Physics Help Forum Heat equation

 Thermodynamics and Fluid Mechanics Thermodynamics and Fluid Mechanics Physics Help Forum

 Feb 17th 2014, 06:55 AM #1 Junior Member   Join Date: Mar 2009 Posts: 3 Heat equation I understand that dq = du + dw. However, there is a version of this equation that goes like this: dq = c_v * dT + pdV I don't understand why the pdV term isn't zero since we are considering volume to be constant. Isn't that what using c_v implies? Please explain. Thank you in advance.
 Feb 18th 2014, 04:42 AM #2 Physics Team     Join Date: Jun 2010 Location: Morristown, NJ USA Posts: 2,347 Such is the nature of partial differential equations. The first term with C_v assumes constant volume, and the second term PdV assumes constant temperature. In essence the equation adds the two effects of changing volume and temp as separate terms. Last edited by ChipB; Feb 18th 2014 at 08:20 AM.
 Feb 19th 2014, 12:15 PM #3 Senior Member   Join Date: Jun 2010 Location: NC Posts: 417 This is always confusing... Hello Princess, Many students get confused by cv. Here is the short answer. cv is a substance property. Values of cv can be (as indicated) determined by a process during which "v" is held constant. Also cv can be calculated. However, once cv is determined at a state, the value belongs to that state and has that value no matter what path is taken through it. Hope this helps... I'm out to dinner... no time. Here are some problems perhaps of interest you... http://www.thermospokenhere.com/tsh/preview.php TSH

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