Re: size of air bubble and inside pressure

size of air bubble and inside pressure

I am a librarian and am trying to find an answer regarding : Does the size 
of an air bubble have anything to do with the pressure inside said air 
bubble?  Do small air bubbles have less pressure, etc.?


Carol,

There is a definite relationship between the size of a bubble and the 
pressure inside of it.  Now, I am considering a bubble like a soap bubble 
where a region of higher pressure is contained within some sort of  
restraining membrane.  There are other cases like  air bubbles underwater. 
In that case the pressure would have more to do with the pressure exerted 
by the surrounding  water than with the shape of the bubble.

The pressure inside a soap bubble is inversely proportional to the radius 
of the bubble.  This means that if you had a bubble one centimeter across, 
its pressure would be twice as high as the pressure of a similar bubble 
that is two centimeters across.  A bubble ten centimeters across would have 
only a tenth as much  pressure as the one centimeter bubble.  

An experiment with bubbles can make it clear that smaller bubbles have 
higher internal pressures than larger bubbles.  On a clean and smooth 
surface, place two bubbles  so that they touch each other.  If the bubbles 
are the same size, then the area where they meet will be flat, indicating 
they have the same pressure.   If the bubbles are different sizes, you 
should see the area bulge in toward the larger bubble, indicating higher 
pressure in the smaller bubble.   It is possible to stick a straw into one 
of the bubbles and change its size by letting air escape or  pushing air 
in.  This should affect the curvature of where the bubbles meet.

The inverse relationship between the size of a bubble and the pressure 
inside it is a general feature of  any bubble where the skin is under 
tension.   Rubber balloons are an example.  It is very difficult to start 
blowing them up, because the pressure of the small balloon is high.   As 
the balloon inflates, it gets easier to put more air in.   You can also see 
this if you connect a large balloon to a smaller one with a straw.  The 
small balloon should force air into the big balloon.

See these links:

http://www.exploratorium.edu/ronh/bubbles/

http://boltzmann.me.psu.edu/~cimbala/Courses/ME033/Lectures/Lecture.02/bubb
le.htm

With regards and best wishes,

Everett