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# Decrease in Temperature and Volume, particle collision with walls question watch

1. A sample of gas (H2) is kept within a container with a piston seal (of mass M) at the top. This piston can move freely. Let's say the pressure on the piston above it is one atmosphere.

If we then put this container in a reservoir of ice then the volume of the gas will decrease, and we wait for its temperature to reduce to 0 degrees C. The final pressure of the gas must be the same as the initial pressure to be able to balance the forces due to the air pressure and the piston's weight.

Now I'm asked if the number of particles per unit area per unit time incident on the walls of the cylinder containing the gas will increase, decrease or stay the same after this decrease in temperature.

I'm not sure how to deduce this. The average speed of the molecules will decrease due to a lower temperature, but also due to a decreased volume containing the gas the distance between the walls also lowers. I know that the pressure is the same in either case. And that the time between collisions of the walls for a single molecules is where l is the distance between the walls and v is the molecule's velocity. So how do I deduce if it's greater, lower or the same?

If anyone needs a diagram because my explanation is poor. Please let me know. Thanks.
2. (Original post by Sasukekun)
A sample of gas (H2) is kept within a container with a piston seal (of mass M) at the top. This piston can move freely. Let's say the pressure on the piston above it is one atmosphere.

If we then put this container in a reservoir of ice then the volume of the gas will decrease, and we wait for its temperature to reduce to 0 degrees C. The final pressure of the gas must be the same as the initial pressure to be able to balance the forces due to the air pressure and the piston's weight.

Now I'm asked if the number of particles per unit area per unit time incident on the walls of the cylinder containing the gas will increase, decrease or stay the same after this decrease in temperature.

I'm not sure how to deduce this. The average speed of the molecules will decrease due to a lower temperature, but also due to a decreased volume containing the gas the distance between the walls also lowers. I know that the pressure is the same in either case. And that the time between collisions of the walls for a single molecules is where l is the distance between the walls and v is the molecule's velocity. So how do I deduce if it's greater, lower or the same?

If anyone needs a diagram because my explanation is poor. Please let me know. Thanks.

Lower temperature -> lower speed

Pressure is related to - speed of molecules and collision rate.

At lower temperature, rms speed of molecules is lowered, so to keep up the same pressure as before, collision rate has to increase!

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