Final answer:
In electric generators, magnetic flux changes from its maximum to zero when the coil is rotated through one-fourth of a revolution, inducing an electromotive force (emf) according to Faraday's law of electromagnetic induction.
Step-by-step explanation:
In electric generators, the magnetic flux through the coil changes when the coil is rotated within a magnetic field. Specifically, when the generator coil is rotated through one-fourth of a revolution, the change in the angle between the magnetic field lines and the area vector of the coil causes the magnetic flux to change from its maximum to zero. This change in magnetic flux induces an electromotive force (emf). According to Faraday's law of electromagnetic induction, a change in magnetic flux through a coil induces an emf. The maximum induced emf occurs when the rate of change of magnetic flux is greatest, which happens when the coil is perpendicular to the magnetic field lines (at a quarter rotation from the maximum flux position).