I understand why you can’t get a perpetual motion machine (that’s one of the few concepts I think I do understand in physics), but practically we dont need a perpetual motion machine, do we? One that will provide energy for oh, say, 10,000,000 or 500,00,000 years will do just fine, right? It seems to me that there ARE sources of energy that will last for quite a while --the sun seems to send out a pretty reliable shitload of energy, and its various forms, such as ocean waves and wind, seems pretty inexhaustible for all intents and purposes. They don’t seem capable of getting used up no matter how much we exploit them. Gravity too seems to provide free energy without much input needed from humans. so–my question is “Is it simply a matter of time until some smart person figures out a way to use these free, practically inexhaustible sources of energy economically, or is it simply not possible to harness such energy sources?”
(Regarding “gravity,” I had the crackpot idea this morning that if we could get the weight of a mountain concentrated on a single small spot, that should provide a lot of energy, but I guess the work involved in setting up that massive amount of weight, and keeping it balanced, would be more than the weight could provide. But I was thinking that that effort would be finite–once you set it up, however difficult that was to do, it would go on provideing the force for a very long time, perhaps long enough to make up for the work invested in in setting it up. Or maybe not. Let’s stick with the sun’s energy, then.)
If the answer is the first option, (" matter of time until some smart guy figures out how to use the sun’s energy…") then wouldn’t we be advised to wait, and forego such dangerous sources of energy, like nuclear, that could destroy (or hopelessly pollute) our civilization before we develop alternative sources of cheap, safe energy? In the alternative, if we have no better chance of these alternatives than we had at transforming lead into gold, then we might as well focus on minimizing the risks of our curent energy sources. I guess another way to frame the question I’m trying to pose here is “Is there any physical law that makes virtually perpetual energy impossible?”
I take it you have actually heard of solar, wind, and wave power? The problem with them is that the energy is spread out over the Earth’s surface, so you need huge arrays of devices to capture it in any quantity.
As for gravity, that is not an energy source in itself. You need to have something that is already high up. With you mountain-on-a-pinpoint idea, obviously the most you could possibly get back from it, however slowly you extracted energy, would be the amount of energy expended in putting the mountain there in the first place.
I think you’re not understanding the idea behind perpetual motion and why it isn’t possible. No one has ever argued that you can’t set up a long-lasting power source and get motion for a long time from that. The argument is that you can’t get ongoing motion without ongoing energy inputs.
As for the mountain idea, just keep the laws of thermodynamics/entropy in mind. No matter how much energy you put into something, you’ll never get more out of it than you put in. In fact, you’re almost guaranteed to get less, even in ideal circumstances.
It’s important to note that force isn’t the same thing as energy.
Energy is force times distance. So you can extract energy from your mountain provided you let it descend. But then you have to expend energy to raise it back up (essentially always more energy than you can extract, as others have noted).
In at least one case, “virtual perpetual motion” was achieved, by a clockmaker named James Cox way back in the 1760’s. He built a clock that whose spring was constanly rewound by changes in barometric pressure. It worked without stopping until sometime in the 1850’s when the museum containing it was dismantled and the clock lost. If this is had not occured, the clock was so well made that it very likely would be working today.
This and other fascinating information about P.M. can be found in the book “Perpetual Motion, the History of an Obsession” by W.J.G.Ord-Hume.
Okay, but how about if it were work you doing anyway? Could you construct a waste dump for heavy items such that it all came to rest on a relatively small point at which the weight of the garbage heap would put [ressure on something that would cause work to be done? It might not be equal to effort of transporting all the concrete, bricks, tschotzshkes etc you were hauling to dump but you’d have to haul them someplace anyway, right? Only this way, you’d be getting gravity to accomplish something for you. It’s kind of an Energy-for-Clunkers program.
Again, pressure alone doesn’t do work. The pressure has to act through some distance.
If you find yourself naturally hauling some weight to a point higher than it really needs to be, you can indeed extract energy from its ensuing descent. Of course, you could also figure out how to haul it no higher than it needs to be, which would mean you’d saved more energy than you could otherwise hope to get back.
A mountain on a pinpoint would be a black hole, or at least some kind of stranglet, assuming it’s stable.
Assuming a way to contain it so it didn’t fall through to the core of the earth and destroy us all, seems like you could get a lot more energy out of it then simply dropping it. If it’s a black hole you could prolly collect hawking radiation. Don’t know what stranglets would be good for.
Look at it this way. Go up to a brick wall. Spend all day pushing against this wall as hard as you can. At the end of the day you will have burned up a lot of energy, but done zero work.
This isn’t true perpetual motion, either, any more than your automatic self-winding wristwatch is. In the latter case, the energy to move the internal workings of the watch is being constantly refreshed from an outside source - your arm moving. In the former case, it is constantly being refreshed by changes in weather, which are essentially driven by the sun. Such a clock would be an ingenious device, and very impressive (because the source of energy would not be apparent) but not a PMM.
Perpetual motion machines are something of a term of art - the expression describes a (non-existent) class of machines requiring no external energy input to keep rotating (or whatever motion is supposed to be involved).
Mere constant movement (of a buoy on the sea, of a whirligig in the wind) does not qualify. To suggest that these things are “virtual” perpetual motion is meaningless. Either a machine is a genuine perpetual motion machine (which is impossible) or it gets its energy from an outside source, and is trivial from an energy budget perspective, albeit wrought with great cunning.
This is the real trouble. In one sense no machine could be perpetual motion because eventually it will wear out. Leaving that aside it’s kind of part of the definition to exclude anything that could actually work. Solar, tidal, geothermal power - they don’t count, gravity or ‘zero-point energy’ they do because so far nobody has found a way to harness the first or to create the second.
External external energy supply does not exclude from perpetual motion because so many of the traditional ones relied on gravity or magnetism. This would be alright if there were any way of giving a vertical energy field a horizontal differential. If something could change gravity or magnetism on just one side then you might have it. It’s possible that electrical feedback could be used to counter magnetism on one side of a motor but the likelihood is that same field would produce counter-acting forces and cancel itself out.
Probably the best bet for an externally powered PMM is a simple heat engine like the drinking bird toy. As a wet head cools, so vapour in the head condenses causing more to evaporate from the body and condense in the head until it overbalances and drops the head in water. That is now warmer and evaporates the liquid to condense in the body where it tips it back and lets the head cool off, drawing the liquid into it. You need a very volatile liquid active in just the right range of temperatures. In theory something like this could use ambient temperature to run some kind of heat engine (maybe on ammonia or freon or even carbon dioxide?) forcing it to condense be rapid expansion once it’s done its work. So it’s kind of perpetual because it runs on normal ambient temperature.
You can do something analogous to shielding gravity if you use a heat engine powered by much bigger differentials in solar heat. You might manage it in the tropics but you’d certainly manage it in airless conditions, one end facing the sun vaporises and forces through a tube with a turbine back to the shadowed end where it condenses. Then it wouldn’t count as a PMM because it works on solar power
What you are talking about here is extracting energy from something with potential energy. That potential energy lies in it being higher up the gravity well than it could be. The energy can be extracted by getting the thing to do some work while falling to a point lower in the gravity well.
How can this be arranged?
Well, case one: you could transport something up, and let it fall down. Using your garbage dump idea, you could haul it all to the top of a platform, and the platform could lower and spin a turbine as it does so. The energy you get out of this will be (allowing for inevitable inefficiences) less than it takes to haul the garbage up onto the platform.
Alternatively, case two: you could transport something to somewhere that it could fall down (rather than hault it up). You could dig a gigantic hole, and put a platform covering the top, then haul your garbage onto the platform and then the platform could lower and spin a turbine as it does so. You wouldn’t have to spend energy raising your garbage onto the platform. But you would instead have to haul all the rock and dirt out of the hole to ground level to dig the hole in the first place. Oops, back to square one again.
Finally, case three: you could do like in case two but use an existing hole (the Grand Canyon, a big open cut mine). It would theoretically release energy. I suspect however that the inefficiencies involved (hauling all your garbage to the edge of the hole, making all the machinery to extract the energy from the falling garbage) would mean you get little or nothing out of it.
Of course, hydroelectric power works on this principle: you extract energy from water which is flowing from high to low. Water is dense, flows nicely and best of all the sun does the job of getting the water back up to the top after it has flowed down to the bottom so you can reuse your turbines over and over again for the same water. Even with all these advantages, there are few places in the world where it is viable to extract hydro power. Case three would have none of these advantages, so it won’t work either.
I used the term “Virtual” with respect to Perpetual Motion in the same context as one would use the term “Virtual” to differentiate “Virtual Reality” from “Reality”. In no way does the first equate to the second. I used the example of the clock to describe a machine that acted like a PPM would be expected to, and for a period of many decades. But no, in no respect was it a real PPM.
A thought. Couldn’t you rigorously define a PPM as a machine that, in operation, violates either the first or second law of thermodynamics? That should about cover all possibilities. It also defines why you should NEVER invest money in one.
Since our best evidence indicates that a PMM can’t exist, this might be slightly better as “a machine that, if it operated, would violate either the first or second law of thermodynamics.”
