# Temperature of ideal gas in a cylinder

1. Nov 22, 2015

### Boomzxc

Qn : https://www.dropbox.com/s/dpcws9q4bjpzvtp/20151123_112037.jpg?dl=0

Why is option A and C wrong or correct?

A :A is plausible as if the piston is suddenly moved inwards, the gas molecules bounce off the piston at higher speeds
Since Temp proportional to rms speed, temp increases
Or
based on 1st law of thermodynamics
The piston suddenly moved in.... So means Q=0 cos no time for heat transfer.
Since compression, W is +ve so U is +ve. That implies T increases.

C : as P increases, V decreases

Thanks a million! !!

Last edited: Nov 22, 2015
2. Nov 22, 2015

### JeremyG

"Suddenly moves inwards" in the question implies that there was little or no heat exchange during this compression process. With this information, apply the First Law and try and deduce for yourself whether A or C is the correct answer.

3. Nov 22, 2015

### Boomzxc

What if the piston is slowly moved inwards? How will the answer change?

4. Nov 22, 2015

### JeremyG

If the piston is moved slowly enough, the temperature will not change. Can you rationalize why?

5. Nov 22, 2015

### Boomzxc

Is it because there's time for heat distribution throughout the gas??

6. Nov 22, 2015

### JeremyG

There's sufficient time during the process for heat to enter or leave the system.

7. Nov 22, 2015

### Boomzxc

If the piston is suddenly moved inwards and the temp increased, and i leave the system there for some time, will the system return to it's prior temperature (room temp) assuming this is carried out at room temp?

8. Nov 22, 2015

### JeremyG

Yes, heat will flow from higher temperature region to lower temperature region. It would be more precise to say that the system and its surroundings reach an equilibrium temperature, which may or may not be the original room temperature. (before the compression)