Introduction to magnetism

A permanent magnet has a magnetic field surrounding it. A magnetic field is envisioned to consist of flux lines that radiate from the North Pole to the South Pole and back to the North Pole through the magnetic material.

Unlike magnetic poles have an attractive force between them.

Two like poles repel each other.

When nonmagnetic materials such as paper, glass, wood or plastic are placed in a magnetic field, the lines of force are unaltered. 

When a magnetic material such as iron is placed in a magnetic field, the flux lines tend to be altered to pass through the magnetic material.

The force lines going from the north pole to the south pole of a magnet are called magnetic flux (φ); units: weber (Wb) The magnetic flux density (B) is the amount of flux per unit area perpendicular to the magnetic field; units: tesla (T)

Ferromagnetic materials such as iron, nickel and cobalt have randomly oriented magnetic domains, which become aligned when placed in a magnetic field, thus they effectively become magnets.