Does Q = enthelpy in Constant Temperature?

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Discussion Overview

The discussion revolves around the relationship between heat (Q) and enthalpy in a constant temperature process, particularly in the context of steam with a specified pressure and dryness fraction. Participants explore theoretical aspects and practical implications related to enthalpy changes during heat transfer.

Discussion Character

  • Homework-related, Technical explanation, Conceptual clarification

Main Points Raised

  • One participant questions whether the enthalpy after heat addition equals the initial enthalpy plus the Q value in a constant temperature process.
  • Another participant states that for an ideal gas at constant temperature, the change in enthalpy is zero.
  • A participant mentions their specific scenario involving steam, expressing uncertainty about how to proceed with the problem.
  • One contribution explains that energy is conserved in isolated systems and suggests that if energy flows into a system, enthalpy should increase by the amount of energy added, although this is not explicitly stated as a fact.
  • A later reply clarifies that for saturated vapor and liquid in equilibrium, adding or removing heat changes the quality, and Q equals the change in enthalpy (ΔH), provided the system remains within the two-phase region.
  • Another participant expresses understanding of the discussion but notes additional questions regarding work in the problem.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between heat and enthalpy, particularly in the context of ideal gases versus real systems like steam. The discussion remains unresolved regarding the specific application of these concepts to the participant's problem.

Contextual Notes

There are limitations regarding the assumptions made about the system, such as the ideal gas approximation and the specific conditions of the steam system, which may affect the applicability of the claims made.

Jameseyboy
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Homework Statement


In a constant temperature process, where heat is added - Does the enthalpy after the heat is added equal the initial enthalpy plus the Q value?


Homework Equations





The Attempt at a Solution

 
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For an ideal gas, if the temperature is constant, the change in enthalpy is zero.
 
For my particular question I have steam with a given pressure and dryness fraction. Really not sure what to do here
 
Enthalpy is a measure of the total energy of a system (relative to some selected "zero point", we have no theory which allows us to determine the absolute energy of any system). In Classical Physics Energy is conserved. It is neither lost nor gained in any isolated system. In Relativity, if you include Mass, Momentum and stress in your definition of Energy, which is the way it works, then Energy is conserved (locally). Meaning the Laws of Thermodynamics are locally obeyed. So, you need to engrave in your soul that unlike vampires, and Magneto, energy can be neither created nor destroyed. Heat is the flow of energy. If energy is flowing into a system, you shouldn't need to ask if enthalpy is increasing by *EXACTLY* the same amount. Of course it is.
Dryness fraction is an engineering term, and until 30 seconds ago, I'd never heard of it.
Try: http://www.engineeringtoolbox.com/wet-steam-quality-d_426.html
or
https://en.wikipedia.org/wiki/Enthalpy–entropy_chart
it seems to be a simple problem of adding components of enthalpy together (computed separately).
 
Jameseyboy said:
For my particular question I have steam with a given pressure and dryness fraction. Really not sure what to do here
No problem. If you have saturated vapor and saturated liquid in equilibrium at a given quality, temperature, and pressure, and you add or remove heat so that the quality changes, then Q = ΔH (provided you don't add or remove enough heat to take you out of the two phase region). Also, ΔS=Q/T.

Chet
 
Ok makes sense so far just wasn't sure.

It also asks for work later in the question.

(these are only for a few marks as well which is why it's frustrating. The harder questions I have a good idea how they go)

I appreciate the help buddy
 

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