26
Aug/094
Aug/094
How Good Is Vacuum Between The Walls Of Vacuum Flask?
The gap between outer and inner walls of vacuum flask is 1cm.
Before the flask was evacuated, the air between the walls was at normal pressure 100kPa and the flask was losing heat at rate 100W.
After air was evacuated and the flask was sealed, the flask is losing heat at rate 0.1W.
Assuming that all heat transfer is due to heat conductivity of residiul air only, ESTIMATE the pressure of residual air between the walls of the flask.
Comments (4)
Trackbacks (0) ( subscribe to comments on this post )
Leave a comment
No trackbacks yet.
8:48 pm on August 26th, 2009
0.1 Pa (not kPa)
The conductivity of air is pretty constant until pressure gets down to 10 Pa, then it drops linearly with pressure. (fewer molecules to convey the heat)
[I also note that at said pressure, the mean free path of 3 angstrom N2 is about 10 cm, which is 10x the space between the walls]
***************
Edit: I’m pretty sure that the best estimate will involve the mean free path and the separation, I’m not quite sure how to get there. So I gave you my best estimate instead.
8:56 pm on August 26th, 2009
The vacuum is very good
9:08 pm on August 26th, 2009
I bet the long winter evenings must just fly by in your house
9:51 pm on August 26th, 2009
Erm…this seems too easy but I’ll state the obvious anyway (understanding these things have a way of being wrong).
Looks like a 1:1 relationship between pressure and heat loss on your setup. 100kPa to 100W. Following that line of reasoning I am guessing the pressure at 0.1kPa to lose 0.1W.
Probably missing something here though…I admit I am no expert in this.