a tricky problem

[flori...@my-deja.com]

Hey there!
One tricky problem!
You have a bubble made from water and soap. This bubble is charged
with the electric charge q and the surface tension is f (the surface
tension of the wall of the bubble... the wall is made as I have said
from water and soap).
Let's say that the bubble is connected to the earth with an resistor R
and a switcher K. Initially the inside and outside the bubble is
vacuum and K is open.
Let's say that somebody is closing the switcher K.
After how much time the radius of the bubble will become zero (0) ?

Thanks,
Florin

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[Charles DH Williams]

In article <8vp19t$vfo$1...@nnrp1.deja.com>, flori...@my-deja.com wrote:

> Hey there!
> One tricky problem!
> You have a bubble made from water and soap. This bubble is charged
> with the electric charge q and the surface tension is f (the surface
> tension of the wall of the bubble... the wall is made as I have said
> from water and soap).
> Let's say that the bubble is connected to the earth with an resistor R
> and a switcher K. Initially the inside and outside the bubble is
> vacuum and K is open.
> Let's say that somebody is closing the switcher K.
> After how much time the radius of the bubble will become zero (0) ?

Well, it depends...I think the real answer is that it *never* will become
zero, the water will evaporate leaving a finite vapour pressure inside the
vacuum which will prevent the bubble from collapsing. However, the bubble
will also loose vapour from its external surface and so will eventually
'burst'.

Of course, if its a homework problem, you probably have to assume that the
vapour pressure of the water is zero and the bubble size minimises the
free energy associated with the surface tension and the electric field,
and you need to know Ohm's law too...

Charles