We are beginning to see the outlines of a big idea : when a gas or other complic

Question

We are beginning to see the outlines of a big idea : when a gas or other complicated statistical system, is left to itself under constant external conditions for a long time, it arrives at a situation where the probability distributions of its physical quantities do not change over time. Such a situation is called thermal equilibrium. We will define and explore equilibrium more precisely later, but already something may be troubling you, as it is troubling Wendy:

Bashiyar: Very good, you say the air does not fall on the floor at room temperature because of thermal motion. Why then doesn’t it slow down and eventually stop (and then fall on the floor), as a result of friction ?

Areej: Oh, no, that’s quite impossible because of the conservation of energy. Each gas molecule makes only elastic collisions with others, just like the billiard balls in first-year physics.

Bashiyar: Oh ? So then, what is friction ? If I drop two balls offthe Tower of Pisa, the lighter one gets there later, because of friction. Everybody knows that mechanical energy is not conserved ; eventually it winds up as heat.

Areej: Uh, um, …

As you can see, a little knowledge proves to be a dangerous thing for out two frictious scientists. Suppose that, instead of dropping a ball, we shoot one air molecule into the room with enormous speed, say, 100 times greater than the average molecular speed. (One can actually do this experiment with a particle accelerator). What happens ?

Explanation / Answer

The first molecule will transfer kinetic energy by colliding to another slower one. After there are two molecules with less speed than initial. After theese molecules will collide with another ones. Such prossece will take place until the molecular speed is equalized.

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