Why does induction current in a fixed frame occur when magnets rotate? What plays in this case the role of outside forces?

For the induction EMF to appear in the frame, it is only important that the magnetic flux penetrating the frame changes over time. If the frame is stationary, the role of external forces is played by the forces acting on free electrons in the frame from the side of the vortex electric field generated by the alternating magnetic field.

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