How do I determine entropy change between two states if given the two temperatures and the two pressures?
For instance when one mole of a monoatomic ideal gas is taken around a Carnot cycle. At point A, the pressure is P1 and the temperature is 500 K. At point B, the pressure is P2 (P1=2*P2) and the temperature is still 500 K. The only information given about point C in the cycle is that it has a temperature of 400 K.
For instance when one mole of a monoatomic ideal gas is taken around a Carnot cycle. At point A, the pressure is P1 and the temperature is 500 K. At point B, the pressure is P2 (P1=2*P2) and the temperature is still 500 K. The only information given about point C in the cycle is that it has a temperature of 400 K.
1 Answer
CONDITIONS OF THE PROCESS
Note that from point
DETERMINING OTHER KNOWN QUANTITIES
Next, consider the ideal gas law for
#\mathbf(PV = nRT)#
#-> PV = RT#
#-># #P_2V_2 = RT#
#-># #P_1V_1 = 2P_2V_1 = RT#
We see that these are equal:
#2cancel(P_2)V_1 = cancel(P_2)V_2#
FINDING SOMETHING WE DO KNOW
We know how the volume changed. Let's focus on that right now. Recall that the change in volume relates back to the expression for reversible (efficient) work:
#\mathbf(w_"rev" = -int_(V_1)^(V_2) PdV)#
Now, since the pressure is NOT constant, let's substitute for
#color(blue)(w_"rev") = -int_(V_1)^(V_2) (RT)/VdV#
# = -RTint_(V_1)^(V_2) 1/VdV#
#= -RT(ln|V_2| - ln|V_1|)#
#= color(blue)(-RT ln|V_2/V_1|)#
At this point, we can find the work. Great. So how does it relate back to the entropy?
RELATING WORK BACK TO ENTROPY
Consider the first law of thermodynamics:
#\mathbf(DeltaU = q_"rev" + w_"rev")#
For an isothermal process (like here!),
Any monatomic ideal gas would have
Thus:
#color(green)(q_"rev" = -w_"rev")#
Next, you'll have to recall that one of the definitions of entropy is:
#\mathbf(DeltaS = (q_"rev")/T)#
where
#DeltaS = (-w_"rev")/T#
#= (Rcancel(T) ln|V_2/V_1|)/cancel(T)#
#= Rln|V_2/V_1|#
Now, we said that the volume of the gas doubled because the pressure of the gas halved at a constant temperature, so naturally we expect the entropy to be positive (as it should be).
What we get for
#color(blue)(DeltaS = Rln2 > 0)#