How to Calculate Work and Heat Transfer in a Polytropic Process of Nitrogen?

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SUMMARY

The discussion focuses on calculating work and heat transfer in a polytropic process involving nitrogen at 100°C and 600 kPa, expanding to a final pressure of 100 kPa with a polytropic index of n=1.2. The initial approach using the equation T2/T1=(P2/P1)^[(n-1)/n] to find T2 and W=nCv(T1-T2) for work is incorrect, as it calculates the change in internal energy instead. To accurately determine work, integration of PdV is required, and the first law of thermodynamics can be applied to find heat transfer once internal energy and work are known.

PREREQUISITES
  • Understanding of polytropic processes in thermodynamics
  • Familiarity with the first law of thermodynamics
  • Knowledge of the ideal gas law and its applications
  • Ability to perform integration in the context of thermodynamic equations
NEXT STEPS
  • Learn how to derive the relationship between pressure, volume, and temperature in polytropic processes
  • Study the integration of PdV for work calculation in various thermodynamic processes
  • Explore the application of the first law of thermodynamics in calculating heat transfer
  • Investigate the properties of nitrogen as an ideal gas under varying conditions
USEFUL FOR

Students and professionals in mechanical engineering, particularly those focused on thermodynamics and energy systems, will benefit from this discussion.

ZLing
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Homework Statement


Nitrogen at 100°C and 600 kPa expands in such a way it can be approximated by a polytropic process with n=1.2. Calculate the work and the heat transfer if the final pressure is 100 kPa.

Homework Equations

The Attempt at a Solution


I used the equation T2/T1=(P2/P1)^[(n-1)/n] to find T2. Then i used W=nCv(T1-T2) to calculate work done. Is this correct? But i don't know how to calculate the heat transfer.
 
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ZLing said:

Homework Statement


Nitrogen at 100°C and 600 kPa expands in such a way it can be approximated by a polytropic process with n=1.2. Calculate the work and the heat transfer if the final pressure is 100 kPa.

Homework Equations

The Attempt at a Solution


I used the equation T2/T1=(P2/P1)^[(n-1)/n] to find T2. Then i used W=nCv(T1-T2) to calculate work done. Is this correct?
No. This is not the work. This is the change in internal energy. To get the work, you need to integrate PdV.
But i don't know how to calculate the heat transfer.
If you know the change in internal energy and the work, then you can use the first law to get the heat.

Chet
 
Chestermiller said:
No. This is not the work. This is the change in internal energy. To get the work, you need to integrate PdV.

If you know the change in internal energy and the work, then you can use the first law to get the heat.

Chet
Hi, does that mean I have to find V1 and V2 first?
 
ZLing said:
Hi, does that mean I have to find V1 and V2 first?
That's one way to start.

Chet
 

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