Heat Transfer: Conduction vs. Convection vs. Radiation

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Discussion Overview

The discussion revolves around the mechanisms of heat transfer, specifically focusing on the differences between conduction, convection, and radiation. Participants explore the conditions under which heat can flow from cold to hot, particularly in the context of radiant heat transfer.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants assert that heat cannot flow from cold to hot, particularly in conduction and convection, but question whether this applies to radiation.
  • Others argue that radiation allows for energy transfer from a hotter object to a colder one, as both objects emit infrared radiation, with the net energy transfer favoring the hotter object due to the Stefan-Boltzmann law.
  • One participant mentions that while heat can flow from cold to hot, it requires work input, as seen in refrigeration systems.
  • There is a discussion about the role of temperature differences and the potential for energy transfer to depend on other factors, including the transparency of materials to infrared radiation.
  • Some participants reference Maxwell's demon to challenge the idea that heat cannot flow from cold to hot, suggesting that there is no law explicitly prohibiting it.
  • Concerns are raised about the conditions under which heat transfer occurs, particularly regarding the emission of photons and the implications of different temperature scenarios.

Areas of Agreement / Disagreement

Participants express differing views on whether heat can flow from cold to hot, particularly in the context of radiation. While some agree that radiation allows for such transfer, others emphasize the need for work input in practical scenarios. The discussion remains unresolved with multiple competing views present.

Contextual Notes

Participants highlight various assumptions, such as the dependence on temperature differences and the nature of the materials involved in radiation. The discussion does not resolve these complexities or the implications of different scenarios.

jobyts
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As I remember learning in school, heat cannot flow from cold to hot. But isn't that true only for conduction and convection? Radiation heat is purely Infrared waves, and there should not be any issue with that. Please correct if I am wrong.
 
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Every object radiates EM energy. When it's infrared radiation, we call it radiant heat.If two bodies are at different temperatures, and both are emitting mainly infrared which is incident on the other, then in general the colder body will receive more radiation than it emits, and thus heat up, even though it's also emitting heat rays which are being absorbed by the hotter body. (Shapes have also to be taken into account.)

But if their temperatures are widely different, then which one heats up may depend on other factors, because the main radiation may not be in the infrared band.
 
Cue the perpetuum mobile using Faraday isolator...
 
Heat an most definitely flow from cold to hot. That's what a refrigerator does after all -_-

The thing is, it will not flow in such a manner spontaneously.
Heat cannot flow from cold to hot without work input.
 
It has never been observed not to flow from hot to cold but there is no law that says it cannot
 
MrXow said:
It has never been observed not to flow from hot to cold but there is no law that says it cannot
You are correct. However, the odds of that happening are very very very small.
 
like 10^-54
 
The OP is asking about radiant heat. Read his query.
 
  • #10
Shooting star said:
The OP is asking about radiant heat. Read his query.
Yes, so let's answer it:

Radiation between a cold and a hot object involves both objects radiating photons towards each other, but since the hot object is hotter, it radiates more (via the Stefan-Boltzmann law) and the net energy transfer is from the hot object to the cold one. How much is determined by plugging both temperatures into the equation.
 
  • #12
russ_watters said:
Yes, so let's answer it:

Radiation between a cold and a hot object involves both objects radiating photons towards each other, but since the hot object is hotter, it radiates more (via the Stefan-Boltzmann law) and the net energy transfer is from the hot object to the cold one. How much is determined by plugging both temperatures into the equation.

I have already given an answer in my first reply.

What if the colder body transparent to infra-red, which the hotter body mostly emits?
 

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