Magnetism and magnetic fields Revision Notes

Understanding the Earth's Magnetism

The Earth itself acts as a massive magnet. This magnetism isn't due to a permanent magnet inside, but rather generated by the movement of molten iron and nickel within the Earth's outer core. These movements create electric currents, which in turn produce a magnetic field that extends far into space. This phenomenon is known as the geodynamo effect. The Earth's magnetic field is essential for life, deflecting harmful solar radiation.

Behaviour of a Magnetic Dipping Compass

A standard compass aligns with the horizontal component of the Earth's magnetic field, indicating direction. A dipping compass, however, is designed to show both horizontal direction and the angle (or 'dip') at which the Earth's magnetic field lines penetrate or exit the Earth's surface. The needle of a dipping compass is free to move in a vertical plane as well as a horizontal one. This allows it to align precisely with the local magnetic field lines. The dip angle varies with geographical location.

Dipping Compass as Evidence for Earth's Core Magnetism

The predictable and systematic 'dipping' of a compass needle across the globe provides crucial evidence that the Earth's magnetism originates from its core. Near the magnetic equator, the compass needle is largely horizontal as the magnetic field lines are parallel to the surface. As one moves towards the magnetic poles, the field lines become increasingly vertical. This causes the dipping compass needle to tilt downwards more significantly, pointing into the Earth at the magnetic north pole and out of the Earth at the magnetic south pole. This global pattern of dip angles cannot be explained by localised magnetism within rocks near the surface; it requires a large-scale, deep-seated source like the Earth's core.

What The Diagram Shows

Gas Pressure on Wall is a physics diagram for Magnetism and magnetic fields. Use it beside the written explanation so the visual supports the exact idea being revised.

What To Explain

The answer should use the diagram to identify the physical relationship, then explain it using the relevant quantity, force, field, wave or energy idea.

Syllabus Link

Private mapping: OCR GCSE physics-a-gateway J249, Magnetism and magnetic fields.

Common Trap

Do not describe the diagram only as a picture. State the concept, then explain what the pattern, relationship, structure, or graph means.

What The Diagram Shows

Light is a physics diagram for Magnetism and magnetic fields. Use it beside the written explanation so the visual supports the exact idea being revised.

What To Explain

The answer should use the diagram to identify the physical relationship, then explain it using the relevant quantity, force, field, wave or energy idea.

Syllabus Link

Private mapping: OCR GCSE physics-a-gateway J249, Magnetism and magnetic fields.

Common Trap

Do not describe the diagram only as a picture. State the concept, then explain what the pattern, relationship, structure, or graph means.