Thermodynamics problem How to find the temperature of a gas.

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Homework Help Overview

The discussion revolves around a thermodynamics problem involving the adiabatic expansion of a gas in an engine. The original poster seeks assistance in determining the final temperature of the gas after it has done work against a piston, given certain parameters such as the number of moles and initial temperature.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants explore the application of the ideal gas law and the First Law of Thermodynamics. Questions arise regarding the necessary parameters, such as Cv, and the relationship between internal energy and temperature change. There is also discussion about the validity of assuming adiabatic conditions.

Discussion Status

Guidance has been offered regarding the application of thermodynamic principles, including the need to determine ΔU and the importance of knowing Cv. Multiple interpretations of the problem's conditions are being explored, particularly concerning the assumptions about the gas and the nature of the expansion.

Contextual Notes

Participants note the absence of specific values for pressure and volume, which complicates the application of certain equations. There is also a concern about whether the adiabatic condition is applicable given the lack of information about the process being quasi-static.

UCstudent
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Thermodynamics problem! How to find the temperature of a gas. Please Help!

Homework Statement



In an engine, 0.45 moles of gas at 1050 degrees K in a cylinder expand adiabatically against the piston. The gas does 3200 J of work against the piston. What is the final temperature of the gas?

Homework Equations



What formula do I use?

The Attempt at a Solution



The only formula I can think to use is

W=(-P)(delta V)

But pressure and volume aren't given, and I can't find them with the given information.
 
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Assume that the gas is ideal and apply the ideal gas law together with the First Law of Thermodynamics.

ehild
 


How do you apply the first law of thermodynamics?
 


UCstudent said:
How do you apply the first law of thermodynamics?
First law: Q = ΔU + W where W = work done by the gas.

Here you are given Q and W. You first have to determine ΔU.

In order to find the change in temperature, however, you have to know the relationship between ΔU and ΔT. That depends on the gas. Are you given the Cv of the gas?

AM
 


What have you learned about adiabatic expansion of a gas? What is the relation among pressure and volume during such a process?
Anyway, you need to know the ratio Cp/Cv. You do not need the initial volume and pressure to solve the problem.
Collect all equations you know and are relevant: The ideal gas law, the adiabatic equation of state (you can derive it using the First Law if you do not know it) and the expression of work done by the gas.

ehild
 


I don't think the adiabatic condition applies here since there is no indication that it is a quasi-static adiabatic expansion. Besides, one would need to know either P or V as well as T. However, we do need to know Cv to solve the problem

AM
 


Andrew Mason said:
I don't think the adiabatic condition applies here since there is no indication that it is a quasi-static adiabatic expansion. Besides, one would need to know either P or V as well as T. However, we do need to know Cv to solve the problem

AM

You are right, we do not need to assume a quasi static process. But we need to know if it is an ideal gas and the number of atoms in its molecules, or Cv for the internal energy. .

ehild
 


Sounds like you have to assume ideal gas
 


Thank you for your help!
 
  • #10


No problem - guess you got it :-)
 
  • #11


I sure did! :) Can you help me with my new problem? lol
 
  • #12


point me to it
 

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