A Loop Entering and Leaving a Field Region

Faraday's law only cares about how flux changes with time, not how much flux there is. As a square loop slides into a field region, the area of the loop that overlaps the field grows steadily, so flux climbs at a constant rate and a steady emf appears, pulling back against whatever pushes the loop in. Once the whole loop is inside, every bit of area already overlaps the field. Moving further doesn't add any new flux, so d(Phi)/dt drops to zero and the emf vanishes, even though the field is just as strong as ever.

Picture a flux versus time graph for the whole trip: flux rises in a straight line while the loop enters, flattens into a plateau while the loop is fully inside, then falls in a straight line of the opposite slope as the loop exits. The slope of that graph is the emf, so the induced current versus time graph is a rectangle: a constant positive value during entry, zero during the plateau, a constant negative value during exit, since the flux is now decreasing and Lenz's law flips the current's direction.

This setup is the cleanest possible model for the general induction story of the unit: motion changes flux, changing flux fights back with an emf, and the fighting stops the instant the change stops. The next step scales this up to rotating coils, where flux keeps changing at every instant and the emf becomes a continuous sine wave instead of a flat rectangle.