What is wrong with a perpetual motion machine?

  • #1
The_Thinker
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I'm just curious, why is it that anye perpetual motion machine that is thought up is discarded immedietly. I know that energy can't be created nor can be destroyed but then again, a few centuries we were absolutely positively sure that the Earth was flat! I mean anything can be wrong, why newton's law's itself was proved wrong and we still learn them in school! How can we be so sure that it can't possibly work!!! Can someone clear this out for me?
 

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  • #2
Integral
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Every real mechanism involves some form of friction. Friction means energy is converted to heat and lost to the environment. Every mechanism requires some form of energy to operate, you cannot retrieve all of the input energy since there are unavoidable losses. Therefore it is impossible to get more energy out of a system then you put in.
...newton's law's itself was proved wrong..

Since when? They still work the last I checked.
 
  • #3
russ_watters
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The laws of thermodynamics have been observed in action for centuries. They have been observed to be correct to an extrordinary level of precision.

And even if they are wrong (and there are several loopholes, mainly in quantum mechanics), that still doesn't show how perpetual motion can work. Its a burden of proof thing - something is only accepted to be true if it is proven true, not if it is not proven false.
 
  • #4
Entropy
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why is it that anye perpetual motion machine that is thought up is discarded immedietly.

Because usually its though up by people who aren't very creditible and don't know what they're talking about. Other than that, name some ideas that have been prematurely discarded.

I know that energy can't be created nor can be destroyed but then again

Not necessarily. Read up in QM.

a few centuries we were absolutely positively sure that the Earth was flat!

We? Many peoples around the world didn't ever think the world was flat. In fact serveral cultures knew the world was round serveral milleniums ago. You're only thinking of a small portion of people in western europe.

I mean anything can be wrong, why newton's law's itself was proved wrong and we still learn them in school!

Correction: Newton was wrong. The only reason they're still used today is because they are simple and work well at low velocities. And yes, anything could be wrong. The only thing we have to go by is probability created by our observations, and probability suggests that a perpetual motion machine is either impossible or far ahead of our understanding.
 
  • #5
Frosty_TAW
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When was newton wrong? Not to challenge you entropy but i have never heard that and i am curious. And the perpetual motion machine, i was told by my physics teacher that it can never happen, due to the fact that no machine can , at this moment in time, have 100% effeciency due to energy converting into heat and so on.
 
  • #6
russ_watters
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The Newton question depends somewhat on your perspective. Newton's laws do work in some cases. Just not every case. Are they wrong, incomplete, obsolete, limited, etc? You pick...
 
  • #7
arildno
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As for the Newton question:
A lot of the triumphs&predictions in QM are based explicitly upon using mathematical procedures where some terms are neglected or simplified.
Does that make QM wrong?

Similarly, you are fully entitled (in fact, obliged) to say that Newton's laws are simplifications (and hence, in a strict sense, wrong).

Usefulness is perhaps a more interesting concept than "truth" by which to gauge the quality of some branch of science..
 
  • #8
T@P
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hey i had an idea for a perpetual motion machine. what about two particles , say a and b, where a is attracted to be and be repels a (with the same force). first of allm is this even a perpetual motion machine? and secondly what is wrong specifically with this example? thanks in advance
 
  • #9
KingNothing
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The theories stated in the original question could not be proven wrong, and in fact they were supported by the information they had at that time. Perpetual is not supported by our current information, and it is actually proven wrong by our current knowledge.

As for T@P's question,

First of all: it's not a machine, and it's not perpetual motion. It is not an example of something that can happen.

Second of all: Also, attraction and repulsion are both forces involving two different objects/particles. Attraction is between two, and repulsion is between two. They don't exist independently. If particle A is net attracted to particle B, then particle B is net attracted to particle A.

EDIT: Just for good measure, it was more than 'a few centuries ago'. In fact, by about 25 AD more than half of the Earth's population generally accepted a theory of it being spherical.
 
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  • #10
Mk
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The Brownian Rachet is a perpetual motion machine postulated by Richard Feynman in a physics lecture at the California Institute of Technology on May 11, 1962 as an illustration of the laws of thermodynamics.

The device consists of a gear with a ratchet, that vibrates under Brownian motion (hence the name) in a heat bath. The idea is that motion in one direction is allowed by the ratchet, and motion in the opposite direction is prevented. Thus, it might be reasoned, the gear will rotate with a small force continuously in one direction.

How does this not work?
 
  • #11
Chi Meson
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THis would be a neat "heat engine" but the collisions of the molecules with one side of the rachet would take away the kinetic energy of those molecules. COnsequently, as the kinetic energy of the rotating rachet increased, the temperature of the gas in the box would decrease. It is not perpetual motion since heat would have to be put into the box for the process to continue. It is a clever idea for getting thermal energy to turn into kinetic, but it in no way violates the conservation principle.
 
  • #12
T@P
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not to sound over persistent, but why can't two particles *theoretically* exist such that one attracts one and the other repels it? if they did exist then the two particle system would basically fly away on its own, and then even more *theoretically* one could harness such molecules allowing one to travel without working for it. Isnt that a perpetual motion machine?
 
  • #13
ZapperZ
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T@P said:
not to sound over persistent, but why can't two particles *theoretically* exist such that one attracts one and the other repels it? if they did exist then the two particle system would basically fly away on its own, and then even more *theoretically* one could harness such molecules allowing one to travel without working for it. Isnt that a perpetual motion machine?

You then first need to violate Newton's 3rd Law, because already you do not have equal but opposite force acting within the system. Such violation causes at least one symmetry-breaking of the underlying space.

Zz.
 
  • #14
The_Thinker
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Well, thx guyz for posting your thoughts on it. And by the way, is there friction when a magnetic train floats over a magnetic rail? I don't know, that's why I'm asking you guyz. And i right of now believe that "friction" presists because our science is still "primitive", if it was absolute, we would be able to attain perfection and take off all the friction, sometime in the future I'm thinking, when computers are advanced enough to prevent frictuinal losses by perfecting the system for perfect efficiecy. I mean all our best engines right now can attain about 45% percent efficeincy, now that can be corrected can't it? Eitherway, the way i c it, there is still scope, physics being as crazy as it nowadays "god" ;) knows what will turn up! huh?
 
  • #15
ZapperZ
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The_Thinker said:
Well, thx guyz for posting your thoughts on it. And by the way, is there friction when a magnetic train floats over a magnetic rail? I don't know, that's why I'm asking you guyz. And i right of now believe that "friction" presists because our science is still "primitive", if it was absolute, we would be able to attain perfection and take off all the friction, sometime in the future I'm thinking, when computers are advanced enough to prevent frictuinal losses by perfecting the system for perfect efficiecy. I mean all our best engines right now can attain about 45% percent efficeincy, now that can be corrected can't it? Eitherway, the way i c it, there is still scope, physics being as crazy as it nowadays "god" ;) knows what will turn up! huh?

There's no fricton in a magnetic rail, but how you do propose to remove friction from the air surrounding these trains, or "friction" from normal electronic resistance, or even the energy loss in meandering vortices in a superconducting maglev? Even in an "ideal" engine, you do not get perfect conversion of energy - just look at the Carnot cycle.

Zz.
 
  • #16
krab
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Most PMM ideas are not really concerned with friction. Their promoters claim that they generate energy and so can easily overcome what little friction there may be. The 2 particles, one attracting, the other repelling, is a good example. If we want examples of machines with so little friction that they continue to move for long periods of time, we have many examples. The solar system is such a machine (at least over human time scales).

So let's concentrate on energy-producing machines. Why do physicists dismiss them out of hand? Maybe an analogy will serve. Physicists have a close working relationship with the concept of energy; as close, I dare say as a layman has with a gold ring he is wearing. Imagine I approach this layman and tell him I have a very special box: if he puts his gold ring in it, wait for a day, and then opens it again, there will be 2 gold rings there. For free. Would this layman believe or be sceptical? Right. He would be sceptical. This is the same scepticism a physicist feels for spontaneously-produced energy.
 
  • #17
russ_watters
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Great analogy, krab.
 
  • #18
T@P
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and yet light moves on its own at an unchanging speed forever. doesn't this contradict a lot of things? isn't it also a sort of pmm?
 
  • #19
stunner5000pt
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Ok first of all newton was NOT wrong, it was just inadequate for the high relativistic velocities that is in QM. Newtons laws are taught in school because newtons laws are based on common sense, and it works for some everyda applications but not for QM.

QM is just a refinement of Newton's laws
 
  • #20
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T@P said:
and yet light moves on its own at an unchanging speed forever. doesn't this contradict a lot of things? isn't it also a sort of pmm?

Not at all. As long as it is traveling it is not doing work.
 
  • #21
Chi Meson
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T@P:

Your statement indicates a lack of understanding of what energy is. This is probably why you are not yet convinced. You are glued to the pre-Galilean idea that to be continuously moving requires a continuous input of energy.

ANY thing that does not encounter a force from something else will travel at an unchanging speed forever.THis is inertia, not PMM.
 
  • #22
bino
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a magnet sitting on a fridge is doing work without anything being put into it.
 
  • #23
Tide
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bino said:
a magnet sitting on a fridge is doing work without anything being put into it.

How do you figure that? Neither the magnet nor the refrigerator are moving.
 
  • #24
Chi Meson
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bino said:
a magnet sitting on a fridge is doing work without anything being put into it.
Again: another case of the statement proving that the person does not understand what they are talking about.

It's like people who think they can speak French because they have listened to other people speaking French. Now they go to a cafe in Paris and arguing with the local folk, telling them "I guess you just don't know how to speak French."

Energy and work are specific things, and from several years of teaching I can tell you that they are NOT what people thought they were before they took a Physics course. Do not use the language of physics to argue against physics until you understand the language.
 
  • #25
mathlete
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Does QM work at low velocities? If not, what makes QM more "correct" than Newton?
 
  • #26
bino
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the magnet is doing work by not falling to the ground.
 
  • #27
Gokul43201
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mathlete said:
Does QM work at low velocities? If not, what makes QM more "correct" than Newton?

Yes, QM works at low velocities (in fact, it's harder to make it work at velocities approaching c) but Newtonian mechanics does not work at small length scales. When you get down to molecular sizes, you can no longer use the rules of Newtonian/Classical Mechanics to calculate behaviors. You need the new rules of QM.

On the other hand, as the length scales increase, QM results approach classical results. This is a requirement for all new theories; that their results match those of an established theory in the regime that the old theory is accurate - and it's called the Correspondence Principle.
 
  • #28
russ_watters
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T@P said:
and yet light moves on its own at an unchanging speed forever. doesn't this contradict a lot of things? isn't it also a sort of pmm?
No. Thats completely different. First off, its not a machine....
bino said:
the magnet is doing work by not falling to the ground.
bino, if you're really serious, google the definition of "work."
 
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  • #29
Gokul43201
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GAWB said:
You're an imbecile.

GAWB, what does this achieve ?
 
  • #30
bino
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sorry i used the wrong analogy a magnet does work by pulling metal toward it.
i am wrong about it just sitting on the fridge.
 
  • #31
bino
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"If the magnet is attracting something from a distance, work is being done by the magnet itself, but this attraction reduces the free energy of the system, something that everything in the Universe is perpetually trying to do. It turns out that the magnet can lower the total free energy of the system if it pulls something that is easy to magnetise, like a paperclip or a piece of iron, closer to it. The loss of free energy is equal to the work done by bringing the paper clip closer to the magnet."
Jon Makar , University of Durham
 
  • #32
GAWB
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bino said:
sorry i used the wrong analogy a magnet does work by pulling metal toward it.

:rolleyes:
 
  • #33
siddharth
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Mk said:
The Brownian Rachet is a perpetual motion machine postulated by Richard Feynman in a physics lecture at the California Institute of Technology on May 11, 1962 as an illustration of the laws of thermodynamics.

The device consists of a gear with a ratchet, that vibrates under Brownian motion (hence the name) in a heat bath. The idea is that motion in one direction is allowed by the ratchet, and motion in the opposite direction is prevented. Thus, it might be reasoned, the gear will rotate with a small force continuously in one direction.

How does this not work?


I have another situation. Consider a cylinder filled with some gas at a given temperature. At the end of the cylinder is a glass door. On the other side of this door is another cylinder at a much higher temperature. Operating the glass door is a man. (Note that the man is indeed a part of the system). It is known that different molecules of the gas have different velocities. What the man does is that whenever a really fast molecule comes, he opens the door. But when a slower molecule comes he does nothing. Also he makes sure that no molecule from the second piston enters the first. Thus all the fast molecules are now transferred to the second piston and the temperature further increases. Does this not disobey the laws of entropy?
 
  • #34
imabug
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siddharth said:
I have another situation. Consider a cylinder filled with some gas at a given temperature. At the end of the cylinder is a glass door. On the other side of this door is another cylinder at a much higher temperature. Operating the glass door is a man. (Note that the man is indeed a part of the system). It is known that different molecules of the gas have different velocities. What the man does is that whenever a really fast molecule comes, he opens the door. But when a slower molecule comes he does nothing. Also he makes sure that no molecule from the second piston enters the first. Thus all the fast molecules are now transferred to the second piston and the temperature further increases. Does this not disobey the laws of entropy?
Maxwell's http://en.wikipedia.org/wiki/Maxwell's_demon]Demon.[/PLAIN] [Broken]
 
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  • #35
siddharth
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That is fantastic!! I never knew that such a paradox already existed!
Anyway the answer to this assumes that erasing the information is a irreversible process. But how could that be? After all forgetting something is a spontaneous process which does not need any external energy. So how will the entropy then increase?
 

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