Confusion regarding the First Law of Thermodynamics

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Discussion Overview

The discussion centers around the interpretation of the First Law of Thermodynamics in the context of a closed system undergoing expansion, specifically when a piston is pulled out, leading to cooling. Participants explore whether the work is done to or by the system during this process and how it aligns with the law's formula under adiabatic conditions.

Discussion Character

  • Debate/contested

Main Points Raised

  • One participant questions whether the work done during the expansion of a closed system is work done to the system or by the system, suggesting it is work done to the system.
  • Another participant argues that the work corresponds to work done by the system, indicating that it is equivalent to reducing the external pressure and allowing the system to perform work.
  • A later reply acknowledges the previous clarification, indicating understanding of the argument presented.
  • Another participant notes that physicists and physical chemists use opposite signs for work done by the system, which must be considered when applying the First Law of Thermodynamics.
  • One participant raises a scenario where it is unclear if the expanding gas does any work, suggesting that if the external force does all the work, the gas will not cool, and thus, the work is not done to the system.

Areas of Agreement / Disagreement

Participants express differing views on whether the work is done to or by the system, indicating that multiple competing interpretations remain unresolved.

Contextual Notes

There are limitations in the assumptions made regarding the work done by the gas and the external force, as well as the conditions under which the gas expands.

Smith
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Is forcing a closed system to expand (e.g. by pulling out a piston), causing it to cool, work done to the system or work done by the system? I assume it was work done to the system, but that means the first law of thermodynamics formula no longer balances if you assume an adiabatic change. With adaibatic, Q = 0, so:
-w = + delta u.
-w is equal to work done to the system, but delta u should be negative, not positive as the formula would make it.

Any ideas what I'm doing wrong.
 
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Smith said:
Is forcing a closed system to expand (e.g. by pulling out a piston), causing it to cool, work done to the system or work done by the system? I assume it was work done to the system
No, it corresponds to work done by the system. It is equivalent to reducing the external pressure and letting the system do the work.
 
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Thanks very much, that makes sense.
 
Physicists and physical chemists use opposite signs for the work done by the system. But that has to be accounted for when writing the first law as ##\Delta E = q + w## or ##\Delta E = q - w##.
 
Smith said:
Is forcing a closed system to expand (e.g. by pulling out a piston), causing it to cool, work done to the system or work done by the system? I assume it was work done to the system, but that means the first law of thermodynamics formula no longer balances if you assume an adiabatic change. With adaibatic, Q = 0, so:
-w = + delta u.
-w is equal to work done to the system, but delta u should be negative, not positive as the formula would make it.

Any ideas what I'm doing wrong.
It is not clear from your scenario whether the expanding gas does any work. If it is the external force rather than the gas that does all the work on the surroundings in expanding the volume for the gas, the gas will not cool. The gas freely expands doing no work. By the first law if Q = 0 and W = 0, then ΔU = 0 which means ΔT = 0. In that case, the work is not done to the system by the external force. Rather the work is done on the surroundings.

AM
 

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