Heat of vaporization conceptual problem

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Discussion Overview

The discussion revolves around the conceptual understanding of when to use the equations for heat transfer in the context of heat of vaporization, specifically the equations Q=mcΔT and Q=mLv. The scope includes homework-related queries and conceptual clarification regarding phase changes and temperature variations.

Discussion Character

  • Homework-related, Conceptual clarification

Main Points Raised

  • One participant questions how to determine when to use Q=mcΔT versus Q=mLv in a physics homework problem.
  • Another participant explains that Q=mcΔT applies to temperature changes within a single phase, while Q=mLv is used during phase transitions at constant temperature.
  • A participant seeks clarification on whether Q=mLv is only applicable when the temperature has changed.
  • It is noted that problems typically involve calculations from an initial temperature (T1) to a final temperature (T2), with a phase transition occurring at a specific temperature (Tt) in between.
  • Participants confirm that Q=mLv should be used for phase changes.

Areas of Agreement / Disagreement

Participants generally agree on the application of the equations in relation to temperature changes and phase transitions, but the discussion remains focused on clarifying the conditions for their use without reaching a consensus on all aspects.

Contextual Notes

Limitations include the assumption that participants have a basic understanding of the concepts of heat transfer and phase changes, as well as the potential for varying interpretations of specific problem scenarios.

Natjack
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When I'm doing the Physics homework the problem for heat of vaporization uses Q=mcΔT and Q=mLv. How do I know when to use which equation through out the problem?
 
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The first formula is related to a change in temperature (hence the presence of ##\Delta T##), when a single phase is heated, while the second is related to the phase transition at constant temperature.
 
So I use that one just when the temperature has changed?
 
Problems usually ask to make calculations going from ##T_1## to ##T_2##, with a phase transition somewhere in there, ##T_1 < T_t < T_2##. You then need to calculate for instance the heat in the liquid phase from ##T_1## to ##T_t##, then the latent heat of vaporization, then heat in the gas phase from ##T_t## to ##T_2##.
 
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so use the mLv for phase changes?
 
Natjack said:
so use the mLv for phase changes?
Yes.
 
Thank you so much for your help
 

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