward tilt is evidence that the magnetic field extends into Earth, rather than simply running across its surface. Observations at different places reveal a large-scale field around the whole planet, broadly like the field around a bar magnet. This is evidence that the main source of Earth's magnetism lies within Earth, in its core.
The comparison with a bar magnet describes the shape of the field; it does not mean that there is a solid bar magnet inside the planet. Most of the field measured at Earth's surface is generated in the liquid outer core. A dipping compass gives visible evidence of the field's direction, while wider measurements establish where its main source lies.
A freely turning magnetic needle aligns with the local magnetic field. A dipping compass can turn vertically: it is roughly horizontal at the magnetic equator, tilts down towards a magnetic pole and is approximately vertical there. This magnetic dip shows that Earth's field has a vertical component and extends into Earth.
The worldwide field pattern provides evidence that Earth's main magnetic field comes from within the planet, principally its outer core. Earth behaves approximately like a bar magnet, but contains no giant permanent bar magnet. Magnetic and geographic poles do not coincide.
Explain what the dipping compass does before drawing the conclusion: its tilt shows that Earth's magnetic field has a vertical as well as a horizontal direction.
Do not say that a compass points exactly towards the geographic North Pole, or that a permanent bar magnet sits inside Earth.
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Earth's magnetic field
The region around Earth where a magnetic object experiences a magnetic force. Its direction and strength vary with location.
Dipping compass
A magnetic needle free to turn vertically as well as horizontally, so it can follow the local direction of Earth's magnetic field.
Magnetic dip
The angle that Earth's magnetic field makes with the horizontal at a particular place.
Put your knowledge into practice — try past paper questions for Physics A
Earth's magnetic field
The region around Earth where a magnetic object experiences a magnetic force. Its direction and strength vary with location.
Dipping compass
A magnetic needle free to turn vertically as well as horizontally, so it can follow the local direction of Earth's magnetic field.
Magnetic dip
The angle that Earth's magnetic field makes with the horizontal at a particular place.