Tuesday, February 10, 2009

Can The Second Law Of Thermodynamics Be Circumvented?

I'm just linkin'. I'm not sayin'...


Anonymous Anonymous said...

Can you say crackpot? Apparently alt.science is an especially well named newsgroup.

(And if you think there's any chance that the guy is right--a quick scan finds an appeal to spring-loaded doors\t. Spring-loaded doors is exactly what maxwell's demon is all about.)


3:05 AM  
Blogger Winston Smith said...

Oh yeah, I'm well aware that it's a crackpot view. And I'm certainly no physicist. But I'm frequently surprised at how un-convincing (admittedly, from the perspective of the non-expert) the arguments in support of the second law are. I'm not saying that it's false, of course. I'm just saying that it sounds more like a hypothesis than physicists tend to admit.

4:56 AM  
Blogger Jim Bales said...

Second law arguments can be tricky. However, I suspect that the scheme proposed would fail.

Why? Well, the author assumes that that there is good heat transfer across the wall, and so the gasses on either side are at the same temperature. This means that the wall itself, and the trapdoor/spring mechanism are also at that temperature.

Temperature is a measure of the average kinetic energy per particle. Since the wall, the trapdoor, the spring, and the balls all have the same temperature, they are all jittering around with the same energy per particle. Since the trapdoor is at the same temperature as the balls, it, too, is making random motions, opening and closing randomly. The trapdoor must have a mass comparable to that of a ball (else the ball could not open the door), so it must take on a range of velocities comparable to that of the balls.

(Why? Because the kinetic energy depends on the mass of the object and its velocity. Since the average kinetic energy of the balls and the trapdoor are the same, and their masses are comparable, the velocity range must also be comparable.

The spring could be made so stiff that the trapdoor can't move appreciably under its random motions, but then the balls won't have enough energy to open it.

Therefore, the trapdoor can neither reliably block the passage of balls in one direction, nor can it reliably permit the passage of balls in the other direction.

Thus, the trapdoor/spring does not give the desired one-way permeability, and the scheme won't work.

This highlights a problem in trying to understand second-law phenomena at the micro-scale. Our intuition as to how things behave (balls, doors, springs, etc.) lead us astray!


12:39 PM  
Blogger Aa said...

Simply open the 'box'* and add energy.

2nd law circumvented.

It happens every day on Earth.

Am I missing something? Or perhaps I'm simplifying too much...

*(make it an open system)

11:15 AM  
Blogger Jim Bales said...

Open the box (i.e., make it an open system) and add energy gets around the *1st* law -- conservation of energy.

The author wants to get around the *second* law -- in particular, the second law formulated as the Carnot relationship, which says that extracting energy from a heat engine requires that the thermal source be hotter than the thermal sink, and that the efficiency goes up as that temperature difference increases.

As I noted in my earlier post, I don't believe this scheme works. And, although I don't consider my comment a definitive disproof, the second law is sufficiently well entrenched that I don't consider it worth my effort to try to create a definitive disproof. However, if the author has an analysis showing that the thermal motions of the trapdoor and spring are small enough to permit the scheme to work, I'd like to see it.


9:50 AM  
Blogger Winston Smith said...

Yeah, I've been meaning to respond in the way Jim just did, Aa.

As I understand it, one way to think of the point of keeping the system closed is that it then emulates the universe as a whole, since there's nowhere for the extra energy to come from in the case of the whole U.

Only if we can get (reliable? Non-random? Non-temporary?) increases in order in the whole system can we say that the second law is false.

Is that right Jim?

10:41 AM  

Post a Comment

Subscribe to Post Comments [Atom]

<< Home