Magnetic fields

A bar magnet surrounded by nickel grains that have formed around the magnet in the shape of its magnetic field.
Iron filings are attracted to a bar magnet

A magnetic field is the region around a magnet where a force acts on another magnet or on a magnetic material. Iron, steel, cobalt and nickel are all magnetic materials and would feel force from a nearby magnet.

Detecting magnetic fields

A magnetic field is invisible, but it can be detected using a magnetic compass. A compass contains a small bar magnet on a pivot so that it can rotate. The compass needle points in the direction of the Earth's magnetic field, or the magnetic field of a magnet.

Magnetic fields can be mapped out using small plotting compasses:

  1. place the plotting compass near the magnet on a piece of paper
  2. mark the direction the compass needle points
  3. move the plotting compass to many different positions in the magnetic field, marking the needle direction each time
  4. join the points to show the field lines

The needle of a plotting compass points to the south pole of the magnet.

A bar magnet with magnetic field lines curving round from the north to south pole. Five small plotting compasses sit on the top line on either side.

The behaviour of a compass shows that the Earth has a magnetic field. The Earth's core, which is made from iron and nickel, produces this magnetic field.

Drawing a magnetic field

A bar magnet, with several curved lines pointing from the north to south poleMagnetic field lines around a bar magnet

The diagram shows these key features:

  • the magnetic field lines never cross each other
  • the closer the lines, the stronger the magnetic field (so the magnetic field from a bar magnet is strongest closest to the poles)
  • the lines have arrowheads to show the direction of the force exerted by a magnetic north pole
  • the arrowheads point from the north pole of the magnet to its south pole