Magnets and Magnetic Field
Magnetic materials are used to make magnets that produce magnetic fields around them. A magnetic field is due to magnetic material or to moving charges. If you place a magnet in a magnetic field (near a magnet for example), it will either be attracted or repelled. Moving charges may also be attracted or repelled by magnets.
Magnetic field lines of a magnet appear to emerge from the north pole and enter from the south pole. In fact they form closed loops with neither a starting nor a finishing point.
A magnet has tow poles: north (N) and south (S) and is therefore called a dipole. If two magnets are held close to each other, they will rotate so that the north and south poles attract each other. Magnets also attract and repel each other: like poles, north and north or south and south, repel each other and unlike poles, south and north, attract each other. We note here similarities with electric charges.
A magnetic field is characterized by its direction and magnitude and therefore is a vector quantity. A magnetic field is represented by magnetic field lines emerging from north pole of the magnet and converging towards its south pole. (see figure below). At each point in space where a magnetic field id present, the magnetic field itself (a vector quantity) is tangent to the field lines.
A Magnetic compass is a small magnet, marked at both ends north (N) and south (S), mounted on a small frictionless rod so that it rotates easily. It is used to detect magnetic fields in general. Early practical applications of magnetic compasses was in indicating directions using the earth magnetic fields.
In SI units, the unit of magnetic field is the Tesla which is equivalent to (Newton × second) / (Coulomb × meter).