Volume work, absolute and relative pressure confusion

In summary, the conversation discusses the calculation of isothermal work done by a gas on a piston in a cylinder. The equation for isothermal work is derived from the equation dW = F(s)ds and includes the absolute pressure and atmospheric pressure. However, it is pointed out that the calculation only considers the work done by the gas and does not include the negative work done by atmospheric pressure. In order to accurately calculate the total work, atmospheric pressure must be taken into account.
  • #1
kandelabr
113
0
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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  • #2
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.
 
  • #3
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.
 

FAQ: Volume work, absolute and relative pressure confusion

What is volume work?

Volume work is the amount of work done by a system as it changes its volume. This can be calculated by multiplying the change in volume by the pressure.

What is absolute pressure?

Absolute pressure is the total pressure exerted by a fluid, including both atmospheric pressure and the pressure exerted by the fluid itself. It is typically measured in units of Pascals (Pa) or pounds per square inch (psi).

What is relative pressure?

Relative pressure is the difference between the absolute pressure and the atmospheric pressure. It is also known as gauge pressure and is typically measured in units of Pascals (Pa) or pounds per square inch (psi).

How do I convert between absolute and relative pressure?

To convert from absolute pressure to relative pressure, simply subtract the atmospheric pressure from the absolute pressure. To convert from relative pressure to absolute pressure, add the atmospheric pressure to the relative pressure.

Why is it important to understand the difference between absolute and relative pressure?

Understanding the difference between absolute and relative pressure is important because it affects how we measure and interpret pressure in various systems. Absolute pressure is necessary for accurate calculations and measurements, while relative pressure is used to measure changes in pressure within a system.

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