Change in internal energy of gas (thermodynamics)

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Discussion Overview

The discussion revolves around the change in internal energy (ΔU) of a gas during thermodynamic processes, particularly under adiabatic conditions. Participants explore the implications of path independence of ΔU and the relationship between work done and changes in pressure and volume.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • One participant asserts that under adiabatic conditions, the change in internal energy is equal to the work done, questioning how this can remain constant despite varying paths taken between two states (A and B).
  • Another participant challenges the notion that multiple paths can exist under adiabatic conditions with differing amounts of work, requesting a specific example to illustrate this point.
  • A participant proposes a scenario where pressure is decreased immediately followed by a reduction in volume at constant pressure, suggesting that the work done would differ based on the path taken.
  • In response, it is argued that one cannot decrease both pressure and volume adiabatically, and that different paths would lead to different final states, temperatures, and work done, thus affecting the change in internal energy.

Areas of Agreement / Disagreement

Participants express disagreement regarding the possibility of varying paths under adiabatic conditions and the implications for work done and internal energy changes. The discussion remains unresolved, with competing views on the relationship between pressure, volume, and internal energy.

Contextual Notes

Participants highlight limitations in understanding the conditions under which adiabatic processes occur, including assumptions about pressure and volume changes and their effects on internal energy.

axe34
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Hi,If the change in U between 2 points (A,B) in a thermodynamic process is always the same despite the path, then please help with the following:

Say I have adiabatic conditions so that delta U = Work done only (Q=0)
Surely I can go through multiple paths between A+B with increased or decreased amounts of work? Then how can the change in internal energy be the same each time?Thanks
 
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axe34 said:
Hi,If the change in U between 2 points (A,B) in a thermodynamic process is always the same despite the path, then please help with the following:

Say I have adiabatic conditions so that delta U = Work done only (Q=0)
Surely I can go through multiple paths between A+B with increased or decreased amounts of work? Then how can the change in internal energy be the same each time?Thanks
Actually, you can't go adiabatically through multiple paths between A and B with increased or decreased amounts of work. If you think you can, please provide one specific example.

Chet
 
Say I want to decrease pressure and volume from A to B.
Could I not drop the pressure immediately, then reduce the volume at this constant pressure. The area (work) of this graph would be less if I did a smooth transition between A + B, would it not?
 
axe34 said:
Say I want to decrease pressure and volume from A to B.
Could I not drop the pressure immediately, then reduce the volume at this constant pressure. The area (work) of this graph would be less if I did a smooth transition between A + B, would it not?
You can't decrease both pressure and volume adiabatically. But, suppose you wanted to decrease the imposed force per unit area immediately, and then let the volume increase adiabatically at this force per unit area. Option 2 is to do a smooth gradual transition between the initial pressure and the force per unit area you used in Option 1. The final temperature and the final volume under Option 2 would be different from the final temperature and the final volume under Option 1, the work would be different, and the change in internal energy (determined by the change in temperature) would be different. You could not reach the same final equilibrium state under Option 2 as you did under Option 1.

Chet
 

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