- #1
Kweh-chan
- 3
- 0
Homework Statement
A real gas obeys Van der Waals‟ equation, which for one mole of gas is
(p + A/V2)(V-B) = RT
and its internal energy is given by
U = CvT - A/V
where the molar heat capacity at constant volume, Cv , is independent of the
temperature and pressure. Show that the relation between the pressure p and
the volume V of the Van der Waals‟ gas during a reversible adiabatic
expansion can be written as
(p + A/V2)(V-B)[itex]\gamma[/itex] = const.
and find the expression for the parameter [itex]\gamma[/itex] in terms of Cv and R .
Homework Equations
(p + A/V2)(V-B) = RT
U = CvT - A/V
Q= U + W
The Attempt at a Solution
There is already a given solution and method for this equation. I worked through this much:
0 = U + W
0 = dU + PdV
dU = (dU/dT)dT - (dU/dV)dV = CvdT + (A/V2)dV
0 = CvdT + (P + A/V2)dV = CvdT + RT/(V-B)
∫R/(V-B) dV = -∫Cv(dT/T)
Rln(V-B) + Cvln(T) = const.
ln(V-B)R + ln(T)CV = const
ln[(V-B)R(T)CV] = const.
(V-B)R(T)CV = const.
I got stuck here and checked the method. My process was right, but according to it, the next line of work is:
(V-B)R(RT)CV = const.
I don't understand where this mystery R comes from. I've tried rearranging the ideal gas equation, and the first given equation to no avail. Could someone please explain how I get this R in the process?
Thanks!