Again with the Force

[uDevil]

Matt,

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OK, now for a couple of questions. uDevil, you mentioned that there is no net force on the molecules. In a pressurized canister, I agree as the molecules are just bouncing around and transferring momentum, but with no net direction of force. There is an equal probability that any molecule will experience an equal force from any direction. But consider what happens when a highly pressurized canister explodes....

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I think you're on the wrong track by thinking about pressure. I say this because diffusion would occur even if there is no pressure difference:

Imagine that you could select a small volume of air in a room and mark each of the molecules in that volume so that you can follow them around the room. Now there is no pressure difference between the selected volume and the rest of the room. (The molecules in the selected volume are of the same type as those in the rest of the room, so there is no difference in partial pressure either.)

Now observe the marked molecules at some later time. They will have become evenly distributed around the room.

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I don't know what you mean by internal temperature. The temperature is independant of the pressure. The kinetic energy of the molecules doesn't depend on the pressure.

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The temperature is not independent of the pressure. PV/nR = T, p is pressure T is temperature.

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I admit this confuses me. I think it depends on how you define the system. For an isolated system (no matter energy or energy can cross the system boundary), there are no (one? anyway the state of the system is fixed once you set it up) independent variables. On the other hand, in an open system, there are 3 independent variables because I am allowed to add or remove gas molecules and energy, and change the volume as I please.

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Increasing the pressure of a gas will increase its temp.

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If you increase the pressure by decreasing the volume and the system is not adiabatic (so heat may cross the boundary), the rise in temperature will be a transient phenomenon and the gas temperature will drop back down to match that of the environment while the pressure remains elevated.

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The gas does work during the expansion....

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I don't see how the gas does work as it is not expanding against an applied force.

BTW, ChrisV merely said he didn't take Physics 1 (we used to call that "bozo physics"). He probably just skipped it as it seems clear to me he has a very good understanding of the topic.

I encourage you to keep thinking about this. Despite what others have said or implied, this is NOT easy.