Detecting magnetic fields

Magnetic field lines can also 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

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

Drawing magnetic field diagrams

The diagram shows the magnetic field around a bar magnet.

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:

  • magnetic field lines never cross each other;
  • the closer the lines are together, the stronger the magnetic field – magnets are strongest at the poles;
  • the lines have arrowheads to show the direction of the magnetic field;
  • the direction of a magnetic field at a point is the direction of the force on a north pole at that point;
  • magnetic field lines point from the north pole of the magnet to its south pole.

Two bar magnets

The magnetic field pattern when two magnets are used is shown in this diagram.

A: Single  magnet, field lines curve, N to S. B: Unlike pole attraction - field lines N of magnet travel to S of second magnet. C: Like pole repulsion - N of two bar magnets, field lines forced apart.Magnetic field lines for fields involving two magnets

Uniform magnetic field

When magnetic field lines are parallel and the same distance apart from each other, we say that the magnetic field is uniform. This is shown in the diagram:

Two bar magnets lie side by side. One is labelled north and the other south. Magnetic field lines travel in straight lines from the north to the south magnet, at right angles to both magnets.Magnetic field lines in a uniform field