magnetism

Meaning: Magnetism arises from moving charges. Currents create magnetic fields, and so do the spin and orbital motion of electrons in atoms. Field lines form closed loops, running outside a magnet from north to south, and isolated magnetic monopoles have not been observed. A magnetic field acts on moving charges and current-carrying wires, not on charges at rest. Because the force is perpendicular to velocity, it changes direction but not speed, so it does no work.

Equation: F = qvB sinθ, where F is the magnetic force (N), q the charge (C), v the speed (m/s), B the field strength (T), and θ the angle between v and B.

On the MCAT: Use the right-hand rule for force direction, and reverse it for a negative charge. A charge entering a uniform field perpendicular to it moves in a circle of radius r = mv/(qB). Passages also use the field around a straight wire, the force on a current-carrying wire, and materials classed as ferromagnetic, paramagnetic or diamagnetic.

Common confusion: Students expect a magnetic field to speed up a charge or to push a stationary one. At rest, v = 0 so F = 0, and the force is perpendicular to v, so kinetic energy stays constant. Speeding a charge up takes an electric field.