Perpetual Motion - is it so damn hard?

Dear Internet: Perpetual Motion from a layman

When you say “free energy” or “a perpetual motion machine” you may mean one of several things:

(i) A machine (eg. solar/wind powered things, plant life, fusion power etc) in a local system (eg. the earth) which run on energy from a source (ie. hydrogen in the sun or on earth) which is effectively infinite <i>with respect to the local system</i> (thanks LSLGuy). Possibly a source no-one knew about before. This IS POSSIBLE and EXISTS.

(ii) A machine which if it lived in perfect isolation could run literally forever, and has zero losses even though it’s impossible to extract any energy form it. eg. one rock orbiting another rock in a newtonian universe. (The only problem is to make it complicated enough to call “a machine” rather than “a rock”) This is POSSIBLE but DOESN’T ACTUALLY EXIST because anything in the universe even if effectively undisturbed, will EVENTUALLY after enough trillion years be interfered with by minute amounts of interstellar dust, protons and neutrons spontaneously decaying, orbits slooowly emitting gravitational radiation[1], etc.

(iii) A machine which runs in a non-isolated environment (eg. on the earth). This is STATISTICALLY IMPOSSIBLE because the second law of thermodynamics observes that all such things always interfere with the environment, and in doing so increase in entropy, which is normally visible as “sort of running down”. This isn’t necessarily obvious, but is true, which is why it’s important. I say “statistically” impossible because it describes what tends to happen. If you start with a room with two different gases in it at two ends, it will eventually mix until the two gases are equally distributed. But it’s always possible (though vanishingly unlikely) that the gases will both suddenly jump to one corner of the room. But you can’t DESIGN that to happen.

And so

Sometimes people use the terms confusingly.

If you talk about finding “free energy” sometimes you mean it in sense (i), which is generally a good thing: even if it’s not literally free, it fulfils the requirements we want to seek, ie. we can have as much of it as we can get. Other times they mean in sense (iii), which means that they’re clearly wrong, even if you can’t point out WHY they’re wrong.

If you say “perpetual motion is impossible” you mean sense (iii). Someone else may mistakenly think (i) and (ii) are a counterexample, in which case they may be correct you should have spoken more clearly when saying what is impossible, but you are quite correct that you’re thinking of something specific which is in fact impossible.

If someone says “isn’t XXXX a counterexample to perpetual motion being impossible” they generally have in mind a machine which either (a) draws power from some effectively but not actually infinite resource, often one which may not be apparent (b) is in case (ii) or © looks like it ought to go on forever, but actually loses energy to the environment slowly as in case (iii), sometimes in a non-obvious way. Either something like “ok, so the water wheel runs the pump and the pump runs the water wheel” forgetting that the efficiency is much much less than 100%. Or something where the efficiency is much closer to 100% and isn’t obvious.

(Or sometimes (iv) complicated quantised physics shenanigans I don’t understand. But that’s not normally what people are thinking of.)

footnotes

[1] I just thought of gravitational radiation today. I’m sure I didn’t include it the last time I had this rant, and would have said that without friction you could have had a system running in isolation for ever. I think you still could CONCEPTUALLY, it just doesn’t happen to work in reality.

I disagree. Perpetual motion is impossible - those that understand the term to mean something else need to be educated.

In any case, in this thread, the OP is talking about a system that would have to violate the laws of thermodynamics to produce an energy output.

It’s a specific technical term, in that it has a specific meaning which is similar to, but not precisely the same as what a normal English speaker unfamiliar with the term would interpret it as. Given that it DOES have a precise meaning, people clearly do need to learn that (stubbornly ignoring meanings, whether for good or bad reasons, is counter-productive). And yet, talking to someone who does NOT know the term, if you want to have any hope whatsoever of conversing, you obviously need to tell them.

I’m not sure how that contradicts what I said the first time, even if I phrased it all (typically for me :slight_smile: ) very badly and sloppily.

:slight_smile: I decided I couldn’t tell, and that appreciating the differences between commonly cited examples was sufficiently useful whichever sort this one was :slight_smile:

Fair enough - I think I misunderstood you as arguing that each of the three possible understandings of the term should be accepted.

Reminds me of college days. My roommate and I made a living taking in each other’s laundry.

Ah, sorry! Yeah, then I see what you mean.

Beowulff, Mangetout, and Sage Rat - you’re all answering the wrong question for me. I see why the second law of thermodynamics rules out heat engines as perpetual motion machines, or at least free energy machines (beowulff does correctly point out perpetual motion machines per se aren’t ruled out, and everybody is responding - as I hoped - to what would have been my question if I’d said “free energy machine” instead; sorry for my sloppy and incorrect misuse of “perpetual motion” when I meant “free energy”).

I am asking, how can one generalize from the 2nd law, which is specifically and ONLY about heat engines that extract useable energy from heat flow, to a discussion of ALL categories of proposed free energy machines?

Entropy is only heat per temperature. Maximally efficient heat engines have to leave untapped a fraction of the incoming heat energy, that is as big a fraction as the outgoing temperature is a fraction of the incoming temperature. My question is, how do you use that to show why somebody’s whirling combination of magnets and weights won’t create endless power? I mean, I expect their machine won’t create the endless power; I just don’t understand how that comes from heat engine rules.

movement looses energy through heat caused by friction

Let’s expand this; although there are plenty of theoretical reversible processes than can occur as part of a cycle, any real world process of converting chemical or nuclear energy to mechanical motion (or momentum) will involve some kind of heat engine, and thus losses due to entropy.

Stranger

Entropy does not only refer to heat. Like I stated, it’s a measure of homogeneity. The difference might be cool here, warm there; it might be high pressure here, low pressure there; or it might be positive charge here, negative charge there. Regardless of what the source of power may be, utilizing that power equates to removing the difference. Using some other power source to restore it simply moves the problem a step back and introduces another way to decrease efficiency (generally).

The First Law covers this. It doesn’t rule out a mechanism that remains in motion forever without energy input; it just rules out a mechanism that remains in motion forever without energy input while you continue to extract energy from it.

The general philosphy of entropy (“shit tends to flow downhill”) says this is probably the case, but entropy as a thermodynamic property of matter definitively limits the maximum efficiency with which one can extract mechanical work from a cycle operating between two heat reservoirs. Whereas there is no theoretical limit (up to 100%) to the efficiency with which one may convert (for example) electrical charge or gravitational potential energy into useful mechanical work, the second law determines the upper bound on efficiency of a heat engine as a function of the reservoir temperatures, and it’s necessarily less than 100%.

I don’t approve of what’s going on here. Bright people with inspired ideas for perpetual motion machines should be given the technical training and funding needed to build them. We’ll never get free energy by teaching them physics.

BTW, here’s my idea: Build enormous drinking-water birds and hook generators to them. I can’t execute this idea myself because I already know how they work.

You mean we could trick the universe into letting perpetual motion occur, if we sneak up on it?

My final - many of you say ridiculous - attemp to save some energy with this buoyancy theme. Lets make this sub look like a javelin. All you need is inside except the rotor. The surface of the spear is also stator magnet - so we can have TWO dynamos (now I hear load laughter). Anyway, the javelin is made of two parts partly inside each other. You have electrodes in 36 grams of brine making us the needed rocket gas (2H2+O2) for two moles. Now in 1000 meters below the surface you light up a spark with rechargeable batteries . As the combustion comes to it’s peak the length of the javelin is increased by an inch or two - you lock it and the system becomes buoyant. During the rise all of the excess heat is off the system and gases back to brine. The combustion must overcome some serious pressures and unlocking the system may require another explosion, but in my opinion, we are not debating about The Laws of Thermodynamics - the heat was meant to leave the system.
I know that the whale defenders oppose this “subvelin”, but in this process of learning physics, I would like to hear some comments on this.

The energy of the explosion has to be sufficient to displace enough water to make the system buoyant. At a depth of 1000 metres, that’s quite a lot of work.

Unless the system has a net energy input somewhere, you won’t (can’t) have enough energy to hand to be able to do this - especially since you’re now talking about throwing away excess heat each cycle.

Try writing it all out as a balance-sheet type operation:

Income:

  1. Electrical energy generated from the motion of the object through the water
  2. Chemical energy (water split to Hydrogen and Oxygen)

Expenditure:
A. Mechanical work required to displace water to make the system buoyant
B. Electrical energy required to split water

Even if your dynamo is 100% efficient, (1) cannot exceed (A), because (1) is driven by the fact that as it descends, the sub displaces less than its own weight of water - that’s why it sinks, and that’s what does the work of pushing it down through the water.
In order to start ascending again, you need to displace some more water so that the sub floats - You can’t win here - you have to push back a bit more than that which pushed you down.

And when you burn the hydrogen, you can’t get back all of the energy you expended splitting it - at least not all in a useful form - and you lose the non-useful heat, sound, etc.

So your sub sinks to the bottom, maybe making some combustible gases on the way down, then maybe ignites an impotent explosion, then it stays on the sea bed.

ETA: We ARE still debating the laws of thermodynamics - until and unless your system attempts to harvest an external energy source (such as tides, temperature gradients, convection currents, solar energy, etc) - which it so far has not attempted to do. It’s only difficult to see the limitations imposed by the laws of thermodynamics here because the Rube Goldberg nature of the contraption obscures them. Money laundering works in much the same way.

Is this some sort of electrical engineering humor?

Also a typo there are loads of them in my text.

Also called a typo - there are loads of them in my text. I would have to be an engineer to be able to reach levels yuo are suggesting. To be quite honest, I’m more aware of my grammatical errors than anything else.

I have to ask at this point… katunari - do you actually accept the reality that you can’t get more energy out of something than has been put in*?

  • In some cases, this might be more energy than you personally put in, the first time around, but if you’re creating a device that stores energy, the store will always be smaller than the work it took to store it, and the process of extracting the energy from store will always also be prone to losses.

I decided to test my theory on me. I’m not that good swimmer or scuba diver - the schema had to be modified to work on the ground. Took me a while to build this apparatus, but I made it. You may call it “penvelin”. This morning I woke up and everything seemed more than OK. It was sunny and windy and there were a few spectators available to witness this event. So I tried to put this gear on and in me and found out that the theory was impossible. There was enough space for only ONE rotor.

You may remember my personal problem - so I have to carry on. Capilarity should come next.