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
Some heat will always be produced to to friction or air resistance. This small amount of energy given off as heat will take away from the amount of kinetic energy, and little by little it will all be converted into heat.
The second law of thermo dynamics prohibits work without loss. Some inefficiency is always part of any system.
A generator “produces” less energy than it uses… It’s more like a “Transformator” rather than a “generator”. Let’s say you are using a diesel generator to produce electricity. Some of the diesel energy goes into electricity, but most of the energy is wasted in heat, sound, gases, etc. You could use some of the electricity to help keep the machine going but what’s the point? You supposedly wants some electricity, why would you water electricity to produce less electricity?
If you use a battery to power a dynamo to produce electricity you’ll end with less energy than you started with. Same as the previous example, you lose energy in friction, heat, noise, etc. You can’t even transmit electricity through cables without lose some of it.
We always lose some energy in the process. It’s sort of like going, “Can I pour water from a bottle into a cup, then pour that water back into the bottle, and keep doing that forever?” You could do it for a while, but eventually you are going to have some of that water evaporate, or spill some, and while it’d be only a few drops lost at a time, eventually you’d run out.
Perpetual motion is sort of like that. 100% efficient energy conversion isn’t possible with our current understanding of physics. So it’d be like “I take 100% of the energy from the generator, but am only able to get 99% of that energy back into it to keep it running.” You lose some energy as heat or light or friction or some other forms, and while you can be very efficient, after a while you will run out of energy that way.
I want to emphasize that generators do not create energy out of nowhere. Generators take existing energy (in the form of motion) and transform it into a different kind of energy (electricity). The total amount of energy does not increase. When the energy becomes electricity, it stops being motion, which means the generator *must* slow down.
There are engines and mechanisms that automatically reset themselves and then go again. For example, your car has revolving parts. Hooked up to a fuel supply, your car could run forever. Let’s even say we could even turn the exhaust into fuel and recirculate it. Even still there are two problems.
First is energy loss. Noise, light, sounds, heat, movement, etc are all examples of energy leaving the system. Therefore, eventually the engine would run out of fuel because energy is constantly being lost in the previously listed forms.
Second, parts break down and need replacing. Engines need maintenance. So on and so forth.
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.
Sure, let’s try it. Let’s say you have a generator that feeds an electric motor that turns the generator. And let’s say the generator produces 100 energies. So the 100 energies goes through some wires to the electric motor, but some of that energy is converted to heat because the wires have resistance. So now the electric motor is fed with 95 energies. Electric motors get hot too so we have losses due to heat, and they make a lot of noise too so we loose some to sound energy so the motor is providing 88 energies to the generator. The generator gets hot as well and makes a bunch of noise so now it is producing 80 energies. So now the cycle repeats and each time you lose more energy to heat, noise, etc.
The main reason why perpetual motion doesn’t work is that in order to compensate for losses due to friction, noise, heat, etc, you have to put in more energy than you get out of the system. And by increasing the amount of energy you put in you also increase the amount of losses, so you have to put in an infinite amount of energy to keep the system running.
ELI5 – the process of making energy or using it has inefficiency that saps the energy. Mechanical processes must fight friction, resistance in conductors saps electrical processes etc.
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