Concept of Magnetism

Magnetism is the phenomenon by which certain materials attract or repel other materials. Every magnet has two poles — North (N) and South (S). Like poles repel and unlike poles attract. Magnetic poles always exist in pairs; a single magnetic monopole has never been observed.

A bar magnet is the simplest form of a magnet. It produces a magnetic field in the space around it, represented by magnetic field lines — closed loops that emerge from N, enter S externally, and travel S→N inside the magnet.

• Ancient Greeks discovered lodestones (magnetite, Fe₃O₄) in Magnesia — a town in ancient Greece — that naturally attracted iron.
• Chinese navigators used magnetite as a compass as early as the 1st century AD.
• In 1600 AD, William Gilbert published De Magnete and proposed that the Earth itself is a giant magnet, explaining why a compass needle points north.
• Hans Christian Ørsted (1820) established the link between electricity and magnetism.

A bar magnet creates a dipole magnetic field. The field is strongest near the poles (dense field lines) and weakest at the centre on the equatorial line.

Key points:

• Field lines emerge from N and enter S externally
• Inside the magnet: direction is S → N
• Lines form closed loops and never intersect
• B_axial = 2 × B_equatorial at the same distance

Earth behaves like a giant bar magnet with its magnetic south pole near the geographic north pole.

Three magnetic elements completely describe Earth's field at any point: