Apparent 2nd law violation? (thermodynamics)

[cesiumfrog]

Gah, Curl is so annoying to talk to. He keeps telling everyone else they need to read his posts again, but hypocritically ignores what everyone else has posted. Multiple people have said the radiation sink will heat up and cease being a sink. Multiple people have said the apparatus description is unclear (being spread over several posts and changing along the way). He fixates on the single criticism that actually does sound dubious, as if that proves his revolutionary idea is beyond criticism.

Not true.
http://en.wikipedia.org/wiki/Radiation_pressure
Also, as you (should) recognize, changing the direction of propagation means the *momentum* has changed. The momentum imparted to a mirror by light is a well-understood phenomenon.

Andy, how patronising, surely we can presume we both know how to calculate radiation pressure: how much wave energy is lost to redshift as the mirror recedes, how much momentum is imparted (by the dance of EM forces on the charges composing the mirror), etc. So I'm a bit confused by your posts. Yes, the mirror is continually changing the momentum of the light beam. Consequently, momentum is perpetually being accrued to the mirror at a known rate. So we can ask, how much power is involved? (A hint would be to ask, other than parity symmetry, how has the beam been altered?) How much kinetic energy has the mirror+anchor received? We know momentum is linear in velocity, but KE is not. Therefore, the lighter the mirror+anchor system, the greater the rate that it will absorb energy from the beam (even though, at least initially, the rate at which it accrues momentum is independent of this). Therefore, as we make the anchor arbitrarily heavy, the rate of energy it absorbs by this process can be made arbitrarily low. But nevermind that. Just mount the mirror on the same optical bench as the source and the receiver. Now it isn't arbitrarily low, it is exactly zero.. And by all means, let the mirror be mounted via a spring. When the device is turned on, sure, for a period energy will be lost compressing the spring, but as the spring compresses it will increase its force, approaching equilibrium with the radiation pressure, and then the elasticity will not sink any further energy. If you think about it, I believe you'll see these were red-herrings rather than the true justifications for the conclusion you're aiming toward. But if I'm not understanding you, please try to help me.

 

[Andy Resnick]

So I'm a bit confused by your posts.

Ok, I'll try again.

Extracting work from thermal radiation is trivial- that's what a solar panel does. The OP claimed (perhaps implicitly) that shaping the radiation field will somehow make this process more efficient- that is, *more work* can be extracted by a collimated beam than by an isotropic radiator- that the efficiency would then be greater than allowed by the second law of thermodynamics. Because solar panels exist.

It's the same faulty reasoning behind 'unlimited' solar concentrators, Archimedes' mirror, etc- by shaping the light beam, *more work* can be extracted (alternatively, more heat is transferred) than could ordinarily be permitted. For example, it is claimed that Archimedes' mirror was able to heat up a ship much faster than would occur by simply being in the sun- like you can burn ants. Similarly, with a solar concentrator I could boil a lot more water than I normally could, and generate much more useful work. To a certain point- given by thermodynamic arguments- this works. Ultimately, this limit is given by the optical invariant (sometimes called the etendue)- a combination of beam divergence and beam diameter.

So I focused on that essential assumption; the change in entropy of the radiation field caused by collimating a portion of a spherical field. IIRC, a collimated beam (with it's low divergence angle) has a lower entropy than a spherical beam- therefore, that loss must be compensated (at least...) by an increase in entropy somewhere else, and the only thing available (that I can think of) is the mirror.

So I proposed a form of 'workless dissipation' which allows the total entropy, before and after the reflection, to remain constant. Perhaps you have an alternative idea?

 

[Curl]

It's not as easy as pointing at the mirror and say it somehow causes an increase in entropy elsewhere. Actually the micro-state definition of entropy you learn from reading books and memorizing formulas can be misleading during thought experiments such as this.

I have another thought experiment which can separate gas particles by velocity (you pick a velocity, and all particles with speeds above this will be put on one side of a container, and the particles with lower speeds will remain in the other half of the container). It is similar to what Maxwell's daemon tries to do, except in my system there is no need for any moving parts and no work needs to be done. It seems like entropy is decreased right? Right. Is the 2nd law violated? No. It turns out that the process is not cyclic, and work CANNOT be done by this separation of gas particles.

The experiment I'm asking about in THIS thread might have some trick like that too, although I cannot tell what it is because the process seems cyclic and work CAN be extracted.

P.S. I'm not talking about solar cells like those in your calculator, I'm talking about a conducting loop (like a coil antenna) which takes advantage of the changing magnetic field present in an EM wave and uses magic called Faraday's law to set up a current. Capacitor, for example, can make use of that current to collect energy.

Also I already said that I know for a fact this cannot work, I'm trying to DISPROVE it, not prove it. However I want a legitimate disproof, I need to point out exactly why this cannot work. It helps me understand thermodynamics.

This might not even be obvious, for example in Maxwell's daemon, it had to be shown that it is IMPOSSIBLE to monitor a daemon at such scales without doing a greater amount of work. This is a deep disproof. Also the ratchet engine, it had to be show that a ratchet with such low sensitivity CANNOT exist. The answers don't come out of some thermo textbook formula, they are often deep. I'm not saying my experiment is deep, but it could be...

 

[cesiumfrog]

P.S. I'm not talking about solar cells like those in your calculator, I'm talking about a conducting loop (like a coil antenna) which takes advantage of the changing magnetic field present in an EM wave and uses magic called Faraday's law to set up a current. Capacitor, for example, can make use of that current to collect energy.

Since when does shining thermal radiation at a wire loop ever produce a useful current? That won't even work to begin with. You'll need more than a capacitor.. And I suppose it's that "more" which is critical in explaining why this process doesn't only proceed in one direction..

 

[Curl]

Finally you are on to something.

A changing B-field produces a current in a loop of wire. A current can be made to do work. Radios are a good example, although they use op amps to make the sound since the energy is very small.

EM radiation has a changing B field (derivative nonzero). It comes from Maxwell's equations.

 

[cesiumfrog]

Radios use rectifiers..
I guess you could hook your loop directly to a resistance heater, that way the analysis is simpler. Of course, it's going to be ridiculously ineffective at extracting power from the radiation even from the beginning (since it's such a bad model for a solar power collector). The larger point is that it also works in reverse: the resistance heater can absorb heat, the thermal movement of the charges therein can cause currents in the loop and create its own radiation, back toward (and limiting the cooling of) your source ball.

 

[Curl]

Don't use a resistance for heating.

The current can go (far away) to an electrical motor that can do work.

 

[cesiumfrog]

The same problem occurs, just through that different mechanism.
(I'd try to spell out the details for you, but you still haven't spelt out the details for me. Let's see, you have a ball, a parabola, a loop, and now a motor, and who knows what you'll add next? Why don't you write down in one post your entire system, and how each part is finally connected? Please attach a diagram? For one example, where goes the radiation left when it isn't all absorbed by the loop? That way we can start giving you specific answers, instead of vague ones.)

 

[Curl]

Okay when I find time I'll make a 3D model and post the binary files and rendered images.