perpetuum mobile =/= energy generation

[Lukraak_Sisser]

This is something I think a lot of people seem to get wrong (based on earlier threads and spam emails).

A perpetuum mobile is a construct that is physically theorethically possible.
IF you were to be able to construct something of totally frictionless materials and IF you were able to place that machine somewhere there are absolutely no interfering forces (so no gravity, no photons, no stray atoms hitting it etc.) THEN any energy you put into the device will stay in the device.

However what such a device does never ever do, is generate energy. You can only tap whatever energy is put in externally, so as a power generator they are inherently pointless, at best just transferring power from A to B, usually losing power in the process, as for harvesting power some form of interaction needs to be introduced.

 

[Aepervius]

Indeed. but most of what is touted today of the net, is not a PMM, but Free Energy Devices, Or Cold Fusion devices. Look at PESN directory for example.

 

[quarky]

There are lots of you-tubes of free energy devices. They are all hilarious. Many are very complicated, and the 'inventors' are quite sincere, which is sad.

 

[sol invictus]

This is something I think a lot of people seem to get wrong (based on earlier threads and spam emails).

A perpetuum mobile is a construct that is physically theorethically possible.
IF you were to be able to construct something of totally frictionless materials and IF you were able to place that machine somewhere there are absolutely no interfering forces (so no gravity, no photons, no stray atoms hitting it etc.) THEN any energy you put into the device will stay in the device.

However what such a device does never ever do, is generate energy. You can only tap whatever energy is put in externally, so as a power generator they are inherently pointless, at best just transferring power from A to B, usually losing power in the process, as for harvesting power some form of interaction needs to be introduced.

People often classify (putative) perpetual motion machines into two kinds:

The first kind violates conservation of energy and would constitute a free energy device.

The second kind conserves energy, but violates the second law of thermodynamics (that entropy increases with time except in equilibrium). If it makes the entropy decrease, it can be used to extract useful energy from thermalized systems, which makes it effectively a free energy device.

You're asserting PMs of the second kind are "theoretically possible" - but they aren't, any more than the first kind is.

To be fair, in your example the entropy is constant, not decreasing - so the second law is saturated rather than strictly violated. According to wiki, a machine that runs forever because of inertia and lack of friction is called a perpetual motion machine of the third kind, although I've never heard that term used.

In any case, we know even the "third" type cannot exist except in perfect equilibrium, because the rate of change of entropy is strictly positive (cannot be zero) away from equilibrium. That means any such machine is physically impossible, unless "machine" is simply another word for perfect thermal equilibrium.

 

[Cuddles]

This is something I think a lot of people seem to get wrong (based on earlier threads and spam emails).

A perpetuum mobile is a construct that is physically theorethically possible.
IF you were to be able to construct something of totally frictionless materials and IF you were able to place that machine somewhere there are absolutely no interfering forces (so no gravity, no photons, no stray atoms hitting it etc.) THEN any energy you put into the device will stay in the device.

However what such a device does never ever do, is generate energy. You can only tap whatever energy is put in externally, so as a power generator they are inherently pointless, at best just transferring power from A to B, usually losing power in the process, as for harvesting power some form of interaction needs to be introduced.

In addition to what Sol said, you're making the same mistake many people make when looking at various different woo - you're looking at what could be theoretically possible if looked at in a rational manner rather than what is actually claimed. In this case, even if it were theoretically possible to have a perfectly frictionless device (how would you build it, exactly?) completely isolated from the rest of the universe, that's not something anyone has actually claimed to have. All the actual claims of perpetual motion have been here on Earth with plenty of friction and other losses. And in those cases, they absolutely could be used to generate free energy, since all you'd have to do would be reduce the losses slightly and you'd be free to extract and use all the saved energy that the device must still be generating.

 

[ehcks]

Frictionless materials can not exist.

Also, the gravitational force has an infinite range, even though it decreases exponentially. You can't find a position in space with no gravitational interference at all.

 

[sol invictus]

Frictionless materials can not exist.

Also, the gravitational force has an infinite range, even though it decreases exponentially. You can't find a position in space with no gravitational interference at all.

It doesn't decrease exponentially, it decreases as a power (1/distance^2 for weak fields).

But apart from that, you're correct and it's a very good point - frictionless materials are physically impossible, they violate the laws of physics. That would be true even without gravity, but gravity alone suffices to make it true. Even the ideal two-body problem in gravity has friction - energy loss to gravitational radiation means the two black holes follow a spiral orbit rather than a closed elliptical one, and will eventually collide and merge to the state of maximum entropy (a single black hole).

 

[ben m]

It doesn't decrease exponentially, it decreases as a power (1/distance^2 for weak fields).

But apart from that, you're correct and it's a very good point - frictionless materials are physically impossible, they violate the laws of physics. That would be true even without gravity, but gravity alone suffices to make it true. Even the ideal two-body problem in gravity has friction - energy loss to gravitational radiation means the two black holes follow a spiral orbit rather than a closed elliptical one, and will eventually collide and merge to the state of maximum entropy (a single black hole).

Gravity alone will not, however, slow down an axially-symmetric spinning object. (Blackbody radiation---or Hawking radiation---will.)

 

[sol invictus]

Gravity alone will not, however, slow down an axially-symmetric spinning object. (Blackbody radiation---or Hawking radiation---will.)

I suppose if it's perfectly axially symmetric and there are no other bodies exerting forces on it, that's true (the symmetry prevents classical gravitational waves). And if it's an extremal Kerr black hole, not even Hawking radiation slows it down, since it has zero temperature (but I think it can still decay via some sort of Schwinger/superradiance process for angular momentum).

But the real world is quantum mechanical, and that means no perfect axial symmetry.

 

[Tubbythin]

To be fair, in your example the entropy is constant, not decreasing - so the second law is saturated rather than strictly violated. According to wiki, a machine that runs forever because of inertia and lack of friction is called a perpetual motion machine of the third kind, although I've never heard that term used.

I would have thought one of third kind would be one that violated the third law of thermodynamics.

 

[sol invictus]

I would have thought one of third kind would be one that violated the third law of thermodynamics.

Not according to wiki. Anyway, there is no such law.

 

[Lukraak_Sisser]

I do realize that no PPM machine can ever be built or exist in the known universe, my point was merely that IF someone managed to build one, it still wouldn't generate energy.

Nice to learn a bit more about physics though, I appreciate that.

It's depressing to see that at least one person in this thread not only does not understand what its about, but seems to see it as a way to sell something that's physically impossible.

 

[Tubbythin]

Not according to wiki. Anyway, there is no such law.

There isn't?

 

[Aepervius]

It's depressing to see that at least one person in this thread not only does not understand what its about, but seems to see it as a way to sell something that's physically impossible.

I was wondering about who you are talking about. Untila careful reading showed me that one of the person I have on "ignore" posted .

 

[quarky]

Years ago, I heard of a flywheel that spun for 2 years. It was charged up to 10,000 rpms (it was a light, fiber wheel), in an evacuated container of helium, on magnetic bearings.
Obviously not pm or anything like it, but impressive, none the less.

 

[sol invictus]

There isn't?

The "third law", at least as sometimes stated, says that the entropy at zero temperature is zero. But that is false - the entropy at zero temperature isn't necessarily zero, it's the log of the degeneracy of the ground state, which need not be zero (although it does need to be the log of an integer).

Sometimes people say the third law is that the entropy is minimized near T=0 - but that's dangerous too, because we know the entropy can decrease with temperature under certain circumstances.

 

[Tubbythin]

The "third law", at least as sometimes stated, says that the entropy at zero temperature is zero. But that is false - the entropy at zero temperature isn't necessarily zero, it's the log of the degeneracy of the ground state, which need not be zero (although it does need to be the log of an integer).

Sometimes people say the third law is that the entropy is minimized near T=0 - but that's dangerous too, because we know the entropy can decrease with temperature under certain circumstances.

I'm not sure this is an argument that the third law doesn't exist. Just that the law isn't strictly true. But the same could be said about Newton's law of gravity or Ohm's law. I can't imagine you'd deny the existence of either of these though.