magnetic
Summary
Magnetic describes fields and forces from moving charges (currents) and from the intrinsic magnetic moments in magnets. A magnetic field exerts a force on a moving charge perpendicular to both its velocity and the field, so it changes direction but not speed.
Detail
Meaning: A magnetic field B (unit: tesla, T) is produced by moving charges, such as current in a wire, and by the magnetic moments of permanent magnets. It exerts a force on a moving charge or on a current-carrying wire. The force is perpendicular to both the velocity and B, so it bends the path without changing speed and does no work. In a uniform field a charge moving perpendicular to B follows a circle of radius r = mv/(qB), where m is mass (kg), v is speed (m/s), q is charge (C). Field lines form closed loops, and isolated magnetic poles have not been observed.
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.
On the MCAT: Expect right-hand-rule questions for direction, mass spectrometer or cyclotron-style passages (predict how r changes when m, v, q or B changes), a velocity selector where qvB balances qE, and the force on a wire, F = BIL sinθ. The field around a long straight wire falls off as 1/r.
Common confusion: Students point the force along B or along v, or think it speeds the charge up. The force is perpendicular to both, so kinetic energy stays constant. A charge at rest, or moving parallel to B, feels no magnetic force. A negative charge feels a force opposite to that on a positive charge.
Sources
- Physics (Cutnell and Johnson)
- Fundamentals of Physics (Halliday, Resnick, and Walker)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.