Thermal Conductance Through Two Materials

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Discussion Overview

The discussion revolves around a thermal conductance homework problem involving heat transfer through two materials, gold and silver, both of the same length and cross-sectional area. Participants explore the implications of steady-state conditions on energy transfer and the role of material conductivity in this context.

Discussion Character

  • Homework-related
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • One participant notes that in a steady state, the conductance would be the same through both gold and silver, despite their differing conductivities.
  • Another participant asserts that while the power transmitted is the same due to energy conservation, the temperature gradient must differ because of the materials' different conductivities.
  • A further inquiry is made about the reasoning behind the power being the same in steady state, questioning the implications of this condition.
  • One participant illustrates a hypothetical scenario to emphasize that differing power levels would violate energy conservation, reinforcing the idea that steady state implies a constant power flow.
  • Another participant defines steady state as involving steady temperatures and a constant rate of energy flow, assuming no losses through the sides.

Areas of Agreement / Disagreement

Participants generally agree that in a steady state, the power transmitted must remain constant, but there is some uncertainty regarding the implications of material conductivity on the temperature gradient and the overall understanding of steady-state conditions.

Contextual Notes

The discussion does not resolve the nuances of how conductivity affects the temperature gradient in steady-state conditions, nor does it clarify the assumptions regarding power loss through the materials.

XianForce
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I had a thermal conductance homework problem where heat flowed through gold, and then silver. Both materials were of the same length and same cross sectional area. The problem mentioned that the energy transfer had reached a steady state, and I found through doing the problem that under these conditions, conductance would be the same through both materials.

Can anyone explain why this is? I thought that the conductivity of the material would still play a role in the rate of energy transfer.
 
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I thought that the conductivity of the material would still play a role in the rate of energy transfer.
It does.
The power transmitted is the same (this follows from energy conservation and the requirement of steady state), but as the conductivity is different the temperature gradient is different.
 
mfb said:
It does.
The power transmitted is the same (this follows from energy conservation and the requirement of steady state), but as the conductivity is different the temperature gradient is different.

Yes, I understand the second part, but why would the power be the same through both? What about something being in "steady state" makes that happen?
 
Imagine 100 W of heat transferred from one side to the center and 200 W transferred from the center to the other side. You now have a device which produces 100 W thermal power out of nowhere and violates energy conservation.
 
By definition...steady state means steady temperatures and therefore steady rate of flow of energy. Ie constant power Through the materials , assuming no power loss through the sides.
 

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