Volume work, absolute and relative pressure confusion

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SUMMARY

The discussion centers on the confusion between absolute pressure and atmospheric pressure in the context of calculating work done by a gas in a closed cylinder with a piston. The user derived the equation for isothermal work but initially neglected the negative work done by atmospheric pressure. The correct approach requires considering both the absolute pressure (p0) exerted by the gas and the atmospheric pressure (patm) acting against the piston to determine the net work done on the piston. The conclusion emphasizes the importance of including atmospheric pressure in work calculations to accurately assess the energy output from the piston.

PREREQUISITES
  • Understanding of thermodynamics principles, specifically isothermal processes.
  • Familiarity with pressure concepts, including absolute and atmospheric pressure.
  • Basic knowledge of work-energy principles in physics.
  • Ability to derive equations related to gas behavior in closed systems.
NEXT STEPS
  • Study the derivation of the ideal gas law and its applications in thermodynamics.
  • Learn about the concept of work done by gases in various thermodynamic processes.
  • Explore the implications of atmospheric pressure in real-world applications, such as engines and pistons.
  • Investigate the use of software tools for simulating thermodynamic processes, such as MATLAB or Python libraries.
USEFUL FOR

This discussion is beneficial for students and professionals in mechanical engineering, physics, and thermodynamics, particularly those involved in designing and analyzing systems that utilize gas expansion and piston mechanics.

kandelabr
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hello.

i can't figure out where I'm wrong. this is the problem:

we have a cylinder, closed with a piston. the absolute pressure inside the cylinder is p0, atmospheric pressure is patm. the air inside expands isothermally to some (specific) volume vend.
i derived the equation for isothermal work directly from this one: dW = F(s)ds.

here it is (sorry for handwriting... uses less time :eek: (will learn TeX one day... :smile:)):
isotherm.jpg


the final equation seems just fine, but you've probably noticed the big fat asterisk. here's why:
p0 is absolute pressure and it exerts F(s0) on the piston. but there is also patm, which does negative work equal to patm * A * (send - s0). this fact hasn't been taken into account anywhere.

where am i wrong (the thermodynamics probably isn't wrong, right? :) )
 
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What you are calculating is the work done by the gas on the piston, not the net work done on the piston. The gas is not subjected to atmospheric pressure--it's contained in the cylinder.
 
oh, i get it.
but if now i want to know how much work i can get from this piston, i need to include atmospheric pressure.
 

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