Magnetic field

A magnetic field is the swirling influence that moving charges create and that other moving charges feel — the partner-in-crime of the electric field. Scatter iron filings around a bar magnet and they line up into graceful loops running from north pole to south: those loops trace the field. Unlike electric field lines, magnetic lines never begin or end anywhere; they always close on themselves in complete loops, because no one has ever found a lone magnetic 'north' charge to start them.

The field B is measured in teslas (T); Earth's field is a feeble 50 microteslas, a fridge magnet musters about 0.01 T, and an MRI scanner roars at 1.5–3 T. A magnetic field grabs only charges that are moving, and it pushes them sideways — perpendicular to both the field and the motion — which is why a current-carrying wire in a field jumps to one side, the seed of every electric motor. Wrap that wire into a coil and you concentrate the loops into a powerful electromagnet you can switch on and off at will.