What is air pressure inside rubber birthday balloon?

The air inside the balloon has to be pressurized in order to stretch the rubber. The same way you blow air into a soap bubble, you need slightly more air per volume in order to apply a greater pressure inside the balloon then outside, thus making the rubber stretch. If you don't have more pressure inside, the rubber won't stretch at all because no force will be applied. 

I'm sorry, but I find this answer unsatisfactory. You are describing the act of inflation, while I am asking specifically about the scalar pressure value in an already inflated balloon.

I am talking about after equilibrium is achieved, you are talking creating differentials to make work. I am in agreement with you, when you blow into a balloon you increase the pressure inside. But, as you state, the immediate result is a change in the interior volume in order to remedy this discrepancy and achieve a static, balanced state--a state where this differential no longer exists, because if it did, the balloon would still be in the act of inflating.

I feel my position is more in tune with the principle of equal and opposite reactions.  There must be equal forces on each side of a point on a balloon not in the process of changing shape--the interior force pushing outward orthogonally, and the exterior force pressing inward in an exact opposite direction.  If these forces were not in balance, the balloon would be changing shape.  The potential energy stored in the elasticity of the balloon and put there by the act of inflation seems to me a separate issue.

I went a bit deeper into the search results looking for information about this, and I believe I have found confirmation of my original position.

Here is an interesting PDF on balloon physics;
http://www.eurekaeducation.net/files/physics/playing_with_balloons.pdf
Which is from a collection;
http://www.eurekaeducation.net/sub_files/physics.html

As described at the top of page three of the "playing_with_balloons" pdf, air pressure in a flexible container approaches that of ambient.  If a discrepancy exists between these pressures, this results in a change in the shape/volume of the balloon to maintain the equilibrium demanded from Boyle's law.

Think of the elasticity of the rubber as capable of storing potential energy.  When air is added to the balloon, work is done to stretch the rubber.  But once it is stretched, it does not require a constant greater pressure or influx of energy to maintain the shape--this would essentially mean free energy or perpetual motion.  As a rock lifted and placed on a table, the force of the rock down on the table & the table back on the rock is independent of the fact that the rock was once on the ground.

My brother has convinced me there is some greater pressure in bubbles and balloons, mainly because of way bubbles behave:

http://www.deepocean.net/deepocean/index.php?science04.php

But bigger the bubble, the less additional pressure it takes,proportional to 1/radius. And, if balloon left to settle, and outgas for days, it will eventually reach equilibrium, but in a typically inflated balloon there is some additional pressure.  How much (with balloons) still a source of debate, but fact that air goes from in to out over time means a pressure differential must be driving it, we think.

cool, this is a great explanation and some good references.

You would be correct if there were no tension forces from the balloon itself.  The pressure inside the balloon is thus higher than ambient.  According to another site it is around 0.05 bar (atm) or 1.4 psi.

 Bob, PhD

Hello,
the pressure in baloon is bigger than atmospheric pressure outside of baloon.
This difference in pressure is caused by the rubber trying to reach status
with minimum potencial energy = you can say balloon is trying to be deflated :).

Pressure in balloon is probably not so much bigger than the atmospheric pressure,
because you are able to blow the ballon with your own lungs.
Men's lungs can produce cca 0.1 atm under normal circumstances see
http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1501025

Taking into account all the previous you can probably easily measure this at home
with tools you can find around and find the exact value - once and for all :).

For measurement you will need:
    a) empty bottle from coke
    b) cca 1-2 meters of some pipe (garden pipe, aquaristic pipe, something like that)
    c) baloon
    d) some water


1) Cut the bottom of the bottle
2) attach the bottle to a pipe - you can tape, glue or just stuck it together
3) fill the bottle with water and let it flow through the other end of pipe
   make sure whole pipe is filled with water
   if you make U with the pipe to have both ends aprox. at the same high you should stop
   flow and have the pipe filled with water

4) blow ballon and close (make few turns on the rubber) it that way that you would be
able to open it again
5) attach ballon to the other end of pipe and open it
6) put the end with bottle higher at such high that no water flows into baloon, but
   no air is escaping from baloon either


7) measure (in centimeters) the difference between the water surface on both ends -
   in baloon and in bottle


Now the values. Assuming that 1 atmosperic pressure is approximately the pressure
of 10 meters high pipe of water
http://www.google.com/search?q=10+meters+in+feet

Each meter (3.2 feet) of water in pipe gives you aprox 0.1 of atmosphere pressure
Each 10cm (3.9 inches) of water in pipe gives you approx 0.01 of atmosphere pressure
http://www.google.com/search?q=10+cm+in+inches

My bet is that it would be 1.056 atm (56cm - 22 inches) :) .

You can use this converter to convert cm or ft you measured, but do not forget to add 1 atm - bottle is open and we are not living in vacuum  right :D.

http://www.sensorsone.co.uk/pressure-units-conversion.html

Happy measuring
Michal Ambroz

we just tested it and the outside pressure was approx. 103 and the interior prressure was 106. We used a vernier gas pressure sensor and logger pro software