How the magnetic flux changes in the loop of a wire in a simple electric generator?
You are right in stating that the source of induced current is the change in flux of the magnetic field. When the loop rotates, the magnetic flux through it does change: the flux is muximum when the plane of the loop is perpendicular to the line of force of the magnetic field, the flux is zero when the plane of the loop is parallel to the magnetic field. Φ = BAcosα, α is the angle between the magnetic field B and the normal vector N of the plane. Even though the vertical wires of the loop are always perpendicular of the magnetic field B (supposed horizontal), the angle between vectors N and B changes when the plane rotates about a vertical axis.
