Understanding Boyle's Law: The Effect of Volume Increase at Constant Temperature

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bobsmith76
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Effect of a volume increase at constant temperature: A constant temperature means that the average kinetic energy of the gas molecules remains unchanged. This in turn means that the rms speed of the molecules, u, is unchanged. If the volume is increased, however, the molecules must move a longer distance between collisions. Consequently, there are fewer collisions per unit time with the container walls, and pressure decreases. Thus, the model accounts in a simple way for Bovle's law


The above sentence is taken from a textbook. What I don't understand is when molecules collide why would the momentum decrease? I would think the momentum would merely be preserved or transferred from one atom to another, just like with billiard balls which are not affected by gravity.
 
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It doesn't say the momentum decreases.

The number of colisions / time with the container walls decreases which gives a less force on the walls and so a lower measured pressure.

Note in an ideal gas you ignore any collisions between the gas molecules.
 
Never mind, I think I got it. It's collisions with the walls of the container, not collisions with other molecules, that's what I was not understanding. If the container is enlarged then the moleculues will hit the container walls less often.
 
bobsmith76 said:
Never mind, I think I got it. It's collisions with the walls of the container, not collisions with other molecules, that's what I was not understanding. If the container is enlarged then the moleculues will hit the container walls less often.

Exactly .