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according to Gay lussac law at constant volume pressure is directly proportional to the temperature
But according to kinetic theory of gases ,molecule of gas are perfectly elastic i.e. no loss of kinetic energy
Then how can temperature will increase?
AND how gases are liquify at high pressure?

To increase temperature at constant volume one must apply heat from outside.  Just because a collision does not result in a net loss of energy to the system does not mean that the speeds of gas molecules colliding with the walls cannot increase if they are hit by a vibrating wall molecule.  That's easy enough to visualize.  

Gases have attractive forces between them, the Van der Waals forces due largely to their fluctuating electric dipole moment.  If the molecules are pressed closely together then these forces dominate their interactions more.  If they're close enough and moving slow enough (lower temperature), then these forces will make the molecules stick together in a liquid.


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Dr. Stephen O. Nelson


I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.


I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.

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