Magnetic field

The magnetic field B is the vector field that captures one half of electromagnetism — the half produced by currents and changing electric fields, and that pushes sideways on anything carrying charge or current. Where the electric field E pushes along its direction, the magnetic field pushes perpendicular to both itself and the velocity of whatever it acts on (the cross-product in qv×B), which is why charged particles in pure magnetic fields move in circles or helices rather than straight accelerations.

There are no magnetic monopoles. Where electric field lines begin and end on positive and negative charges, magnetic field lines always close on themselves — they have nowhere to begin and nowhere to end. This is the content of ∇·B = 0, one of Maxwell's four equations, and is observed to be an exact law in nature: every "north pole" comes paired with a "south pole" on the same magnet, and cutting the magnet in half just produces two new dipoles. The microscopic source of B is always a moving charge or an intrinsic angular-momentum spin, never a pole sitting alone.

B has units of tesla in SI (kg·s⁻²·A⁻¹) and gauss in CGS (1 T = 10⁴ G). Earth's surface field is about 50 microtesla. A refrigerator magnet is around 5 millitesla at the surface. An MRI machine runs at 1.5 to 3 tesla. The strongest steady fields ever built in a laboratory reach about 45 T. Pulsed fields can briefly hit a few thousand tesla. The magnetic fields near neutron stars exceed 10⁸ T, distorting atoms into pencil-shapes and rewriting how matter works.