Sign convention for work of a gas

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Discussion Overview

The discussion revolves around the sign convention for work done by a gas during expansion, particularly in the context of thermodynamic processes. Participants explore different interpretations of work in relation to the First Law of Thermodynamics.

Discussion Character

  • Debate/contested

Main Points Raised

  • Some participants propose that when a gas expands, the work done by the gas is positive, as it performs work on the environment.
  • Others argue that the sign of work can depend on the convention used, with some defining work done by the system as negative and work done on the system as positive.
  • A participant notes that the First Law of Thermodynamics can be expressed differently based on the chosen sign convention, leading to variations in the interpretation of work.
  • Another participant mentions that the common convention in science is that work done by the system is negative, which aligns with the idea that the system loses energy during work.
  • References to external sources, such as engineering thermodynamics literature, are provided to support the discussion of conventions.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the sign convention for work done by a gas. Multiple competing views remain regarding the definitions and implications of work in thermodynamic contexts.

Contextual Notes

The discussion highlights the dependence on definitions and the potential for confusion arising from different sign conventions in thermodynamics. There are unresolved aspects regarding the application of these conventions in various contexts.

anigeo
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if a gas is expanding,what will be the sign of the work done by it?
 
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anigeo said:
if a gas is expanding,what will be the sign of the work done by it?
Positive, since the gas is doing positive work on the environment. The key word is 'by'. If, instead, you wanted the work done on the gas, it would be negative in the case of expansion.
 
Where a "sign convention" enters the picture is when talking simply about the work W in a thermodynamic process. For some people, books, etc., W means the work done by the system, W = PΔV, and the First Law of Thermodynamics is ΔU = Q - W. For others it means the work done on the system, W = -PΔV, and the First Law is ΔU = Q + W.
 
anigeo said:
if a gas is expanding,what will be the sign of the work done by it?

The common convention in science is that work done by the system is negative. This is understandable because work has units of energy and if a system does work he loses energy via ΔU = W = -PΔV (adiabatic process).

This is also the convention in engineering

http://en.wikibooks.org/wiki/Engineering_Thermodynamics/First_Law
 
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