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 Thermodynamics and Fluid Mechanics Thermodynamics and Fluid Mechanics Physics Help Forum Jan 4th 2019, 11:22 AM #1 Junior Member   Join Date: Jan 2019 Posts: 2 Enthalpy formula Hi guys, I am relatively new in Thermodynamics as I am an electrical engineer. Right now, I am looking for an exact enthalpy calculation formula. The Gas is N2 with Temperature 27C and Pressure 124.73 bar. From a paper the enthalphy is 288.28 kJ/kg. I also found the same answer from online calculator ( http://www.peacesoftware.de/einigewe...tickstoff.php7 ). However, I don't know how to calculate it. I can describe all attempts enthusiastically that I have done but I am afraid it might waste your time. Could you please help me to find the formula for this enthalphy? Thank's in advanced Donny   Jan 5th 2019, 10:57 AM #2 Junior Member   Join Date: Dec 2018 Location: The 'Milky Way' Galaxy Posts: 13 As far as I know absolute value of enthalpy can't be calculated. Only change in enthalpy of rxns can be calculated. Donny likes this.   Jan 6th 2019, 08:49 AM #3 Member   Join Date: Sep 2014 Location: Brasília, DF - Brazil Posts: 32 Chapter suggestion Donny, I suggest you read Chapter 45 of The Feynman Lectures on Physics - Vol I topsquark likes this. __________________ Work on: General thermal systems Cryogenics Micro-drop fluid mechanics   Jan 8th 2019, 01:38 AM   #4
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 Donny, I suggest you read Chapter 45 of The Feynman Lectures on Physics - Vol I

Hi mscfd,

Thank you for your suggestion. I read the book and I can see the point. However, I am lost in translation in equation (45.1). I don't know how to elaborate delta f become delta x and delta y.   Jan 8th 2019, 03:01 AM   #5
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 Hi mscfd, Thank you for your suggestion. I read the book and I can see the point. However, I am lost in translation in equation (45.1). I don't know how to elaborate delta f become delta x and delta y.
Donny, eq. 45.1 is the definition of the total derivative. You can easily find a topic about this content in a Calculus book.

$\displaystyle f(x_1,x_2,x_2,....,x_n)$
$\displaystyle df=dx_1\frac{\partial f}{\partial x_1}+dx_2\frac{\partial f}{\partial x_2}+dx_3\frac{\partial f}{\partial x_3}+.....+dx_n\frac{\partial f}{\partial x_n}$

Anyway, if the internal energy can be defined as a function of the temperature and volume (this is not the only form): $\displaystyle U(T,V)$

$\displaystyle dU=dT\left(\frac{\partial U}{\partial T}\right)_V+dV\left(\frac{\partial U}{\partial V}\right)_T$

The letters under the parentheses mean that the variation of the derivative is for V or T constant. In pure mathematics this information is redundant, since it is a partial derivative, for a physicist or an engineer who is in the laboratory measuring, it is important to know which property should remain constant during the variation.
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Last edited by mscfd; Jan 8th 2019 at 05:09 AM.  Tags enthalpy, formula, pressure, temperature 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 tom89 Advanced Thermodynamics 2 Jun 4th 2018 09:49 AM Rome Thermodynamics and Fluid Mechanics 1 Oct 9th 2017 12:23 PM simina11 Thermodynamics and Fluid Mechanics 2 Feb 22nd 2015 05:22 PM rimmer Thermodynamics and Fluid Mechanics 1 Mar 9th 2010 03:54 AM Natla Energy and Work 0 Nov 6th 2008 04:19 PM