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 Thermodynamics and Fluid Mechanics Thermodynamics and Fluid Mechanics Physics Help Forum Jul 8th 2014, 07:34 PM #1 Member   Join Date: Jun 2014 Posts: 67 Problem about entropy Combining first and second law of thermodynamics we can get the following equation TdS=dU-PextdV First question: Is this equation available for irreversible process that dS≠dQ/T? Second question:If the system temperature Tsys is smaller than the surrounding temperature Tsur, which temperature should we put in the equation? I have this question because sometimes people use Tsur instead of Tsys (e.g. , 19:45) but the equation is supposed to describe changes in the system.   Jul 9th 2014, 12:08 PM #2 Senior Member   Join Date: Jun 2010 Location: NC Posts: 418 Gibbs Equation... Hi Kelsiu, Just some thoughts... TdS = dU -pdV is one of the Gibb's Equations (Tds = dH + Vdp is the second Gibb's Equation - for simple compressible substances). Some call the Gibb's Equations, "property relations." My understanding is these equations are not a combination of the First and Second Law. Were they a "combination" they would simply be an extension of those laws and therefore NOT constitute independent information. For the mass equation (some say "conservation of mass equation") For the momentum eqn, energy eqn ... All properties relate directly the matter of the system. The Second Law is unique in that its "system" is the universe. The properties of its equation belong some to the system, some to the surroundings. I have a page about Entropy. Sorry it is in a "draft" stage. Perhaps of use. http://www.thermospokenhere.com/wp/0...t_entropy.html Good luck with your studies! TSH   Jul 9th 2014, 06:03 PM   #3
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 Originally Posted by THERMO Spoken Here Hi Kelsiu, Just some thoughts... ....................... I have a page about Entropy. Sorry it is in a "draft" stage. Perhaps of use. http://www.thermospokenhere.com/wp/0...t_entropy.html Good luck with your studies! TSH
Thank you for the answer and very good page about entropy. I have no doubt that the equation is a property relation describing equilibrium states from end point to end points. What confuses me is that in a video lecture (, 19:45) the lecturer obvious put TdS>dU+PdV for irreversible process and specially indicate T is Tsur.

And in applying the first law work done is smaller in irreversible process than in reversible process, we cannot say dW=−pdV, but if we indicate P is Pext (assume constant external pressure for simplicity) we can put dW=−PextdV and so we can say dU=dQ-PextdV. In fact I posted this question because I guess he is doing something like putting Pext in dW=−pdV. Is it correct?  Tags entropy, problem Thread Tools Show Printable Version Email this Page Display Modes Linear Mode Switch to Hybrid Mode Switch to Threaded Mode Similar Physics Forum Discussions Thread Thread Starter Forum Replies Last Post burgess Advanced Thermodynamics 1 Jul 21st 2014 07:33 AM evan.kimia Thermodynamics and Fluid Mechanics 1 May 2nd 2010 08:52 PM arbolis Thermodynamics and Fluid Mechanics 1 Jun 7th 2009 09:54 AM arbolis Thermodynamics and Fluid Mechanics 9 Jun 6th 2009 03:36 PM einstein123 Energy and Work 3 Jan 4th 2009 06:58 AM 