|MadSci Network: Physics|
One explanation is that magnetic field generated by the microwave oven penetrates the faraday cage and interferes with the operation of the DC motor. The magnetic field is not strong enough to damage the fan but is strong enough to affect its motion. Since the magnetic field is of high frequency which reverses its direction 2.45x10^9 times in one second, the DC motor cannot response to it fast enough to make a motion. (The static magnetic field of the DC motor itself can be ignored if it is much smaller than the strength of the magnetic field due to the microwave oven.)
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