Why Temperature increases by increasing pressure?

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Discussion Overview

The discussion revolves around the relationship between temperature and pressure in gases, particularly how temperature increases with pressure when volume decreases. Participants explore the underlying mechanisms at the microscopic level, considering both theoretical and practical implications.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants suggest that the temperature of a gas is related to the kinetic energy of its molecules, which increases as pressure rises due to more frequent collisions with the container walls.
  • One participant uses a billiards table analogy to illustrate how imparting kinetic energy to gas molecules can occur through mechanical action, questioning if energy delivery depends on collisions with the walls.
  • Another participant emphasizes that compressing a gas rapidly increases its temperature, indicating a non-equilibrium process, while also noting that if the system interacts thermally with the environment, the average energy becomes independent of volume after equilibration.
  • It is mentioned that even slow compression of a gas in an insulated piston-cylinder arrangement will result in heating, challenging the notion that speed is a factor in temperature increase.
  • Participants discuss the concept of quasistatic processes, where compression is performed slowly enough to maintain thermal equilibrium, suggesting that temperature may not change significantly in such cases.
  • One participant references adiabatic processes, where temperature changes are governed by specific equations, while another contrasts this with free expansion, where no work is done and temperature remains relatively constant.
  • There is a mention of the conservation of energy in relation to heating during compression and cooling during expansion, particularly in the context of refrigeration cycles.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the relationship between temperature, pressure, and volume changes in gases. There is no consensus on the mechanisms at play or the conditions under which temperature changes occur.

Contextual Notes

Some discussions involve assumptions about the conditions of gas behavior, such as whether the process is adiabatic or quasistatic, and the implications of thermal interactions with the environment. These factors remain unresolved within the discussion.

suhagsindur
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Temperature of gas is due to kinetic energy of molecule & pressure is due to molecule collide to container wall with some kinetic energy.
If I decrease volume of container it increase pressure & Temperature. For pressure it is clear that more no. of molecule is struck to container wall due to decrease in container area & so increase pressure.

But how the Temperature increases?
How kinetic energy of molecule increase by decreasing volume? Which phenomena is responsible for it at microscopic level?
 
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Take a billiards table where the balls are spread out on the table. Now sweep your arm across the table. What happens to the balls in the way? They move. You've just imparted kinetic energy to them.
 
russ_watters said:
Take a billiards table where the balls are spread out on the table. Now sweep your arm across the table. What happens to the balls in the way? They move. You've just imparted kinetic energy to them.

According to this, it is the mechanical action that puts the walls to move that delivers energy to the gas, is it?

Therefore, if one is able to displace the wall a little bit so that during this process no collision happens there will be no energy delivered. I am correct thinking this way?

Best Regards

DaTario
 
DaTario said:
According to this, it is the mechanical action that puts the walls to move that delivers energy to the gas, is it?

Therefore, if one is able to displace the wall a little bit so that during this process no collision happens there will be no energy delivered. I am correct thinking this way?

Best Regards

DaTario

Take russ watters analogy a step further.Imagine the gas trapped in a cylinder by a piston.If the piston is stationary the molecules bounce off with the same KE.If the gas is compressed by moving the piston the molecules bounce off with increased KE and the gas warms up.The faster the compression the greater the temperature rise.
 
DaTario said:
According to this, it is the mechanical action that puts the walls to move that delivers energy to the gas, is it?

Therefore, if one is able to displace the wall a little bit so that during this process no collision happens there will be no energy delivered. I am correct thinking this way?

Best Regards

DaTario
Sounds like a maxwell's demon type question.
 
As stated by russ_watters, if you compress rapidly, the temperature will increase. This is a non-equilibrium process. However if the system thermally interacts with the environment [http://en.wikipedia.org/wiki/Canonical_ensemble" ], then once equilibration with the environment takes place, the average energy is independent of the volume :

If the system thermally interacts with the environment, then after the system has reached thermal equilibrium, the average energy is given by (see http://en.wikipedia.org/wiki/Equipartition_theorem" ) :

[tex] AverageEnergy = \frac{3}{2} NkT[/tex]

where N is the number of particles. i.e. the mean energy, after equilibration, is independent of the volume.
 
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It's not a question of speed. If you compress an insulated piston-cylinder arrangement at an arbitrarily slow speed, it will heat up.
 
Yes you are correct. I was not trying to imply that speed was required for heating it up.

Just to add, if the system is thermally interacting with the environment & is initially in equilibrium, then we can ensure that it remains arbitrarily close to equilibrium during compression by ensuring that the compression procedure is performed arbitrarily slowly. c.f. Reif (Fundamentals of statistical and thermal physics)

"Such a process is said to be 'http://en.wikipedia.org/wiki/Quasistatic_process" ') that the system requires to attain equilibrium if it is suddenly disturbed. To be slow enough to be quasi-static implies that one proceeds slowly compared to the time [tex]\tau[/tex]"
 
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In a diesel engine, I believe for air PV1.4 = constant and TV0.4 = constant during compression.. Look at the equations in

http://www.ent.ohiou.edu/~thermo/Intro/Chapt.1_6/Chapter3c.html

Bob S
 
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  • #10
@Bob S : what you have stated is true for http://en.wikipedia.org/wiki/Adiabatic_process" processes (i.e. processes where no heat is transfered).

For a quasistatic process where the system is thermally interacting with the environment (i.e. heat transfer allowed & compression performed infinitely slowly), the temperature of the system does not change.
 
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  • #11
For the opposite case, in the case of a "free expansion" of a gas, such as removing a panel separating a chamber with gas from chamber with no gas (vacuum), no work is done and there is very little change in temperature.
 
  • #12
Just considering the temperature increase in terms of heat, it is a result of the conservation of energy. A gas absorbs heat in expansion and must release same in compression.
Consider the heating by compression and cooling by expansion in a refrigerator
 

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