Filling Flat Tire - Adiabatic Reversible Process

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Discussion Overview

The discussion revolves around the thermodynamic analysis of a nearly flat bicycle tire being pumped up, approximated as a reversible adiabatic compression process. Participants explore the implications of various assumptions regarding pressure, volume, and temperature during the process, as well as the relevant equations to apply.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about the volumes involved, questioning whether the volume of air used to fill the tire is relevant and how it relates to the constant volume assumption made by the professor.
  • Another participant clarifies that the volume of the tire is constant, but the volume of the air before and after compression is not, highlighting ambiguity in the problem regarding the measurement of final pressure.
  • A participant suggests that the equations provided can be used if certain assumptions about the state of the gas are made, particularly regarding the initial and final conditions of pressure and temperature.
  • There is a discussion about whether atmospheric pressure could reach 5 bar to fill the tire without work, questioning the nature of the reversible process and the relationship between internal and external pressure.
  • One participant recommends using the ideal gas law (PV=NkT) to derive relationships between initial and final states, suggesting a method to solve for the final temperature.
  • Another participant mentions the need to find the initial volume using the adiabatic identity, indicating a step in the problem-solving process that remains unresolved.

Areas of Agreement / Disagreement

Participants do not reach a consensus on several key aspects, including the interpretation of the problem's conditions, the relevance of certain variables, and the appropriate equations to apply. Multiple competing views remain regarding the assumptions necessary for solving the problem.

Contextual Notes

There are limitations regarding the assumptions made about pressure measurements and the definition of constant volume, as well as unresolved steps in the mathematical derivation needed to reach a solution.

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Homework Statement


A nearly flat bicycle tire becomes noticeably warmer after it has been pumped up. Approximate this process as a reversible adiabatic compression. Take the initial P and T of the air (before it is put into the tire) to be 1.00 bar and 298.0K. The final volume of air (after it is in the tire), is 1 L and the final pressure is 5.00 bar. Calculate the final temperature (be sure to state your assumptions).

P initial = 1.00 bar
V initial = ?
T initial = 298K

P final = 5.00 bar
V final = 1 L
T final = ?

moles = ?



Homework Equations



Cval dT = P dV

P 1-γ T γ = constant

P internal = P external (since it is reversible)


The Attempt at a Solution



The biggest issue I am having is figuring out the volumes. My professor gave us a hint saying that we can assume constant volume, but what does he mean - I know we can assume the tire volume is constant, but the volume of air used to fill the tire can't be the same as the volume of the tire can it? Is that volume even relevant.

The first equation I derived but if volume is constant like my professor was saying, dV will be zero which ruins that equation. I found the PV=constant online but it seems way too easy. Is this assuming everything else is constant?

I don't care if someone comes out and gives me a direct answer I just need a little guidance. The homework is due Wens so I have a bit of time. Thank you!

There are just a ton of variables unaccounted for that could affected pressure which makes it really confusing to me - temp, moles, volume, etc.
 
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Volume of the tire is constant, volume of the air before and after compression is not.

Question is ambiguous, as it doesn't state whether the final pressure is measured at 298 K or immediately after pumping (when the air is hotter). That's where you have to assume something - just state it in the final result.
 
Thanks for the reply!

I would assume since the gas is at lower pressure when it is 298k that it is before it is pumped. Does this mean I can use the equations above?

Also, does the gas have to be pumped to fill the tire if it is a reversible process Pinternal=Pexternal? I was thinking in this fake model that atmospheric pressure may go to 5 bar to fill the tire without any work?
 
you've got to use PV=NkT, which holds both before and after compression. You then have two equations you can combine (eliminate Nk) to something like which can be solved for Tf.
that means Pi*Vi=Pf*Vf*Ti/Tf.

If course, first you must find the value for Vi first by using the adiabatic identity P^γ V =const.
 

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