The equation:
emf = dBA / dt
Shows that an emf may be induced by a change in B or A. I'm confused about the situation in which A is changed to induce an emf, though.
My book describes a situation in which a ring is dropped through a magnetic field such that as it falls, the area of the ring is perpendicular to the magnetic field.
The book considers 'A' to be changing here, but I don't understand why you'd say that the area is changing if the area of the ring stays the same throughout the fall. I get the idea that an emf is induced because the circumference of the ring cuts through the field lines, though.