Thermodynamics Work from pressure

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Homework Help Overview

The problem involves calculating the work done on air during isothermal compression as it is pumped into bicycle tires, with specific parameters including pressure and volume reduction factors.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss the use of the ideal gas law and the work equation, with some attempting to simplify the equations involved. Questions arise regarding the appropriate values for variables such as n and how to apply pressure ratios instead of volume directly.

Discussion Status

Participants are exploring different approaches to the problem, with some providing guidance on using pressure ratios and clarifying the need for specific variables. There is an ongoing exchange of ideas without a clear consensus on the method to be used.

Contextual Notes

There is uncertainty regarding the volume needed for calculations and the value of n, as well as the implications of working with pressure ratios in the context of the problem.

Logan McEntire
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Homework Statement


The tires on a bicycle require an air pressure of 80 psig. When isothermally pumped up the bicycle tires, the volume of the air (that was originally in the atmosphere) is reduced by a factor of 5.7. Please determine the work that must be done on each lbm of air that is pumped into the tires. Please report your answer in lbf-ft/lbm.
.

Homework Equations


W=nRT*ln(V2/V1)
Pv=nRT

The Attempt at a Solution


My attempt might be very wrong, but this is what i tried.
I simplified Pv=nRT to T=PV/nR
Plugged that into W=nRT*ln(V2/V1)
W=PV*ln(V2/V1)
I know the standard pressure (14.7psi) and the pressure inside the tire (80psig+14.7psi=94.7psi), but I don't know the volume of to plug in for the V.
 
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Use nRT, with T equal to room temperature.
 
What do I use for n?
 
For isothermal compression P1/P2 =V2/V2, instead of using volume, you can use pressure ratio
 
Logan McEntire said:
What do I use for n?
You are trying to get the work per pound mass. So you don't need to know n. You are solving for W/n.
 

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