6.3 Magnetic Fields
Magnetic fields have effects not only on charged particles, but on many other things. Before we investigate these, it is first necessary to find out a little bit about what a magnetic field is. A magnetic field is simply a region around a magnetic body or current-carrying conductor, in which a force exists.
The direction of a magnetic field determines how different objects will act in the field. It is indicated by imaginary lines called flux lines. Flux lines, for the sake of convention, are said to always flow from the north pole (i.e. of a magnet) to the south pole. The number of flux lines a field contains indicates the strength of the field (the more lines, the stronger the field).
Here is a diagram of the flux lines around and through a magnet, indicating the direction of the magnetic field. Since these lines are imaginary, an infinite number of them exist, even though only a few have been drawn. Also, the magnetic field extends in all directions infinitely, but with distance becomes so weak as to be negligible.