Polytropic process where n is not given

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    Polytropic Process
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Discussion Overview

The discussion revolves around a problem involving a polytropic process for a balloon containing ammonia, where the pressure is proportional to the square of the diameter. Participants explore how to determine the value of the polytropic index \( n \) and calculate the work done during the heating process, given specific initial conditions and final pressure.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Homework-related

Main Points Raised

  • One participant presents a problem involving a balloon with ammonia, noting the initial conditions and the need to find the work done in a polytropic process.
  • Another participant suggests that if pressure is proportional to the square of the diameter, then diameter is proportional to the square root of pressure, prompting a discussion on the relationship between volume and diameter.
  • A participant expresses uncertainty about how to relate volume to pressure in the context of the problem.
  • Another participant provides a formula for volume in terms of diameter and encourages finding a relationship that fits the form \( PV^n = \text{constant} \) to determine \( n \).
  • One participant calculates a potential value for \( n \) as 15.37 based on their simplifications, but this is later revised.
  • A later reply indicates that the participant corrected their approach and found \( n = -\frac{2}{3} \), which they state provided the correct relation for their calculations.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the value of \( n \) initially, as different calculations and approaches are presented. However, one participant later claims to have resolved their confusion and found a value for \( n \).

Contextual Notes

Participants express uncertainty regarding the relationships between pressure, volume, and diameter, and how these relate to the polytropic process. The discussion includes various assumptions and simplifications that may affect the calculations.

Who May Find This Useful

This discussion may be useful for students or individuals studying thermodynamics, particularly those interested in polytropic processes and the relationships between pressure, volume, and temperature in gases.

canadiansmith
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Hi I really need help figuring out this problem. A balloon behaves such that the pressure inside is proportional to the diameter squared. It contains 2 Kg of ammonia at 0 degrees celsius and 60% quality. The balloon and ammonia are now heated so that a final pressure of 600 kPa is reached. Considering the ammonia as a control mass, find the amount of work done in the process.
the answer is 117.19 KJ
i was able to find the initial volume from the quality and mass
I got Vinitial = 0.3483
I can't find a way to get a value for n in the polytropic process equation
W = (P2V2-P1V1)/ 1-n
 
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canadiansmith said:
Hi I really need help figuring out this problem. A balloon behaves such that the pressure inside is proportional to the diameter squared. It contains 2 Kg of ammonia at 0 degrees celsius and 60% quality. The balloon and ammonia are now heated so that a final pressure of 600 kPa is reached. Considering the ammonia as a control mass, find the amount of work done in the process.
the answer is 117.19 KJ
i was able to find the initial volume from the quality and mass
I got Vinitial = 0.3483
I can't find a way to get a value for n in the polytropic process equation
W = (P2V2-P1V1)/ 1-n


If the pressure is proportional to the square of the diameter, then the diameter is proportional to the square root of the pressure. Now the volume is proportional to what power of the diameter? ... does this help?
 
Well not really. The question you asked is exactly the thing I am having problems with. I found that V=(4/3)pi(d/2)^3 but I am not sure if what I am trying to do is to get Volume to the power of something to equal pressure or some random constant.
 
canadiansmith said:
Well not really. The question you asked is exactly the thing I am having problems with. I found that V=(4/3)pi(d/2)^3 but I am not sure if what I am trying to do is to get Volume to the power of something to equal pressure or some random constant.

You're almost there. You are trying to find an n for which PVn = constant. So if

V=(4/3)pi(d/2)^3

and

d \propto sqrt(P) → d = d1(P/P1)^0.5

then

V=(4/3)pi(d1(P/P1)^0.5/2)^3

Now collect the P and the V on the same side of the equation, and put it in the form PVn = constant. What is n?
 
after simplifying I got (p^1.5)V = 3.9116*10^-5 which gives me 0.3483^n = 9.1052*10^-8
so n would be 15.37?

I got V1 = 0.3483
P1= 429.6
 
Thank you so much for your help. I figured out what I was doing wrong. I got n = -2/3 now. Which gave me my relation. Thanks again
 
Good luck!

BBB
 
Last edited:

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