P-V Diagram: Computing Work and Determining Sign of Work

Click For Summary
SUMMARY

The discussion focuses on calculating work using P-V diagrams in thermodynamics, specifically addressing the sign of work during gas compression. Participants clarify that the work done by the gas is negative when the change in volume (dV) is negative, as indicated by the equation W_b = ∫_1^2 PdV. The absolute pressure (P) is always positive, which influences the interpretation of work values based on the convention used.

PREREQUISITES
  • Understanding of P-V diagrams in thermodynamics
  • Knowledge of boundary work calculations
  • Familiarity with the concepts of absolute pressure and volume changes
  • Basic calculus for integration in work calculations
NEXT STEPS
  • Study the principles of thermodynamic work and energy transfer
  • Learn how to interpret P-V diagrams for various thermodynamic processes
  • Explore the implications of positive and negative work in gas systems
  • Review integration techniques for calculating work in different scenarios
USEFUL FOR

Students and professionals in physics and engineering, particularly those studying thermodynamics and fluid mechanics, will benefit from this discussion.

Saladsamurai
Messages
3,009
Reaction score
7

Homework Statement


Photo9.jpg


I have plotted the info as such

Photo7.jpg


I can compute the work by adding the area of the triangle to the area of the rectangle, but my question is is all of this work negative? It is compressed so I am assuming it is?
 
Last edited:
Physics news on Phys.org
Hi Saladsamurai,

Is your book asking for the work done by the gas, or the work done on the gas?
 
Saladsamurai said:

Homework Statement


Photo9.jpg


I have plotted the info as such

Photo7.jpg


I can compute the work by adding the area of the triangle to the area of the rectangle, but my question is is all of this work negative? It is compressed so I am assuming it is?

I can't see your diagram while I'm at work, however, based on what you wrote if the dV is negative, then the boundary work will be negative. If the dV is positive, then the boundary work is positive. Remember that P is absolute pressure and is thus always positive.

W_b = \int_1^2 PdV

Like alphysicist eluded to, it may be written as a positive value based on the convention taken.

CS
 

Similar threads

Thermodynamics: P-V diagram question
  • · Replies 2 ·
Replies
2
Views
2K
How can voltage between unlike charges decrease with proximity?
  • · Replies 4 ·
Replies
4
Views
1K
Correct Derivation of the Adiabatic Condition? (PV Diagram)
  • · Replies 4 ·
Replies
4
Views
1K
Sound Wave Interference and Finding the Path Differences with Diagrams
  • · Replies 20 ·
Replies
20
Views
5K
Confusion over acceleration of gravity as positive/negative
  • · Replies 10 ·
Replies
10
Views
2K
Calculating acceleration of a prism and block connected to a wall through a rope and pulley
  • · Replies 4 ·
Replies
4
Views
1K
Solving Homework: Find Final State, Mass, Work & P-v Diagram
  • · Replies 4 ·
Replies
4
Views
1K
Change in the energy content of an isobaric process
  • · Replies 2 ·
Replies
2
Views
2K
Understanding a Velocity-Time Graph
  • · Replies 14 ·
Replies
14
Views
6K
Tension & Circular Motion Question - Looking for speed
  • · Replies 5 ·
Replies
5
Views
974