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This might be dumb question but I'm so curious to know if this actually works or is impossible. I was researching how you could generate electricity from magnets and copper wire and also have read how the hydroelectric generators work and it seems like they share the same concept. So if we are able to generate electricity by moving the turbine with water then why not have a some sort of motor which will make the turbine spin by getting power from the generator. I mean first there needs to be some sort of initial mechanical source to make the turbine spin then once the turbine starts moving and the generator gives power, the other motor gets power and it will take over the spinning of the turbine. So that way it will auto spin itself and basically means free energy. Can someone explain why this is not possible?

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    $\begingroup$ Variations on "then why not have a some sort of motor which will make the turbine spin by getting power from the generator." are the plan for most every perpetual motion machine ever proposed. Conservation of energy and the second law of thermodynamics kills them all. $\endgroup$ – dmckee Mar 15 '15 at 4:17
  • $\begingroup$ You will always lose power due to friction. With a hydroelectric power plant, you gain an advantage. You start with a liquid with great kinetic potential (confined to a small region) and get electricity which is easily transported (sometimes many miles) to do useful work. $\endgroup$ – LDC3 Mar 15 '15 at 4:30
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    $\begingroup$ There's no such thing as a free lunch. The same holds for energy. $\endgroup$ – hft Mar 15 '15 at 5:46
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As with virtually all perpetual motion machines, the reason becomes obvious once you consider the thermodynamic efficiency of the components involved. No turbine is 100% efficient, and also no motor is 100% efficient. This means that out of the initial energy you put in to make the turbine spin, only a certain percentage will be converted to electricity, with the rest being converted to heat. Then, out of that electricity, only a certain percentage is converted to mechanical energy to drive the turbine again. Then we go round again, losing some of that energy to heat until pretty soon the whole thing stops turning.

Of course, it would work just fine if you had an engine or a turbine that was more than 100% efficient, putting out more energy than you have to put in to drive it. But then, that's exactly the reason why we know that efficiencies over 100% are impossible: we observed that perpetual machines seem to be impossible, and from that Carnot derived his thermodynamic limits. The resulting theory has stood the test of time since the 19th century.

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If you hook up a motor to your generator output, the motor would not be able to spin fast enough to produce sufficient current to drive the generator in the first place. This is a typical case of law of conservation of energy. You may also need to check Lenz's law first. It says: "If an induced current flows, its direction is always such that it will oppose the change which produced it."(from Wikipedia) This further explains the law of conservation of energy in a system of a conductor and changing magnetic field.

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