Faraday's law of induction

Meaning: A changing magnetic flux through a loop of wire induces an emf. Flux changes if B changes, if the loop's area changes, or if the angle between B and the loop's normal changes, as in a rotating generator coil. Lenz's law gives the direction: the induced current's own field opposes the change in flux.

Equation: emf = −NΔΦ/Δt, with Φ = BA cos θ. Here emf is in volts (V), N is the number of turns (unitless), Φ is magnetic flux in webers (Wb = T·m²), B is field strength in teslas (T), A is loop area in m², θ is the angle between B and the loop's normal (degrees or radians), and t is time in seconds (s). The minus sign expresses Lenz's law.

On the MCAT: Expect a magnet moved toward a coil, a loop pulled out of a field, or a rotating coil. You predict whether the emf rises or falls, or find the direction of the induced current. On a graph of Φ versus t, the emf is proportional to the negative slope, so a steeper line means a larger emf. Doubling the turns doubles the emf.

Common confusion: Students assume a large flux or a strong field produces an emf. Only a change in flux does. Constant flux, however large, gives zero emf, and at the instant the flux graph is flat, even at a maximum, the emf is zero.