Re: [Phys-l] Greenhouse effect / 2nd law

[John Denker <jsd@av8n.com>]

On 04/25/2011 02:43 PM, Folkerts, Timothy J wrote:
> I have been involved in a few discussions of global warming.  There
> seems to be a school of thought that has concluded that second law
> precludes the greenhouse effect.  They are convinced that IR cannot
> go from a cold object to a hot object.
>
> For example, here is one incarnation ...
>
> "Over and over again the interpretation goes something like this…..
> A hot object transfers heat to a colder surface for example 100J.
> The colder surface transfer heat to the warmer surface say 99J.
> This they say satisfies the second law of thermodynamics.
> Their reasoning is that the NET HEAT is from hot to cold.
> I hope by now everyone who has paid attention knows that this is nonsense."

Gaaack.

Minor point:  I don't see what that has to do with the greenhouse 
effect, but I'm not going to worry about that for now.

Main point:  Try this:  Consider the following apparatus:


Position 1:
                      ....HHH....
  ......HHH......WWW......CCC......


Position 2:
                      ....HHH....
           ......HHH......WWW......CCC......


Position 3:
                      ....HHH....
                    ......HHH......WWW......CCC......

  
In position 1, the upstairs hot thing is facing a very cold thing.
The hot thing is giving off tons of IR.  The cold thing is giving
off practically no IR.

Then we slide the lower part of the apparatus all the way to
position 3.  It slides like the valve in a trumpet.

In position 3, the upstairs hot thing is facing a downstairs hot
thing that is equally hot.  They are in equilibrium.  The upstairs 
hot thing is giving off the same amount of IR as before.  It *has* 
to be giving off the same amount of IR, because it doesn't know or 
care what it is facing.

In position 2, the upstairs hot thing is facing a warm thing WWW
that is only slightly less hot than the hot thing.  This situation
is almost in equilibrium because the warm thing is giving off
almost as much IR as the hot thing.

Returning to position 3:  This is not only possible, it is the 
usual case in thermal equilibrium.  Heat out equals heat in. 

FWIW dynamic equilibrium is the usual case for chemical equilibrium, 
when a reaction sits at equilibrium without going to either extreme,
for instance when the water vapor in the head space of a sealed
bottle is in equilibrium with the liquid water in the bottle.
The rate of the reaction proceeding to the left is equal to the 
rate of the reaction proceeding to the right:

   H2O(g) <--> H2O(l)                 [1]

This is a /dynamic/ equilibrium.  It is /not/ true that the vapor
molecules are forever vapor and the liquid molecules are forever
vapor.  There is dynamic exchange.  This is closely analogous to
the exchange of IR in the original question.

If you want another example, there is

  H2O(l) <-->  H^+(aq) + OH^-(aq)

That is, there is about 10^-7 mole of dissociated water in each
liter of water, in equilibrium.  That's what pH means.  This is 
/dynamic/ equilibrium, in the sense that every molecule will 
(sooner or later) spend some time dissociated.  You can easily 
observe this using D2O.




> I have tried a few things, but does anyone here have any brilliant
> insights on how to respond to this? Or is trying to explain
> thermodynamics to a general audience on the web simply a hopeless
> task?

If your goal is 100% agreement, that's hopeless.  Those who want
to understand will find a way to understand.  Those who want to
misunderstand will find a way to misunderstand.