# What is wrong with a perpetual motion machine?

the magnet is doing work by not falling to the ground.

Gokul43201
Staff Emeritus
Gold Member
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.

russ_watters
Mentor
T@P said:
and yet light moves on its own at an unchanging speed forever. doesnt this contradict alot of things? isnt 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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Gokul43201
Staff Emeritus
Gold Member
GAWB said:
You're an imbecile.

GAWB, what does this achieve ?

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.

"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

bino said:
sorry i used the wrong analogy a magnet does work by pulling metal toward it.

siddharth
Homework Helper
Gold Member
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?

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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siddharth
Homework Helper
Gold Member
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?

krab
bino said:
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.
You are (now) quite right. But what has this to do with Perpetual Motion?

The_Thinker said:
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?

You can't prove a theory like Newtons wrong. You may only present a theory that is more correct and thereby showing how it is less accurate. But Newons laws present the very fundamentals that we use to quantify, so it will never be proven wrong, unless you give up comparison and counting in general itself. Without those two things, which are at the heart of his laws, you can't anthying sensable to humanity.

If you are patient and study all those laws, it will bring you to the energy concept. You'll learn about the different forms energy takes. When you account for all these forms, you'll begin to see what it means. It gets clearer and clearer as you go further.

Lets humor ourselves and say we created a perpetual motion machine. It would be a machine that would have motion for ever right? This means a machine must have motion, but no force can act upon it. That would mean it would move perpetually.

We make the machine. It's two mirror with a few photons bouncing back and forth. The photons stay in the mirrors perfectly. No outside force acts upon the photons or the mirrors. That's a perpetual motion machine.

What happens when you try to use some of the energy? You must take out a packet of energy. This means a force must acts upon your perpetual machine.

To grab a photon from your machine, you must exert a force upon your machine. Once you exert a force upon it you are exerting energy upon the machine.

So how does any machine ever give energy perpetually if first a force must act upon it to get energy from it?

______________________________
Nothing happens without a collision.

if someone were able to make some kind device that could harness that pulling or pushing of magnets then we could get a perpetual machine.

krab
So now you no longer believe the Makar quote you gave in post 31?

Nah, magnets run out of power, it takes a while, but yes they do.

Where do electrons get their constant orbital motion from? In superconducters how does electricity flow without resistance? Superfluidity, how do superfluids flow without friction? In the quantum world, their is no friction?

Magnets lose their properties eventually.... This i did not know, is it true...?

krab
Magnets may eventually lose their field, but that is entirely beside the point. The point is that the field is conservative. That means it takes a certain energy to set up a configuration of separated magnet and paper clip (e.g.). You get some of that energy back when the paper clip moves toward the magnet, but then you'll have to input the energy to separate them again. This is essentially how a motor works. Costly electrical energy in, mechanical energy out. There is no free lunch and no PPM.

but if you were able the set them up in such a way so that the magnets are always being pushed or pulled for as long as the magnets have they power. then you could get free energy. granted is would not be forever because magnets dont last that long but it would be a long time.

krab