Describe Faraday's law qualitatively, compute an induced EMF from a changing magnetic flux, and connect this to how generators and motors work.
Last week showed that moving charge creates magnetism; this week reveals the reverse, arguably even more useful: a CHANGING magnetic field through a loop of wire induces an electric current in that loop, a phenomenon called electromagnetic induction. The key quantity is magnetic flux, Phi = B*A*cos(theta), measuring how much field passes through a loop of area A. Induction only happens when this flux is actively CHANGING over time -- a strong but perfectly steady field through a stationary loop induces nothing at all.
Faraday's law quantifies the induced EMF (electromotive force, essentially a voltage): EMF = N * (change in flux) / (change in time), where N is the number of loops of wire, since more loops means more induced EMF for the same changing flux. Lenz's law adds the direction: the induced current always flows in the direction that OPPOSES the change causing it. This exact principle drives electric generators (mechanically spinning a coil through a field to produce electricity) and, run in reverse, electric motors (using current in a field to produce motion, from last week's force law).
Mastery looks like: They compute an induced EMF from a changing magnetic flux using Faraday's law, and explain qualitatively why a CHANGE in flux, not just its presence, is what matters.
Common stumbles: Believing a strong, unchanging magnetic field alone will induce a current; forgetting to include the number of loops (N) in the EMF calculation.