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.

 

[sophiecentaur]

I put more blame on the poor explanations than on those misinterpreting them.

Agreed. The teachers / explainers need to be capable of stating things in a way that's resistant to misinterpretation. The basic statement that heat flows from hot to cold can lead to the assumption that a hot object 'knows' that there's a cold object nearby. It's necessary to include the net effect of heat loss and heat gain, loss being entirely due to the temperature of the object itself and gain being equal to the radiant energy arriving. That arriving energy is due to one or many objects and their temperatures. This process is easiest to grasp when radiant heat is concerned. It' nothing like as obvious when convection is involved because it actually involves moving material carrying the energy.

 

[DEvens]

For questions 1 and 2, yes they will radiate. For 3, the definition of "significant" is required.

I used an online calculator for this.

The amount of heat radiated strongly depends on the surface finish. If it was highly polished, at 30C, it could be as little as 36 Watts per square meter. If it was nearly solid black it could be as high as 479 Watts per square meter. But note that the sliced fruit will also radiate. And it is unlikely to be polished. So it will not likely heat up much.

Pumping vacuum on the chamber will boil water out of the fruit and so cool it. If your goal were to get heat to the fruit to warm it, then you want the inner surfaces to be dark and textured. If you want the fruit cool you want the inner surfaces to be as smooth and polished and shiny as possible.

 

[sophiecentaur]

How does heat radiate differently in a vacuum ?

Why should it? The emission process is ended once the radiated photons have left the surface, they just keep going till they hit something (air molecule or solid surface). Up, down or sideways, the radiation process is the same. But of course, absorption of heat from surroundings will depend on what's actually out there - but not gravity.

A forest fire. . . . .

You've now left the ideal world of basic Physics and all the other modes of heat transfer can come into play. The fire is propagated by adding sources of chemical energy which burn when the received heat raises the temperature to 'burning point'.

In a fruit drier, the fruit will drop in temperature as it loses water (washing on a washing line) The container surface will cool and the vacuum pump will extract air and water vapour - doing work, of course. The walls will reach an equilibrium temperature lower than ambient but be radiating / convecting heat to the room.

Each of the basic mechanisms can be considered separately but it's the net effect that determines the temperatures and heat flow. A list of all these effects (with directions) will be quite long in the real world.