Does compressing a gas mean the temperature has to change?

Click For Summary

Discussion Overview

The discussion revolves around the relationship between gas compression and temperature changes, particularly in the context of thermodynamics. Participants explore various scenarios, including adiabatic and isothermal processes, and the implications of the first law of thermodynamics.

Discussion Character

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

Main Points Raised

  • One participant questions whether compressing a gas necessarily results in a temperature change, particularly in a well-insulated system, referencing the first law of thermodynamics.
  • Another participant suggests that temperature increases with compression due to increased particle collisions, although they acknowledge potential misconceptions.
  • Several participants reference the ideal gas law (PV = nRT) to support the idea that temperature typically rises with increased pressure during compression.
  • A participant notes that sudden compression leads to a pressure increase that exceeds expectations based on volume reduction alone, suggesting a complex relationship between pressure, volume, and temperature.
  • One participant introduces the concept of adiabatic versus isothermal processes, indicating that temperature changes depend on the specific path taken in phase space.
  • Another participant states that if work is done on the gas, the internal energy increases, which could imply a temperature rise for an ideal gas.

Areas of Agreement / Disagreement

Participants express differing views on whether temperature change is a guaranteed outcome of gas compression. While some argue that temperature generally increases, others point out exceptions based on the specific conditions of the compression process.

Contextual Notes

Participants reference various thermodynamic principles and equations, but there is no consensus on the necessity of temperature change during gas compression, highlighting the complexity of the topic.

Who May Find This Useful

This discussion may be useful for students and enthusiasts of thermodynamics, particularly those interested in the behavior of gases under compression and the implications of different thermodynamic processes.

zzinfinity
Messages
47
Reaction score
0
Hi,
I'm taking a thermodynamics class and am curious about how changes in volume of a gas affect temperature.

I have the relations

Q-W=ΔU+ΔKE+ΔPE
and
Δu= ∫c(T)dT


If we take we take Q, ΔKE and ΔPE to be 0 this leaves us with.

-W= ∫c(T)dT

So does this mean any time you compress a gas (do work on it) there has to be a temperature change? At least with a well insulated system?
 
Science news on Phys.org
Temperature is directly related to the average kinetic energy of a substance. This kinetic energy can harvest itself in translations, vibrations, and rotations. If you compress a bunch of particles you are most definitely increasing the number of collisions. If you imagine millions of collisions per second you will see a picture in your head of extremely fast vibrations. With that, you can see that the average speed of particles is increased because of how rapidly they are hitting each other, thus temperature increases.

Disclaimer: I may have misconceptions based on this, this is just my intuition.
 
PV = nrt so yes temperature usually rises as compression pressure increases.

There is a discussion I have seen in these forums and someone knew of an exception
which was explained, but as a general rule its true.
 
Naty1 said:
PV = nrt so yes temperature usually rises as compression pressure increases.

In this scenario volume is decreasing as well.
 
zzinfinity said:
Hi,
I'm taking a thermodynamics class and am curious about how changes in volume of a gas affect temperature.

I have the relations

Q-W=ΔU+ΔKE+ΔPE
and
Δu= ∫c(T)dT


If we take we take Q, ΔKE and ΔPE to be 0 this leaves us with.

-W= ∫c(T)dT

So does this mean any time you compress a gas (do work on it) there has to be a temperature change? At least with a well insulated system?
One funny thing about sudden compression is that the pressure does not double when you compress to half the volume, but more than double. As the temperature rise under sudden compression, the pressure will also rise in addidion to the compression rate found by the change in volume.

Vidar
 
Low-Q said:
One funny thing about sudden compression is that the pressure does not double when you compress to half the volume, but more than double. As the temperature rise under sudden compression, the pressure will also rise in addidion to the compression rate found by the change in volume.

Vidar

And if pressure rises due to the temperature will the temperature rise again due to the pressure?
 
zzinfinity said:
So does this mean any time you compress a gas (do work on it) there has to be a temperature change? At least with a well insulated system?

Depends on the path the gas follows in phase space. Adiabatic compression, where everything is well insulated and you don't add heat to the system, does result in an increase in temperature following PV^γ = constant together with the equation of state, where γ is the heat capacity ratio that depends on the gas and is 5/3 for an ideal gas. If you extract heat during compression, you have isothermal compression where T does not change, but in general and classically at least, T will rise.
 
zzinfinity said:
Hi,
I have the relations

Q-W=ΔU+ΔKE+ΔPE
The first law is:

[tex]\Delta Q = \Delta U + W[/tex]

where W is the work done by the gas. The ΔKE and ΔPE are part of ΔU.

So if ΔQ = 0, then ΔU = - W. So if work is done ON the gas (as in a compression), ΔU will be positive. If U is proportional to T then ΔT will necessarily be positive. This would be the case for an ideal gas.

AM
 

Similar threads

  • · Replies 28 ·
Replies
28
Views
4K
  • · Replies 20 ·
Replies
20
Views
4K
  • · Replies 5 ·
Replies
5
Views
4K
  • · Replies 109 ·
4
Replies
109
Views
10K
  • · Replies 2 ·
Replies
2
Views
3K
  • · Replies 4 ·
Replies
4
Views
1K
  • · Replies 60 ·
3
Replies
60
Views
11K
Replies
6
Views
2K
  • · Replies 8 ·
Replies
8
Views
3K
  • · Replies 22 ·
Replies
22
Views
6K