Internal energy of a substance.

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Discussion Overview

The discussion revolves around the concept of internal energy in substances, particularly focusing on ideal gases and the behavior of internal energy during isothermal processes. Participants explore the implications of temperature on internal energy and examine specific cases such as the condensation of substances from saturated vapor to saturated liquid.

Discussion Character

  • Conceptual clarification, Debate/contested, Technical explanation

Main Points Raised

  • Some participants assert that for an ideal gas, the internal energy depends solely on temperature, leading to the conclusion that during an isothermal process, the change in internal energy is zero.
  • Others challenge this by stating that during condensation from saturated vapor to saturated liquid, the system is not an ideal gas, suggesting that the change in internal energy is not zero.
  • A participant notes that while the statement holds true for ideal gases, real gases exhibit a slight dependence of internal energy on volume, referencing the Joule-Thomson effect.
  • Another participant mentions the distinction between a 'true' ideal gas and what some refer to as a 'perfect' gas, emphasizing that internal energy in a 'true' ideal gas only depends on temperature, assuming a constant number of molecules.

Areas of Agreement / Disagreement

Participants generally agree that for ideal gases, internal energy is temperature-dependent and that the change in internal energy during isothermal processes is zero. However, there is disagreement regarding the behavior of internal energy during condensation, with competing views on whether it can be treated as an ideal gas.

Contextual Notes

Participants highlight the limitations of applying ideal gas behavior to real gases, particularly during phase changes, and note the dependence of internal energy on volume in non-ideal conditions.

Who May Find This Useful

This discussion may be useful for students and professionals interested in thermodynamics, particularly those exploring the properties of gases and phase transitions.

shenjie
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Hi all, I was taught that internal energy of an ideal gas in depending on the temperature which mean during isothermal process, the change in internal energy is equal to zero. Is this statement true?

How about the change in the internal energy of a substance when it is condensed from saturated vapour to saturated liquid isothermally?

Sincerely thanks to those who help me :)
 
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shenjie said:
Hi all, I was taught that internal energy of an ideal gas in depending on the temperature which mean during isothermal process, the change in internal energy is equal to zero. Is this statement true?
Yes.
How about the change in the internal energy of a substance when it is condensed from saturated vapour to saturated liquid isothermally?

No. During the condensation, it is not an ideal gas. It's part vapor and part liquid.
 
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shenjie said:
Hi all, I was taught that internal energy of an ideal gas in depending on the temperature which mean during isothermal process, the change in internal energy is equal to zero. Is this statement true?

That's true for an ideal gas, but in real gases there's a slight dependence of internal energy on volume, i.e. the partial derivative ##\left(\frac{\partial U}{\partial V}\right)_{T,n}## is not exactly zero. This is related to the Joule-Thomson effect.
 
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yeah, for a 'true' ideal gas, the internal energy only depends on the temperature (assuming the number of molecules is constant). I think some people call this 'true' ideal gas a 'perfect' gas.
 

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