heat rising?

[YT2095]

a simple question and hardly merits a whole thread but does anyone know if Heat Rises in a vacuum?

 

I know it doesn`t in zero g, but I`m thinking about a Light bulb or thermionic valve type vacuum, does it matter if a bulb/tube is right way up or upside down as far as glass and base heating is concerned?

[DrP]

I Think that 'heat rises' normally means through convection - which obviously wont happen in a vacuum. Heat transfer by radiation and conduction through contact would be the only mechanisms which would work in a vacuum. I don't think they are effected by orrientation (up and down) rather than from path of least resistance from hot to cold and radiation in a straight line.

 

I know it doesn`t in zero g, but I`m thinking about a Light bulb or thermionic valve type vacuum, does it matter if a bulb/tube is right way up or upside down as far as glass and base heating is concerned?

 

I would say no based on the energy transfer here being radiated and conducted alone - no convection inside the bulb. There would be convected heat from the bulb, which would go upwards of course, but in the vacuum of the bulb I can't see how orientation would effect the heating of the casing or base one way or the other.

[Klaynos]

I agree with DrP

[YT2095]

Thanks both

[timo]

Slightly related, potentially interesting in this scope, not checked by me for actual physics content or credibility: http://www.compadre.org/informal/index.cfm?Issue=2 , "Flames in Space".

[YT2095]

yeah, I`v seen that Zero g effect before, the interesting part is that when the flame goes out, it re-ignites as soon as you move it into fresh air again, although I assume there is some time limit before it ceases to do that.

[insane_alien]

the time limit is how long it takes to cool down below autoignition temperature.

[John Cuthber]

A related effect means it's very imortant to have fans on spacecraft. People do not spontaneously "reignite".

[CaptainPanic]

Any heat rising is indeed through convection. In other words: in absence of molecules, molecules cannot go up. That sounds a little silly, but that's what it comes down to.

 

Heat doesn't rise itself, it's the fact that the gas (its molecules) is heated. Hot gas expands, (gets a lower density). The lower density gas rises. Cold gas, with higher density, will fall.

 

That's also why you cannot take a hot air balloon up into space.

 

For your information, light bulbs have low pressure inert gas (nitrogen or neon for example) inside the bulb, but no absolute vacuum, so there will be some convection inside the bulb.

[Mr Skeptic]

So long as you don't have a complete vacuum nor zero g, heat will rise. The top of a lightbulb will get warmer than it would if it were the other way around.

 

In a complete vacuum, the effect would be reversed (but so tiny you wouldn't notice it): photons going against the gravitational field would be red-shifted, while photons going with the gravitational field would be blue-shifted. So you could say that in a vacuum, heat falls! However, you would need a pretty impressive gravitational field to have a significant effect.