A New Thermodynamics

Blog: Intermolecular collisions are inelastic

By Kent W. Mayhew


Inelastic Collisions are the Norm

Traditional kinetic theory wrongly considers that all collisions between gaseous molecules are elastic, i.e. energy is conserved. For any elastic collision, the relative velocity of the two colliding masses before (collision) equals the minus of the relative velocity after the collision. Another way of considering an elastic collision is that both the momentum and kinetic energy are cosnserved. This is different than an inelastic collision where only the momentum is conserved.

From classical mechanics we know that when two dissimilar objects, i.e. masses M1 and M2, collide then their total momentum is conserved, therefore in terms of their velocities (v1 and v2):              

v2=(M1/M2)v1      Or     v1=(M2/M1)v2                1.10.36

A better understanding of elastic vs inelastic collisions is given in pdf to the right which is an appendix given in my book "New thermodynamics: Say no to entropy".

  This author has shown that kinetic theory is better explained by realizing that collisions are inelastic in which case momentum is mechanically conserved but kinetic energy is not i.e. the total kinetic energy after the collision does not equal the total kinetic energy after the collision. See papers on kinetic theory. see blog on kinetic theory.

Furthermore, collisions between gas molecules with different masses is not readily envisioned. This is all discussed in more detail in my book. So although a plausible solution for elastic collisions exists, the assertion of their reality remains questionable. The more logical solution becomes that intermolecular collisions are not elastic, and that kinetic theory retains its absolute validity simply because the gas is sufficiently dilute that the predominate energy exchange is the surrounding wall molecules imposing their kinematics onto the gas molecules. 

Now imagine; when gaseous molecules do collide that heat is given off, hence such collisions are not elastic, but energy remains conserved. If inelastic collisions occur within a closed system, then the other gas molecules and/or the surrounding wall molecules should absorb any collision derived heat that is given off. Accordingly, such collisional generated heat becomes part of the equilibrium state between molecular collisions and vibrations, plus the emission and absorption of radiation.

Importantly, this helps formulate an explanation for viscous dissipation, and/or natural P-T system relationships that being molecular collisions are generally not elastic therefore heat is readily given off, as well as Joule’s weight experiment. The implication being that intermolecular collisions even in the condensed matter states are not necessarily elastic, as was previously envisioned. This again is in agreement with both natural P-T system relationships and molecular viscous dissipation.

Furthermore it has been determined that collisions between photons and electrons are inelastic5,6,7,8. With that being the case then why would anyone even believe that collisions between molecules are anything but inelastic! This also bodes the question; to what degree are collisions between thermal radiation and condensed matter elastic?

Note the concept of inelastic collisions is at odds with Avogadros hypothesis, and the ideal gas law because these are based upon tradiational kinetic theory. In this author's new understanding of kinetic theory, since wall molecules tend to massive in comparision to small gas molecules i.e. montaoomic, diatomic, triatomic, then the walls impose thier kinematics onto these relatively small gas molecules, if the gas is sufficiently dilute. And this explains why such small gas molecules seemingly adhere to traditionally accepted kinetic theory.   



Copyright Kent W. Mayhew

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PdF file is Appendix B8 of my book concerning Elastic Collisions
This website is copyright of Kent W. Mayhew who in 2018 resides in Ottawa Ontario Canada
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