magnetic field

Meaning: A magnetic field B is a vector field produced by moving charges (currents) and by magnets. Outside a magnet, field lines run from north to south pole and form closed loops. A long straight wire carrying current makes field lines that circle the wire (right-hand rule). The field exerts force on moving charges, and that force is perpendicular to both velocity and B, so it does no work.

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

On the MCAT: A charged particle enters a field: find the force direction with the right-hand rule (reverse it for a negative charge). If v is perpendicular to B, the path is a circle of radius r = mv/(qB), and speed and kinetic energy stay constant. Related questions use a current-carrying wire, F = ILB sinθ, or compare radii for different masses or charges.

Common confusion: B does not push along the field the way an electric field does. A charge at rest, or moving parallel to B (sinθ = 0), feels no magnetic force.