Answer to Question #131588 in Mechanics | Relativity for Katerina

Question

Someone had an idea for an electric fan that costs nothing to run. the electric motor which turns the fan also turns a generator. this produces electricity for the motor, so no battery or mains supply is needed explain why this idea will not work

Answer 1

The problem here is that there is no 100% efficient energy transfer in any mechanism. Electrical generators have moving parts that experience friction, part of the energy is lost for the friction. Also, during rotation, the fan resist the air (as a viscous fluid) and part of energy is lost for making work on air flows. Some losses in electric circuit of generator are present as well. Namely, Foucault's currents and hysteresis in rotor. Overall, all these losses make this machine (motor, fan and generator) to lose all the energy and stop after some finite time.