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Q#1 When a gas is rapidly compressed (say by pushing down apiston) its temperatu

ID: 1676364 • Letter: Q

Question

Q#1 When a gas is rapidly compressed (say by pushing down apiston) its temperature increases. When a gas expands against apiston, it cools. Explain these changes in temperature using thekenetic theory, in particular nothing what happens to the momentumof molecules when they strike the moving piston. Q#1 When a gas is rapidly compressed (say by pushing down apiston) its temperature increases. When a gas expands against apiston, it cools. Explain these changes in temperature using thekenetic theory, in particular nothing what happens to the momentumof molecules when they strike the moving piston.

Explanation / Answer

When the piston is moving downward, toward an approachingmolecule, the molecule is moving faster relative to the piston(just like a car moving in the opposite direction appear to bemoving very fast) and the molecule moves with the same speed,relative to the piston when it bounces off. Therefore, a moleculewill move faster when it impacts a piston moving toward it... andfaster molecules mean an increase in temperature. . Exactly the opposite happens when the piston is moving awayfrom the gas. The relative speed of the molecules to the piston isslower than their actual speed, so their speed after they bounceoff the piston is slower... and the temperature of the gasdecreases.