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linford86
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Homework Statement
This is from "Equilibrium Statistical Physics" by Plischke and Bergerson, problem 1.1:
"Consider a Carnot engine working between reservoirs at temperatures T1 and T2. The working substance is an ideal gas obeying the equation of state [ [tex]PV=Nk_BT[/tex] ], which may be taken to be a definition of a temperature scale. Show explicitly that the the efficiency of the cycle is given by
[tex]\eta=1-\frac{T_2}{T_1}[/tex]
where [tex]T_1>T_2[/tex].
Homework Equations
The first law of thermodynamics, the ideal gas law, and a few other things. I'll introduce these in my partial solution; I hope that's not inappropriate.
The Attempt at a Solution
Well, I began by noting that the efficiency is defined as [tex]\eta=\frac{W}{Q_1}[/tex]. We also know that [tex]W=Q_1+Q_2[/tex], using the convention that heat leaving the system is negative. For the sake of reference, I will define that the Carnot engine moves through points A, B, C, and D, and that AB is isothermal at temperature [tex]T_1[/tex], BC is adiabatic, CD is isothermal at temperature [tex]T_2[/tex], and DA is adiabatic. For the process from A to B, since it is isothermal, the internal energy does not change. By the first law of thermodynamics, we have that:
[tex]dU=0=\bar{d}Q-\bar{d}W[/tex] and, thus, [tex]\bar{d}Q=\bar{d}W[/tex]
Next, we have that [tex]\int_A^B \! dQ=\int_A^B P \, dV=Nk_BT\ln(\frac{V_B}{V_A}[/tex]. Likewise, [tex]Q_2=Nk_BT\ln(\frac{V_D}{V_C})[/tex]. Simplifying the equation for [tex]\eta[/tex], we have:
[tex]\eta=1+\frac{\ln(\frac{V_D}{V_C}}{\ln(\frac{V_D}{V_C}}[/tex]
I understand that, for an adiabatic process, [tex]TV^\gamma=(constant)[/tex] and that [tex]\gamma=\frac{C_D}{C_V}[/tex], but I have no idea how to use this information to further reduce [tex]\eta[/tex] to the desired result --
[tex]\eta=1+\frac{T_2}{T_1}[/tex]
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