# eli5 why are 100% efficient systems impossible?

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I know the *base* of the story, every single system doing any kind of work leads to a certain loss of energy, basically what makes perpetual motion machines impossible, as without pumping more energy into the system, the one it already have will eventually all be exchanged in the form of friction and whatnot.

But I want to go to the *fundamentals*, why is it like that? Why is it impossible? Why can’t we imagine a hypothetical machine made with whatever unobtainium we might need that is actually, mathematically energetically perfectly efficient? *Why did heat beat us? xd*

In: 3

Can we imagine one? Sure. Some real systems even come pretty close, like planets orbiting the sun.

But there are so many things that may carry off the energy of the system. Friction, various waves, the expansion of space, and so on. Odds are very good at least one of them interacts with your machine.

100% efficiency is possible in only one case: when you want to convert some energy to heat.

The fundamental law in the action here is the second law of thermodynamics. It basically states that energy tends to spread equally and to do something usefull you always have to move energy from high concentration to low concentration while the way back is impossible.

Something has to happen within the system for it to be a system. As soon as you imagine a perfect system say, A affects B and B affects A. Even if you’d say, all energy from A is transferred to B and back again you’d have to deal with the fact that Both A and B prefer the lowest state of energy. That results in an equilibrium where both A and B will share the energy. This will simply result in a stand still at the moment that A has transferred half of the energy to B because at that point they are doing exact the same thing in opposite direction resulting in an energetically perfect situation where nothing really changes. If we then bring in the concept of entropy A and B will expel eachother in order to get rid of the impasse. This however means that there is loss of energy within the system, breaking with our paradigm of perfect energy transmission. In the end you will always come back to find loss of energy because of it. Some outside force would have to keep A and B together but as soon as you would introduce this phenomenon ‘C’ it too has to use energy to do so. If we have a phenomenon D to introduce that energy, C and D have to be in the same situation as A and B. This could go on endlessly and all factors you add will have to be compensated in one way or another constantly expanding what we try to define as the system. So the fundamentals are probably entropy enabled by directing energy outward. I’m curious if this makes sense to you and if it is what you were asking for

Fundamentally, you can’t have 100% efficient systems because you can’t 100% control what direction atoms are going.

Heat is random motion of atoms. And, since it’s random, there’s always *some* of the motion going in a direction you don’t want.

The only way to get an atom to not go anywhere you don’t want (not counting quantum weirdness) is to cool it to absolute zero, but now it has no thermal energy and you can’t do anything useful with it in a thermodynamic sense.

The second you add any energy it starts wiggling and you can’t completely control how it bounces off it’s surroundings so it’s *always* going to dump some kinetic energy into the things it interacts with, which is just heat transfer at the molecular level. And since energy is conserved and *some* of it must end up us heat, you’re never going to get 100% of it to do non-heat (i.e. anything useful).

If you *do* want heat, then you can get 100% efficiency, as others have said, because that’s the only place you can guarantee all the energy will eventually go.

From the moment two objects are interacting in some way with each other, they’re exchanging energy. It’s not just about simply friction

The problem with perpetual motion machines though is not that they’re impossible but that they’re useless. Aside from making cool desk toys, a theoretical perpetual motion machine would be in perfect equilibrium, containing exactly as much energy as it needs to perform its action and losing none of it in the process. If you tried to harness that energy, then you’d be inevitably be taking energy from that system, disturbing the equilibrium and making the machine eventually stop. So we don’t just need a machine that loses no energy but a machine, or process, that produces more energy than it consumes, which is physically impossible.

There’s a lot more at play than just friction, no interaction of any sort is lossless. It goes from a higher energy state to a lower energy state, otherwise it’s not moving or doing anything at all. Remove air friction and there’s material friction, remove material friction by for example using magnets but even magnetic interactions are not lossless. Remove magnets somehow even just gravity is not a lossless exchange of energy. Of course we usually think of gravity on a huge cosmic scale in which there’s essentially limitless reserves of energy but even they are not in fact limitless. Sure Launching a rocket from earth does not make a noticeable difference but in actuality it does, it ever so slightly takes away from the earth’s momentum.