11. ₁. A Carnot heat engine contains 8.00 moles of oxygen gas (assume CVm= 2.5R). The temperature of the heat source is 500.0 °C and the temperature of the cold reservoir is 25.0 °C. Calculate the thermodynamic efficiency, V₁, V2, V3, V4, P3, P4, w and q for each step of the cycle and the total heat and work of the cycle if P = 20.0 bar and P₂ = 5.00 bar.
A carnot cycle consists of 4 steps.
1) reversible isothermal expansion (P1,V1,TH to P2, V2,TH)
2) reversible adiabatic expansion (P2, V2, TH to P3, V3, TC)
3) reversible isothermal compression (P3, V3, TC to P4, V4, TC)
4) reversible adiabatic compression (P4, V4, TC to P1, V1, TH)
P1 = 20 bar = 20x0.987 atm = 19.74 atm {1 bar = 0.987 atm)
P2 = 5bar = 5x 0.987atm = 4.935 atm
TH = 500oc = 500+273 = 773 K
TC = 25oc = 25+273 = 298K
Cv = 2.5R = (5/2) R
Cp = (7/2) R
= Cp/Cv = 7/5
It follows ideal gas equation
P1 V1 = nR TH
V1 = (nR TH)/ P1 = (8mole x 0.082 L atm mol-1 K-1 x 773 K) / 19.74 atm = 25.69 L
V2 = (nR TH) / P2 = (8mole x 0.082 L atm mol-1 K-1 x 773 K) / 4.935 atm = 102.75 L
again TH = Tc (as adiabatic process)
V3 = x V2 = = 70.18 L
So, P3 = = = 2.785 atm
again,
(as adiabatic process)
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