Final answer:
When a magnet and a disk are both spinning at the same angular velocity, an induced emf is generated due to the cutting of the magnetic field across the disk. This phenomenon is called motional emf.
Step-by-step explanation:
When a magnet and a disk are both spinning at the same angular velocity, there is an induced emf (electromotive force) due to the magnetic field produced by the magnet cutting across the disk. This phenomenon is known as motional emf. The induced emf is given by the equation emf = Blv sin(theta), where B is the magnetic field, l is the length of the side wires of the disk, v is the linear velocity of the disk, and theta is the angle between the velocity and the magnetic field. So, when both the magnet and the disk are spinning at the same angular velocity, the induced emf is generated, with its magnitude determined by the strength of the magnetic field, the velocity of the disk, and the angle between them.