Magnets, magnetic fields and compasses

Understand the fundamental principle that unlike magnetic poles attract each other, while like poles repel.
Describe the diverse applications of permanent and temporary magnetic materials, specifically cobalt, steel, iron, and nickel.
Analyze the physical and operational differences between permanent magnets and induced magnets.
Map the shape and direction of magnetic fields surrounding bar magnets, and relate the local strength of the field to the density of the magnetic field lines.
Describe the practical use of plotting compasses to experimentally map magnetic fields, including the Earth’s own magnetic field.

Magnetic poles and magnetic materials

A bar magnet has two poles: north and south. Two magnets can exert forces on each other without touching. Unlike poles attract, so a north pole and a south pole pull towards each other. Like poles repel: two north poles or two south poles push apart.

Iron, cobalt and nickel are magnetic metals. Steel contains iron and is also a magnetic material. Being a magnetic material is not the same as already being a magnet: an unmagnetised piece of iron is attracted to either pole of a magnet, but does not initially have fixed north and south poles of its own.

Permanent magnets and induced magnets

A permanent magnet retains its magnetism and produces its own magnetic field. Steel can be used to make permanent magnets because it retains magnetism. Permanent magnets are useful in compasses, where a magnetic needle must remain magnetised, and in loudspeakers and electric generators, where a lasting magnetic field is needed.

Cobalt and nickel are also used as ingredients of permanent-magnet alloys. For example, Alnico contains aluminium, nickel, cobalt and iron. Alnico permanent magnets are used in electric guitar pickups and electric motors, providing a lasting magnetic field. These uses involve cobalt and nickel within an alloy, rather than necessarily as pure metals.

An induced magnet forms when a magnetic material is placed in another magnetic field. The material becomes magnetised, developing a north pole and a south pole. For example, bringing the north pole of a permanent magnet near an unmagnetised iron object induces a south pole at the object's nearer end and a north pole at its farther end. The nearer unlike poles attract, pulling the iron towards the magnet.

Temporary magnetic materials lose most or all of their induced magnetism when the external field is removed. Iron is useful as a temporary magnetic core in an electromagnet: it strengthens the magnetic effect while the electromagnet is operating, but does not need to remain strongly magnetised afterwards. This is the practical contrast with a permanent magnetic material such as steel: sometimes a lasting magnet is useful, and sometimes magnetism needs to be switched on and off.

Representing a magnetic field

A magnetic field is a region where a magnet or magnetic material experiences a magnetic force. Field lines are a drawing convention that represents this invisible field; they are not physical threads surrounding a magnet.

Around a bar magnet, the field lines curve from one pole to the other. Outside the magnet, arrows point from north to south. At any point, the arrow shows the direction in which the north pole of a small compass would point.

The concentration of lines represents field strength. Closely spaced lines show a stronger field; widely spaced lines show a weaker field. A bar magnet's field is strongest near its poles and becomes weaker farther away.

A uniform magnetic field has the same strength and direction throughout a region. It is represented by straight, parallel, equally spaced field lines. An approximately uniform field can be produced in the central part of a narrow gap between broad, facing north and south poles. Near the edges, the lines curve and the field is no longer uniform.

Three nested, non-crossing field lines arch above a bar magnet and three below it. Outside the magnet, arrows run from the north pole on the left to the south pole on the right. At the top of the outer arch, a compass’s north-seeking end points right, tangent to the field line. Lines are closer near the magnet and farther apart away from it. A second panel shows broad opposite poles with straight, parallel, equally spaced field lines across their central gap, representing an approximately uniform field; the lines curve near the edges. Schematic, not to scale. — Field-line arrows show direction; line spacing represents relative strength. The central gap between facing opposite poles can have an approximately uniform field.

Mapping a field with a plotting compass

A plotting compass contains a small magnetic needle that can turn freely. Its north-seeking end points along the local magnetic field. One compass reading therefore gives the field direction at one position; moving the compass lets you trace the field's shape.

To map a bar magnet's field, use a bar magnet, paper, a pencil and a plotting compass:

Place the magnet on the paper and draw around it. Label its north and south poles so its position is recorded.
Put the compass close to the north pole. Allow the needle to settle, then mark the positions of its two ends.
Move the compass so that its south-seeking end is at the previous mark made by the north-seeking end. Mark the new position of the north-seeking end.
Repeat to make a chain of points leading towards the south pole.
Join the points with a smooth curve and add an arrow in the direction the north-seeking end pointed.
Start at several different positions to trace other field lines.

Move the compass along its previous north-end mark, record the next direction, and join the points to trace a field line.

The needle lies along the field line at each position. Near a pole, different traced lines crowd together; farther away, they spread apart. This connects the measured directions to the field pattern and its relative strength.

A compass and the Earth's magnetic field

Away from nearby magnets and magnetic objects, a freely turning compass needle still settles in a particular direction. Its north-seeking end points roughly towards geographic north. The needle is responding to the Earth's magnetic field.

You can demonstrate this with plotting compasses on paper away from magnets, iron objects and electrical equipment. Mark the direction of the north-seeking end at several positions. Across a small area, the needles point in approximately the same direction because the Earth's field is approximately uniform on that scale. Turn a compass away from that direction and release it: the needle returns to alignment with the field.

This behaviour provides evidence that the Earth has a magnetic field associated with a magnetic core. A compass is itself a small magnet, so its repeated alignment shows that a magnetic influence exists even when no nearby bar magnet is present. It does not mean that the core is a solid permanent bar magnet.

The north-seeking pole of a compass is attracted towards the Arctic region. Since unlike poles attract, that region behaves as a magnetic south pole, despite being near geographic north. Geographic directions and magnetic pole types must therefore be distinguished.

Unlike poles attract; like poles repel.
Magnetic materials: iron, steel, cobalt and nickel.
Permanent magnets retain magnetism and produce their own field. Uses include compasses, loudspeakers and generators; steel is a permanent magnetic material.
Cobalt and nickel are ingredients of Alnico permanent magnets, used in guitar pickups and electric motors.
Induced magnets become magnetised in an external field. Temporary materials, such as iron, lose most or all magnetism when it is removed; iron is useful in electromagnet cores.
The nearer end of an induced magnet becomes the opposite pole to the inducing pole, causing attraction.

Field patterns

Bar magnet: curved lines, pointing north → south outside the magnet.
Closer lines mean a stronger field; strength is greatest near the poles and decreases with distance.
Uniform field: same strength and direction; straight, parallel, equally spaced lines.

Compasses

A plotting compass's north-seeking end shows the local field direction.
Trace a field by marking needle positions, moving the compass along those marks, joining the points and adding direction arrows. Repeat from different starting points.
Away from nearby magnetic influences, compasses align with the Earth's field and point roughly north: evidence for the Earth's magnetic core.
Near geographic north, the Earth behaves as a magnetic south pole.

Attraction alone does not prove that an object is a magnet: an unmagnetised magnetic material is also attracted. Repulsion by a known magnet is evidence that the object is itself a magnet.

When drawing magnetic fields, include arrows from north to south outside the magnet. Field lines must not cross.

Show a uniform field using straight, parallel, equally spaced lines with arrows pointing in the same direction.

In a compass investigation, use the north-seeking end of the needle to mark the field direction.

A region where a magnet or magnetic material experiences a magnetic force.

One of the two regions of a magnet, called north and south, where its magnetic effect is strongest.

A material that is attracted to a magnet and can become magnetised, such as iron, steel, cobalt or nickel.

A magnet that retains its magnetism and produces its own magnetic field without needing an external magnetic field.

A magnetic material that becomes magnetised when placed in another magnetic field. A temporary induced magnet loses most or all of its magnetism when that field is removed.

A magnetic field with the same strength and direction at every point.

A small compass whose freely turning magnetic needle shows the direction of the magnetic field at its position.