# Is This a Case of Motional emf?

by MS La Moreaux
Tags: case, motional
 PF Gold P: 963 Strictly, no. Whether the emf is classed as motional or not depends on your reference frame. If your frame is one in which the magnet is stationary and the wire moves, you have a motional emf. It can be thought of as arising from the qvxB forces on the charge carriers in the conductor, as the conductor moves through space. If you are in a frame in which the magnet moves and the wire is stationary, but part of a circuit, there will be an emf in that circuit according to the Faraday/Maxwell equation $$\oint Edl = -\int\int \frac{\partial B}{\partial t}dA.$$ The line integral is evaluated around the closed loop of the circuit, and the surface integral over any surface bounded by the circuit. So the force on the charge carriers is qE, arising from the electric field produced by the changing magnetic field. Faraday's description of the phenomenon – the emf arises when the magnetic flux linking a circuit changes – fits in either reference frame.