Specific heat at constant pressure formula help

  • Thread starter yecko
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  • #1
yecko
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Homework Statement


f2vSXtq.png

https://i.imgur.com/f2vSXtq.png

Homework Equations


Kjy1Tzh.png

https://i.imgur.com/Kjy1Tzh.png

The Attempt at a Solution


In this question, the pressure is different at different point, in other words it is not constant throughout the system. Why the solution use c(p) (or "enthalpy" h which specified for constant pressure system) as part of the formula used?

Thank you very much!
 

Answers and Replies

  • #2
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Homework Statement


View attachment 211514
https://i.imgur.com/f2vSXtq.png

Homework Equations


View attachment 211515
https://i.imgur.com/Kjy1Tzh.png

The Attempt at a Solution


In this question, the pressure is different at different point, in other words it is not constant throughout the system. Why the solution use c(p) (or "enthalpy" h which specified for constant pressure system) as part of the formula used?

Thank you very much!
For an ideal gas, enthalpy is independent of pressure. Doesn't the open system version of the first law require you to use enthalpy?
 
  • #3
yecko
Gold Member
277
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For an ideal gas, enthalpy is independent of pressure. Doesn't the open system version of the first law require you to use enthalpy?
Is that imply enthalpy is used just because the first law of thermodynamics is applicable?
And using Cp is not contradict to the situation of pressure changed, but rather Cp is used because it can trace out the enthalpy?
Thank you very much!
 
  • #4
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Is that imply enthalpy is used just because the first law of thermodynamics is applicable?

Yes. The open system version of the first law tells us that, for your problem the change in enthalpy is equal to the change in kinetic energy.
And using Cp is not contradict to the situation of pressure changed, but rather Cp is used because it can trace out the enthalpy?
Thank you very much!
Yes.
 

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