According to Van der Waals rule if T>Tinv, then repulsion works between the molecules of gas which causes a temp. decrease during expansion (Joule Thomson effect ) and when T
In neutral molecules like Ar or N2, the electron distribution is such that there is no permanent dipole or similar imbalance *on the average*. But *at each instant*, the electrons have to be somewhere, and their locations in space may be such that a temporary electric dipole moment, quadrupole moment, etc., results. If another molecule is near, the temporary electric moment will affect its electron distribution, which gives rise to a weak attraction that is approximately proportional to r^(-6) [r: center-center distance].
Look for "London forces", "van der Waals forces" or "dispersion forces" on the Internet if you want to know more about this effect.
Please note that these forces always act between molecules; this is not a matter of temperature! But whether these forces can affect the molecules of a gas enough to cause a positive Joule–Thomson effect or condensation, that depends on density and temperature.
In neutral molecules like Ar or N2, the electron distribution is such that there is no permanent dipole or similar imbalance *on the average*. But *at each instant*, the electrons have to be somewhere, and their locations in space may be such that a temporary electric dipole moment, quadrupole moment, etc., results. If another molecule is near, the temporary electric moment will affect its electron distribution, which gives rise to a weak attraction that is approximately proportional to r^(-6) [r: center-center distance].
Look for "London forces", "van der Waals forces" or "dispersion forces" on the Internet if you want to know more about this effect.
Please note that these forces always act between molecules; this is not a matter of temperature! But whether these forces can affect the molecules of a gas enough to cause a positive Joule–Thomson effect or condensation, that depends on density and temperature.
Dear Sir, but there is no description about repulsion force............
The repulsion force is caused by Pauli's principle, namely that no electrons may share the same quantum state. Let us consider helium for simplicity, which has a fully occupied 1s shell (2 electrons differing in spin quantum number only): If you place 2 helium atoms side-by-side, you have 2 electrons with the quantum state 1s/spin-up and 2 with 1s/spin-down, which is forbidden. Consequently, the two 1s niveaus “split” to form two new orbitals, sigma and sigma*. Each of these is occupied by 2 electrons (spin-up and spin-down), and so Pauli's principle is rescued.
The sigma orbital has a lower energy than the original 1s orbitals; if occupied, it causes a chemical bond. The sigma* orbital has a higher energy and causes repulsion. The size of the energy shift depends on the distance of the atoms.
The catch is that the niveau splitting is not symmetric: The energy increase of sigma* outweighs the energy decrease of sigma. Thus a pair of helium atoms has a higher energy than two single ones, and consequently tries to fly apart.
Dear Vinay Tyagi,
have a look at the famous paper from Lennard-Jones 'Cohesion'.
http://www.owlnet.rice.edu/~cz1/prog/lj/lj.pdf
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