- #1
Telemachus
- 835
- 30
The problem says: Consider an ideal gas of N particles in a spherical vessel of radius R. A force acts directly over the molecules and is directly proportional to the distance to the center of the sphere ##V(r)=\alpha r##. Calculate the pressure of the gas, and the density of particles at the surface.
So I set the Hamiltonian: ##\displaystyle H= \sum_i^N \frac{p_i^2}{2m}+\sum_i^N \alpha r_i##
Then, as the potential is independent of the momentum, I can take ##Z=\frac{1}{N!}Z_1^N##, where ##Z_1## represents the partition function for only one molecule.
Then:
##\displaystyle Z_1=\frac{1}{h^3}\int_{-\infty}^{+\infty}e^{-\beta \frac{p^2}{2m}}\int_{0}^{2\pi}\int_{0}^{\pi}\int_{0}^{R} dr d \phi d \theta r^2 \sin \theta e^{-\beta \alpha r}##
Now, the pressure in the canonical ensemble is proportional to the derivative with respect to the volume of the natural logarithm of the partition function. And the thing is I don't have any volume anywhere in there (I've done the integral, and I don't get the volume), so I get a pressure equal to zero. Something is wrong with what I've done.
Help please.
So I set the Hamiltonian: ##\displaystyle H= \sum_i^N \frac{p_i^2}{2m}+\sum_i^N \alpha r_i##
Then, as the potential is independent of the momentum, I can take ##Z=\frac{1}{N!}Z_1^N##, where ##Z_1## represents the partition function for only one molecule.
Then:
##\displaystyle Z_1=\frac{1}{h^3}\int_{-\infty}^{+\infty}e^{-\beta \frac{p^2}{2m}}\int_{0}^{2\pi}\int_{0}^{\pi}\int_{0}^{R} dr d \phi d \theta r^2 \sin \theta e^{-\beta \alpha r}##
Now, the pressure in the canonical ensemble is proportional to the derivative with respect to the volume of the natural logarithm of the partition function. And the thing is I don't have any volume anywhere in there (I've done the integral, and I don't get the volume), so I get a pressure equal to zero. Something is wrong with what I've done.
Help please.