User:Marko Petek/sandbox: Difference between revisions

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Changed from ("Heat and entropy" section):

"...repeatedly colliding and therefore exchanging energy so that their individual speeds are always changing, even being motionless for an instant if two molecules with exactly the same speed collide head-on, before another molecule hits them and they race off, as fast as 2500 miles an hour. (At higher temperatures average speeds increase and motional energy becomes proportionately greater.)"

To just:

"...repeatedly colliding and therefore exchanging energy so that their individual speeds are always changing. Assuming an ideal gas model, average kinetic energy increases linearly with temperature, so the average speed increases as a square root of temperature."

The bold part in the given excerpt was wrong. In a minimalistic model such as the ideal gas, kinetic energy is the only component contributing to the total energy of the system. It follows, that the kinetic energy of the system is conserved. Interactions between molecules are treated as perfectly elastic collisions - kinetic energy and momentum are both conserved. The situation, where two molecules with non-zero combined kinetic energy before the interaction have no kinetic energy after it, is therefore an impossibillity.

The molecules would be moving away from each other with the same speeds after the interaction in this specific case - conservation of KE and momentum are easily imaginable.

sprayer_faust 05:02, 10 February 2012 (UTC)