On Fri, Feb 03, 2012 at 08:50:40PM +0100, Evgenii Rudnyi wrote: > > I guess that you have never done a lab in experimental > thermodynamics. There are classical experiment where people measure > heat of combustion, heat capacity, equilibrium pressure, equilibrium > constants and then determine the entropy. If you do it, you see that > you can measure the entropy the same way as other properties, there > is no difference. A good example to this end is JANAF Thermochemical > Tables (Joint Army-Naval-Air Force Thermochemical Tables). You will > find a pdf here > > http://www.nist.gov/data/PDFfiles/jpcrdM9.pdf > > It is about 230 Mb but I guess it is doable to download it. Please > open it and explain what is the difference between the tabulated > entropy and other properties there. How your personal viewpoint on a > thermodynamic system will influence numerical values of the entropy > tabulated in JANAF? What is the difference with the mass or length? > I do not see it. > > You see, the JANAF Tables has started by military. They needed it to > compute for example the combustion process in rockets and they have > been successful. What part then in a rocket is context dependent? > > This is the main problem with the books on entropy and information. > They do not consider thermodynamic tables, they do not work out > simple thermodynamic examples. For example let us consider the next > problem: > > ----------------------------------------------- > Problem. Given temperature, pressure, and initial number of moles of > NH3, N2 and H2, compute the equilibrium composition. > > To solve the problem one should find thermodynamic properties of > NH3, N2 and H2 for example in the JANAF Tables and then compute the > equilibrium constant. > > From thermodynamics tables (all values are molar values for the > standard pressure 1 bar, I have omitted the symbol o for simplicity but > it is very important not to forget it): > > Del_f_H_298(NH3), S_298(NH3), Cp(NH3), Del_f_H_298(N2), S_298(N2), > Cp(N2), Del_f_H_298(H2), S_298(H2), Cp(H2) > > 2NH3 = N2 + 3H2 > > Del_H_r_298 = Del_f_H_298(N2) + 3 Del_f_H_298(H2) - 2 Del_f_H_298(NH3) > > Del_S_r_298 = S_298(N2) + 3 S_298(H2) - 2 S_298(NH3) > > Del_Cp_r = Cp(N2) + 3 Cp(H2) - 2 Cp(NH3) > > To make life simple, I will assume below that Del_Cp_r = 0, but it is > not a big deal to extend the equations to include heat capacities as well. > > Del_G_r_T = Del_H_r_298 - T Del_S_r_298 > > Del_G_r_T = - R T ln Kp > > When Kp, total pressure and the initial number of moles are given, > it is rather straightforward to compute equilibrium composition. If > you need help, please just let me know. > ----------------------------------------------- > > So, the entropy is there. What is context dependent here? Where is > the difference with mass and length? > > Evgenii >
The context is there - you will just have to look for it. I rather suspect that use of these tables refers to homogenous bulk samples of the material, in thermal equilibrium with a heat bath at some given temperature. If we were to take you at face value, we would have to conclude that entropy is ill-defined in nonequlibrium systems. More to the point - consider milling whatever material you have chosen into small particles. Then consider what happens to a container of the stuff in the Earth's gravity well, compared with the microgravity situation on the ISS. In the former, the stuff forms a pile on the bottom of the container - in the latter, the stuff will be more or less uniformly distributed throughout the containers volume. In the former case, shaking the container will flatten the pile - but at all stages the material is in thermal equilibrium. In your "thermodynamic context", the entropy is the same throughout. It only depends on bulk material properties, and temperature. But most physicists would say that the milled material is in a higher entropy state in microgravity, and that shaking the pile in Earth's gravity raises the entropy. Furthermore, lets assume that the particles are milled in the form of tiny "Penrose replicators" (named after Lionel Penrose, Roger's dad). When shaken, these particles stick together, forming quite specific structures that replicate, entraining all the replicators in the material. (http://docs.huihoo.com/reprap/Revolutionary.pdf). Most physicists would say that shaking a container of Penrose replicators actually reduces the system's entropy. Yet, the thermodynamic entropy of the JNAF context does not change, as that only depends on bulk material properties. We can follow your line of thinking, and have a word entropy that is only useful in certain contexts, then we'll need to make up a different word for other contexts. Alternatively, we can have a word that applies over all macroscopic contexts, and explicitly qualify what that context is. The underlying concept is the same in all cases though. It appears to me, that standard scientific usage has become to use the same word for that concept, rather than coin different words to describe the same concept in all the possible different contexts that there are. -- ---------------------------------------------------------------------------- Prof Russell Standish Phone 0425 253119 (mobile) Principal, High Performance Coders Visiting Professor of Mathematics hpco...@hpcoders.com.au University of New South Wales http://www.hpcoders.com.au ---------------------------------------------------------------------------- -- You received this message because you are subscribed to the Google Groups "Everything List" group. To post to this group, send email to everything-list@googlegroups.com. To unsubscribe from this group, send email to everything-list+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/everything-list?hl=en.
