Why is perpetual motion impossible? For example, can’t you just redirect some of the energy produced from a generator into keeping itself running?
In: Physics
There is no 100% efficiency in energy.
For your generator example, we have two electric motors, we pick two of a type that when powered they turn, and when turned they make power. We start by spinning the link between them to set every thing in motion. The two motors are the exact same size so it should just go forever right? Let’s say the generator is putting out some 1000 “energy units” at the speed it’s turning, and the motor requires 1000 energy units to spin at that speed. But there’s losses.
For one, friction. The rotors of the motors aren’t getting a free ride, they have bearings or bushings they ride on, and you have two of them spinning. Well now we just lost 10 energy units to that in the motor and in the generator, so 20 units down the drain. Now we have the resistance of the wire running from the generator to power the motor to turn the generator. We just lost another 10 power units there. And the heat the motor is putting out, we’re losing 70 energy units just to heating up the copper windings and the metal shell. Now we realize on top of all this, electrical load on a generator shows up as physical resistance to turning; but we won’t worry about that we already have enough going on, or the losses to running their cooling fans either.
So your generator at the speed you spun it up to is putting out 1000 energy units, but since some of that energy is being lost to heat and friction the motor turning it is only doing the work of 900 units of power — remember it is required to get 1000 to maintain the speed it is going. So it slows down and since it slows down the generator slows down and is now putting out less energy units, this happens continuously (and quickly) until everything stops.
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