Internal energy change in isobaric system

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Discussion Overview

The discussion revolves around the internal energy change and work done in an isobaric system, particularly in the context of an ideal gas. Participants explore the relationships between heat transfer, internal energy, work, and specific heat capacities.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Mathematical reasoning

Main Points Raised

  • One participant expresses confusion about calculating the heat transfer (ΔQ) in an isobaric system and relates it to pressure and volume changes.
  • Another participant suggests using the first law of thermodynamics, stating that ΔQ = ΔU + W, and proposes that for quasi-static isobaric compression, work done (W) can be expressed as PΔV.
  • A participant notes that for an ideal gas, internal energy (U) is a function of temperature (T), implying that changes in internal energy (dU) can be related to changes in temperature (dT).
  • There is a query about the direct relationship between specific heat capacity at constant pressure (Cp) and changes in internal energy (dU), seeking a way to express dU in terms of Cp and dT.

Areas of Agreement / Disagreement

Participants appear to agree on the fundamental relationships involving internal energy and temperature for ideal gases, but there is uncertainty regarding the specific connections between Cp, dU, and ΔQ. The discussion remains unresolved regarding how to express dU in terms of Cp and dT without splitting ΔQ.

Contextual Notes

Participants have not fully explored the implications of their assumptions regarding the ideal gas behavior, and the relationships between specific heat capacities (Cp and Cv) remain under discussion without definitive conclusions.

cooper607
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hi all, i have a confusion about the internal energy change and work done in a isobaric system...
suppose i want to find the delQ in isobaric system in terms of P & V...now i may assume the delW part would be pdv=p(V2-V1)...as i m compressing the gas...
but what happens to the du?
and is the specific heat capacity at const pressure somehow related to the du?
please help
 
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cooper607 said:
hi all, i have a confusion about the internal energy change and work done in a isobaric system...
suppose i want to find the delQ in isobaric system in terms of P & V...now i may assume the delW part would be pdv=p(V2-V1)...as i m compressing the gas...
but what happens to the du?
and is the specific heat capacity at const pressure somehow related to the du?
please help
Use the first law:

ΔQ = ΔU + W where W is the work done by the system.

If the compression is slow enough, you could treat it as a quasi-static isobaric compression such that W = PΔV.

To find ΔQ all you have to do is find ΔU. If this is an ideal gas, that is easy to do because ΔU is related to ΔT (how?) and ΔT is related to Δ(PV) by the ideal gas law. Or you could just use Cp and the change in T to determine ΔQ.

AM
 
well as for ideal gas U is only function of T, so i think we can replace dU with dT...but i just don't want to split the delQ here...
so i wanted to know is there any direct link between Cp and dU so that somehow i can replace the dU in terms of Cp & dT?
 
cooper607 said:
well as for ideal gas U is only function of T, so i think we can replace dU with dT...but i just don't want to split the delQ here...
so i wanted to know is there any direct link between Cp and dU so that somehow i can replace the dU in terms of Cp & dT?
What is the relationship between ΔU and ΔT (for an ideal gas? What is the relationship between Cv and Cp?

AM
 

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