Undergrad Is this proof of cp - cv correct

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SUMMARY

The discussion centers on the derivation of the relationship between specific heats, cp and cv, in thermodynamics. The equation cp - cv = (dU/dV)T(dV/dT)P + P(dv/dT)P is established, with the condition that dV is zero leading to simplifications in the expressions. The participants emphasize the importance of starting with the correct differential forms, specifically dU = Cv dT - [P - T(∂P/∂T)V] dV and dH = dU + d(PV) = Cp dT + [V - T(∂V/∂T)P] dP. The discussion concludes that the initial approach to the derivation was incorrect due to improper handling of the volume change.

PREREQUISITES
  • Understanding of thermodynamic concepts such as specific heat capacities (cp and cv)
  • Familiarity with differential calculus in the context of thermodynamics
  • Knowledge of partial derivatives and their applications in thermodynamic equations
  • Experience with the first law of thermodynamics and enthalpy definitions
NEXT STEPS
  • Study the derivation of the first law of thermodynamics and its implications for specific heats
  • Learn about the Maxwell relations and their applications in thermodynamic analysis
  • Explore the concept of enthalpy and its relationship to internal energy in thermodynamic processes
  • Investigate the implications of volume changes on thermodynamic equations and state functions
USEFUL FOR

This discussion is beneficial for students and professionals in thermodynamics, particularly those studying heat transfer, chemical engineering, and physical chemistry. It is especially relevant for anyone looking to deepen their understanding of specific heat capacities and their derivations.

planck999
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cp=(dU/dT)P+P(dv/dT)P
cv=(dU/dT)V
cp-cv=(dU/dT)P+P(dv/dT)P- (dU/dT)V=(dU/dV)T(dV/dT)P+P(dv/dT)P- (dU/dV)T(dV/dT)V
since dV is zero (dU/dV)T(dV/dT)V is zero.
Hence
cp-cv=(dU/dV)T(dV/dT)P+P(dv/dT)P

I expanded both dU/dT and since one of them has no change in volume it is zero. is it acceptable? Did I multiply and divide both expressions by dV?
 
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This is done incorrectly. Start with $$dU=C_vdT-\left[P-T\left(\frac{\partial P}{\partial T}\right)_V\right]dV$$and $$dH=dU+d(PV)=C_PdT+\left[V-T\left(\frac{\partial V}{\partial T}\right)_P\right]dP$$
 
Chestermiller said:
This is done incorrectly. Start with $$dU=C_vdT-\left[P-T\left(\frac{\partial P}{\partial T}\right)_V\right]dV$$and $$dH=dU+d(PV)=C_PdT+\left[V-T\left(\frac{\partial V}{\partial T}\right)_P\right]dP$$
Thanks
Chestermiller said:
This is done incorrectly. Start with $$dU=C_vdT-\left[P-T\left(\frac{\partial P}{\partial T}\right)_V\right]dV$$and $$dH=dU+d(PV)=C_PdT+\left[V-T\left(\frac{\partial V}{\partial T}\right)_P\right]dP$$
 

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