magnetic force

Meaning: A magnetic field pushes on a charge only if the charge is moving across the field. The force is perpendicular to both v and B (right-hand rule: fingers along v, curl toward B, thumb gives the force on a positive charge; reverse it for a negative charge). Because it is perpendicular to the motion, it does no work and cannot change speed. It only bends the path, giving a circle of radius r = mv/(|q|B) when v is perpendicular to B.

Equation: F = qvB sinθ, where F is force (N), q is charge (C), v is speed (m/s), B is field strength (T), and θ is the angle between v and B. For a wire, F = ILB sinθ (I in A, L in m).

On the MCAT: Typical items use a mass spectrometer or velocity selector (qE = qvB, so v = E/B), ask which way a proton or electron is deflected, or ask how the radius changes with mass, speed or charge.

Common confusion: The force is not along B or v the way an electric force is along E. A charge at rest, or moving parallel to B (θ = 0), feels no magnetic force.