conservation of total_enthalpy and total_energy
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Following conclusions based on thermodynamic definitions could be useful when checking for conservation of total_energy or total_enthalpy in a system.
Definitions: Enthalpy = h = Internal_Energy + Pressure * Volume = U + PV........... (1) Total_Energy = E = Internal_Energy + Kinetic_Energy + Potential_Energy = U + 0.5(u^2) + PE........ (2) Total_Entahlpy = H = Enthalpy + 0.5*(u^2) = h + 0.5(u^2)................. (3) Work done: Constant Volume: 1W2= 0 Constant Pressure: 1W2= Pressure * Volume_Change = P(V2-V1)........ (4) E = H - PV [from (1), (3), (2), neglecting PE]........ (5) First Law of Thermodynamics: Heat_Change = Work_done + Energy_change ===> 1Q2 = 1W2 + E2 - E1 For a constant volume process(W=0) and adiabatic process (Q=0): E2 - E1 = 0 ; change in total_energy is conserved. ===> From (5): (H2-H1) - (PV2 - PV1)=0 So, for a constant volume and adiabatic process, change in total_energy is zero. For constant pressure process: 1Q2 = P(V2- V1) + (U2- U1) + 0.5(u2^2 - u1^2) [from (5), (2),(4) and neglecting PE] 0 = H2 - H1 [from (3), adibatic process 1Q2=0] So, for a constant pressure and adiabatic process, change in total_enthalpy is zero. |
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