Magnets and the Earth’s magnetic field

The magnetic field created by a magnet

A bar magnet has a north pole (N) and a south pole (S). Outside the magnet, the field lines emerge from the north pole and converge at the south pole, forming closed curves (they close inside the magnet). Two poles of the same name repel each other; two poles of opposite names attract each other.

Characterising the magnetic field vector

At a given point, the magnetic field vector B is characterised by:

• a direction: that of the tangent to the field line at that point
• a sense: given by the arrows on the field line
• a magnitude: denoted by B, expressed in tesla (T)

The closer together the field lines are, the stronger the field is at that point.

The Earth’s magnetic field

The Earth behaves like a huge magnet: it produces a magnetic field that deflects part of the solar wind and enables compasses to find their bearings.

Quantity	Approximate value
Earth’s magnetic field (France)	approximately 4.7×10⁻⁵ T
Field near a common magnet	10⁻³ to 10⁻¹ T

Common misconception

Please note: the Earth’s geographic North Pole is close to the magnetic South Pole (and vice versa). This is why the north end of a magnetic needle, attracted by a south pole, points towards the geographic North Pole. Confusing the magnetic North Pole with the geographic North Pole is a very common mistake.