Magnetic field

Permanent magnets (bar) or electric magnets (straight wire, solenoid,...) create magnetic field. Moving electric charge (current) gives rise to magnetic field. Unlike electric field lines, the magnetic field lines are closed loops i.e. no magnetic monopole exists. There are always both South pole and North pole.

The magnetic field at any point:

• direction tangential to field line and towards South pole (outside the magnet)


• strength proportional to density of field lines (also called magnetic flux)
  

The geomagnetic field due to plasma streams (volcanic electric currents) inside the Earth and the field geometry pretty similar to the field of gigantic bar magnet. The separation between geomagnetic south and geographic north is about 11 degrees. Solar plasma flow (solar wind) reshapes the dipole field:

In the presence of many fields, the total field is a vector sum of the component fields.