Heat radiation in vacuum

[berkeman]

How does heat radiate differently in a vacuum ?

Can you say how the three heat propagation mechanisms (conduction, convection and radiation) are affected by the setup described in the OP (the Original Post that started the thread)? Thanks.

 

[sophiecentaur]

How does heat radiate differently in a vacuum ?

No differently. The rate of heat transfer will be proportional to the temperature difference of the two surfaces and their emissivity / absorptivity - and slightly modified / delayed by any intervening gas. The rate of burning will depend on the temperature but the process can be affected greatly by the forced convection when the wind is significant. The wind will 'drive the fire' in its direction. Flammability of the materials will also affect the speed of travel. With damp trees, the fire can self-damp because the water takes too long to be boiled off. That could be treated as the rate of availability of fuel.

You can't reduce the behaviour of a forest fire to just one or two mechanisms but the 'fire triangle' requirement rules.

 

[sophiecentaur]

It's how they interpret "heat flows from the hotter to the colder".

The words "more" or "net" should be included in their statement. No resulting implied paradox there.

 

[haruspex]

The words "more" or "net" should be included in their statement. No resulting implied paradox there.

Who are "they" here? Google "second law heat flow" and you will find plenty of apparently authoritative sources that say such as "heat always flows spontaneously from a hot object to a cooler object, and never in reverse on its own" (https://openbooks.lib.msu.edu/colle...dynamics-heat-engines-and-their-efficiency-2/).
No mention of "net" there.

 

[haruspex]

it requires more heat for a longer period to bring available materials to a radiant temperature

I did not understand that bit. What is a "radiant temperature"? Isn’t that anything above absolute zero? And do you mean more heat or a higher temperature?

Do you just mean the flow of heat is slower if radiation is the only mechanism?

 

[sophiecentaur]

Who are "they" here? Google "second law heat flow" and you will find plenty of apparently authoritative sources that say such as "heat always flows spontaneously from a hot object to a cooler object, and never in reverse on its own" (https://openbooks.lib.msu.edu/colle...dynamics-heat-engines-and-their-efficiency-2/).
No mention of "net" there.

The heart radiated from a surface depends just on its temperature and emissivity. The heat absorbed depends on the energy arriving and its absorptivity. The net flow is the balance between the two and (btw) for a given surface is the emissivity is the same value as the absorptivity. If that were not true, it would contravene the laws of thermodynamics.
If you insist that the only relevant statement is that heat flows from hot to cold, it does not make clear that heat is absorbed and emitted from both surfaces and Stefan's Law. It also fails to make clear that some of the 'heat' will be emitted and travel for ever - in an expanding universe, never hitting another object.

Many statements are made about Science that are incomplete and confusing. "Nature abhors a vacuum" applies everywhere until you scratch the surface and realise there are better ways of putting it.

 

[haruspex]

The heart radiated from a surface depends just on its temperature and emissivity. The heat absorbed depends on the energy arriving and its absorptivity. The net flow is the balance between the two and (btw) for a given surface is the emissivity is the same value as the absorptivity. If that were not true, it would contravene the laws of thermodynamics.
If you insist that the only relevant statement is that heat flows from hot to cold, it does not make clear that heat is absorbed and emitted from both surfaces and Stefan's Law. It also fails to make clear that some of the 'heat' will be emitted and travel for ever - in an expanding universe, never hitting another object.

Yes, yes, I know all of that as you do. But I have come across people who have read the oversimplified "heat flows from a hotter body to a colder, not the other way", from sites that ought to get it right, and come away with the belief that it is all one way, not merely one way on balance. I put more blame on the poor explanations than on those misinterpreting them.