Does Decreasing Volume and Internal Energy of a Gas Affect Heat Flow and Constant Pressure?

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Discussion Overview

The discussion revolves around the effects of decreasing volume and internal energy of a gas on heat flow at constant pressure. Participants explore the implications of these changes in the context of both ideal and real gases, referencing thermodynamic principles and laws.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant suggests that as the volume decreases and the internal energy of the gas decreases, heat will flow out of the gas, leading to a negative heat value.
  • Another participant proposes that for an ideal gas, the pressure remains constant if the volume and temperature are proportional during the cooling process, referencing the ideal gas law.
  • A question is raised about the behavior of real gases, with a participant noting that real gases also follow state equations that relate pressure, volume, and temperature.
  • Clarification is sought regarding the relationship between volume and pressure, with a participant initially misunderstanding the proportionality statement, later correcting it to indicate that volume and temperature are proportional according to Charles' law.

Areas of Agreement / Disagreement

Participants express differing views on the implications of decreasing volume and internal energy on heat flow and the conditions under which pressure remains constant. There is no consensus on the effects when considering real gases versus ideal gases.

Contextual Notes

Participants reference the first law of thermodynamics and Charles' law, but the discussion includes some misunderstandings about the relationships between variables, which remain unresolved.

Who May Find This Useful

Individuals interested in thermodynamics, gas laws, and the behavior of gases under varying conditions may find this discussion relevant.

Mohammed Alqadhi
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A gas in a cylinder with constant pressure, the gas cooled down and its internal energy decreased as well as its volume. The heat Q will be flowing into the gas or out of the gas?

My try for the solution: As the volume decrease the work done by the gas will be negative.
The gas cooled down, so its internal energy will be negative too, and use the first law equation, the heat will be negative and will flowing out of the gas, but what make me stop and think is:
If the internal energy and the volume decreased and the heat is flowing out of the gas, how the pressure is still constant?
 
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Mohammed Alqadhi said:
The gas cooled down, so its internal energy will be negative too, and use the first law equation, the heat will be negative and will flowing out of the gas, but what make me stop and think is:
If the internal energy and the volume decreased and the heat is flowing out of the gas, how the pressure is still constant?
Assume it is an ideal gas: PV=nRT. The pressure remains constant if the volume and the pressure temperature are proportional during the cooling process.
 
Last edited:
Ok, that means heat will be flowing out of the system.
What would be If we consider it as a real gas?
 
The real gases also obey some state equation, relation among pressure, volume and temperature. With proper heat transfer, the temperature and volume can be adjusted in the way that ensures constant pressure.
 
ehild said:
Assume it is an ideal gas: PV=nRT. The pressure remains constant if the volume and the pressure are proportional during the cooling process.

I am having trouble trying to understand what you mean by this statement !
First of all I understand that a gas under constant pressure obeys Charles' law...there are only 2 variables, V and T...Charles' law.
You suggest that if volume and pressure are poroportional (!) then the pressure remains constant, I do not get this.
 
lychette said:
You suggest that if volume and pressure are poroportional (!) then the pressure remains constant, I do not get this.
Sorry, I meant volume and temperature being proportional: V/T = const.
 
ehild said:
Sorry, I meant volume and temperature being proportional: V/T = const.

i.e Charles' law
 
lychette said:
i.e Charles' law
Yes.
 

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