Magnetic Field
Definition
The invisible influence around magnets and moving charges that pushes on other magnets and on moving charges.
Symbol: · Unit: tesla (T)
Technical definition
A vector field, denoted B, produced by moving charges and intrinsic magnetic moments. It exerts a force F = qvB sin θ on moving charges — always perpendicular to their velocity — and is measured in teslas.
Example
Earth's magnetic field is only about 50 microteslas — a million times weaker than an MRI magnet at 1.5 T — yet it is strong enough to swing compass needles worldwide and deflect the solar wind around the planet.
Magnetic fields do not push on stationary charges. No matter how strong the magnet, a charge sitting still feels zero magnetic force — only moving charges feel F = qvB. This is also why magnetic forces do no work on a charge moving in a circle: the force is always perpendicular to the motion, so it changes direction, never speed.