A region where a stationary magnet experiences a force. Magnetic lines of force are imaginary lines in space along which a hypothetical solitary magnetic north pole will travel. It has no effect on a static charged particle, if the magnetic field too is unchanging. It is a changing
magnetic field that can accelerate charged particles. Field strength is measured in teslas and for comparison a good permanent magnet produces about 1 T, the magnetic field on the earth's surface is about 4 x 10-5 T, and interplanetary fields are around 5 x 10-9 T.