APPLIED THERMODYNAMICS 4.8
Relation between Cp & Cv :
Consider a perfect gas being heated at constant pressure from T1 to T2. According to non flow equation Q1-2 = (U2 – U1) + W 1-2 ------ (1) Also for perfect gas, U2 – U1 = m Cv (T2 – T1) In constant pressure process, the work done by the fluid, W 1-2 = p (V2 – V1) = m R (T2 – T1) [ pV = mRT, and p1 = p2 = p in this case] Heat transferred in the constant pressure process is given as Q1-2 = m Cp (T2 – T1) By equating the three expressions in (1) m Cp (T2 – T1) = m Cv (T2 – T1) + m R (T2 – T1) m Cp (T2 – T1) = (T2 – T1) (m Cv + m R) m Cp = m(Cv + R) Therefore Cp = Cv + R or Cp – Cv = R Dividing both sides by Cv, we get
Similarly, dividing both sides by Cp, we get
S.A.Q.5: How the enthalpy changes of a chemical reaction is calculated? _____________________________________________________________________________ _____________________________________________________________________________ ______________________________________________________________ Specific Heat of a Pure Substance Specific heat at constant volume Specific heat at constant volume, cv of a pure substance is defined as the ratio of change of the specific internal energy of the substance with temperature, when the specific volume is held constant. Symbolically it is written as,
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