OCR GCSE Physics A (J249) — Magnetism and magnetic fields
Have you ever wondered why a compass always points the same way, no matter where you are in the world? A magnetic compass contains a tiny bar magnet, known as the compass needle, which is mounted on a free-moving pivot. Because it can rotate freely, this needle will automatically interact with any magnetic forces nearby.
When there are no other magnets around, a compass needle always aligns itself with the magnetic field lines of the Earth. It points North/South wherever you are on the planet.
This happens because the Earth behaves as though it has a massive bar magnet buried at its centre, creating a global magnetic field. This is a region where magnetic forces act on other magnets and magnetic materials. The strength of this field is called the magnetic flux density () and is measured in Tesla (T).
It is crucial to understand that the Earth's geographic poles are opposite to its magnetic poles. The geographic North Pole (the Arctic) actually acts as a magnetic South Pole, while the geographic South Pole (Antarctica) acts as a magnetic North Pole. Since opposite poles attract, the north-seeking pole of a compass needle is naturally attracted towards the geographic North Pole.
The consistent behaviour of a compass provides direct causal evidence that the Earth generates its own internal magnetic field. The fact that the needle lines up with the magnetic field lines of the Earth globally proves the source of this magnetism must come from within the planet.
Scientists know this magnetic field originates in the Earth's core. Specifically, the outer core is filled with molten iron and nickel. As these liquid magnetic materials move through convection currents, they generate electrical currents that produce the global magnetic field.
Further evidence comes from the angle of dip (also called magnetic dip). If a compass is allowed to pivot vertically, the needle tilts downwards as you travel closer to the poles. At the magnetic poles, the angle is 90° and the needle points vertically downward, aiming directly at the magnetic core.
You can use a small plotting compass to reveal the invisible field lines around any magnet or to map the Earth's local field.
Students often incorrectly assume that the geographic North Pole is also the magnetic North Pole. In reality, the geographic North Pole is a magnetic South pole, which is exactly why the north-seeking pole of a compass is attracted to it!
When asked to explain how a compass provides evidence for the Earth's core, you must explicitly state that the needle "lines up with the magnetic field lines of the Earth" and "points North/South wherever you are".
If you are asked to draw magnetic field lines around the Earth, ensure your arrows point away from the geographic South Pole (which is the magnetic North) and towards the geographic North Pole (which is the magnetic South).
Always use the specific terms "aligns" or "interacts" when describing what a compass needle does in a magnetic field.
Magnetic compass
A device containing a small permanent bar magnet that is free to pivot and align with a magnetic field.
Bar magnet
A rectangular permanent magnet that has a north pole at one end and a south pole at the other.
Compass needle
The small bar magnet inside a compass that rotates to indicate the direction of magnetic fields.
Magnetic field
A region around a magnet where a force acts on another magnet or on a magnetic material such as iron or nickel.
Magnetic field lines
Invisible lines used to map out the direction and strength of a magnetic field.
Magnetic flux density
The measure of the strength of a magnetic field, measured in Tesla (T).
Geographic North Pole
The northernmost point on the Earth's axis of rotation, which magnetically acts as a South Pole.
Magnetic South Pole
The pole of a magnet where magnetic field lines point inwards; on Earth, this is located near the geographic North Pole.
Geographic South Pole
The southernmost point on the Earth's axis of rotation, which magnetically acts as a North Pole.
Magnetic North Pole
The pole of a magnet where magnetic field lines point outwards; on Earth, this is located near the geographic South Pole.
Earth's core
The innermost part of the Earth, consisting of a solid inner core and a liquid outer core made of iron and nickel, which generates the Earth's magnetic field.
Angle of dip
The angle between the Earth's magnetic field lines and the horizontal plane of the Earth's surface.
Plotting compass
A small compass used to systematically map the magnetic field pattern around a magnet.
Put your knowledge into practice — try past paper questions for Physics A
Magnetic compass
A device containing a small permanent bar magnet that is free to pivot and align with a magnetic field.
Bar magnet
A rectangular permanent magnet that has a north pole at one end and a south pole at the other.
Compass needle
The small bar magnet inside a compass that rotates to indicate the direction of magnetic fields.
Magnetic field
A region around a magnet where a force acts on another magnet or on a magnetic material such as iron or nickel.
Magnetic field lines
Invisible lines used to map out the direction and strength of a magnetic field.
Magnetic flux density
The measure of the strength of a magnetic field, measured in Tesla (T).
Geographic North Pole
The northernmost point on the Earth's axis of rotation, which magnetically acts as a South Pole.
Magnetic South Pole
The pole of a magnet where magnetic field lines point inwards; on Earth, this is located near the geographic North Pole.
Geographic South Pole
The southernmost point on the Earth's axis of rotation, which magnetically acts as a North Pole.
Magnetic North Pole
The pole of a magnet where magnetic field lines point outwards; on Earth, this is located near the geographic South Pole.
Earth's core
The innermost part of the Earth, consisting of a solid inner core and a liquid outer core made of iron and nickel, which generates the Earth's magnetic field.
Angle of dip
The angle between the Earth's magnetic field lines and the horizontal plane of the Earth's surface.
Plotting compass
A small compass used to systematically map the magnetic field pattern around a magnet.
