What is a magnetic field?

A vector quantity

A magnetic field is a physical quantity that describes the magnetic influence present at a point in space. It is denoted by B and is expressed in tesla (T). Unlike a simple number, the magnetic field is a vector: it has a direction, a sense and a magnitude (norm).

Detecting a magnetic field

A magnetic field can be detected using a compass: the magnetised needle aligns itself with the direction of the local field. Near a bar magnet, iron filings align themselves along curves called field lines, which emerge from the north pole and converge at the south pole on the outside of the magnet.

Sources of magnetic fields

Source	Example
Permanent magnets	bar magnet, U-shaped magnet
Electric currents	wire carrying a current, coil
The Earth	Earth’s magnetic field

Common pitfall

Do not confuse an electric field with a magnetic field: the electric field acts on any charge, even a stationary one, whereas the magnetic field only exerts a force on a moving charge (and therefore on an electric current). Another common mistake is to give only the value of B (for example, B = 5 × 10⁻³ T) without specifying its direction or sense; this is an incomplete answer, as B is a vector.