# Isobaric Process - Entropy in Terms of Pressure and Volume

Written by Jerry Ratzlaff on . Posted in Thermodynamics

Isobaric process is a thermodynamic process where the pressure is kept constant, $$\Delta p = 0$$.

## Isobaric process - entropy in terms of pressure and volume Formulas

 $$\large{ S = \Delta S \; \left( - n\; R \right) \; \left[ ln \left( \frac{p_f}{p_i} \right) \right] }$$ $$\large{ S = \Delta S \; \left( n\; R \right) \; \left[ ln \left( \frac{V_f}{V_i} \right) \right] }$$

### Where:

 Units English Metric $$\large{ S }$$ = entropy $$\large{\frac{Btu}{lbm-R}}$$ $$\large{\frac{kJ}{kg-K}}$$ $$\large{ \Delta S }$$ = change in entropy $$\large{\frac{Btu}{lbm-R}}$$ $$\large{\frac{kJ}{kg-K}}$$ $$\large{ R }$$ = molar gas constant $$\large{ \frac{lbf-ft}{lbmol-R} }$$ $$\large{ \frac{J}{kmol-K} }$$ $$\large{ ln }$$ = natural logarithm $$\large{dimensionless}$$ $$\large{ n }$$ = number of moles $$\large{dimensionless}$$ $$\large{ p_f }$$ = final pressure $$\large{\frac{lbf}{in^2}}$$ $$\large{Pa}$$ $$\large{ p_i }$$ = initial pressure $$\large{\frac{lbf}{in^2}}$$ $$\large{Pa}$$ $$\large{ V_f }$$ = final volume $$\large{in^3}$$ $$\large{mm^3}$$ $$\large{ V_i }$$ = initial volume $$\large{in^3}$$ $$\large{mm^3}$$