# What is air pressure inside rubber birthday balloon?

Consider a normal rubber balloon inflated to a reasonable size; where the initial resistance of the rubber to stretching is overcome, and the balloon is not so full that the rubber can no longer stretch (therefore behaving more like a rigid than flexible container). Where the balloon has freedom to form itself to a size and shape without reaching the extreme limits of the material.

The rubber of the balloon obviously is stretched and would like to return to a smaller size, but is prevented by the air inside. I believe that the air pressure inside such a freely shaping container would be equal to the air pressure outside. My brother believes that the air inside must be under greater pressure than ambient, a position I feel has absurd consequences, such as balloon air being heavier than outside air (greater pressure means denser, means negative buoyancy). I am talking only about a sealed balloon not undergoing any current changes--my point being the balloon would continue to reshape itself until an equilibrium with ambient was achieved.
Is the air in sealed balloon which is neither over nor under-inflated at a greater pressure than the air outside the balloon? What about the air inside a soap bubble?

"Is the air in sealed balloon which is neither over nor under-inflated at a greater pressure than the air outside the balloon? What about the air inside a soap bubble? " Was your question and Hanerlio correctly understood the question but didn't answer the question specifically. I will attempt to loosley convey the static properties of a balloon. As it appears that even after reading a PDF on balloon physics, you have failed to grasp the conscept of pressure.

The answer to the question is Yes. The pressure inside the balloon is greater than the outside of the baloon. It is true as you mentioned that there is an equilibrium maintaining a somewhat consistant shape of the balloon, however, this has little to do with pressure. The reason is simple. The elastic is exerting a force per unit area on the gas inside balloon. This makes it potentially difficult to blow one up. Simultaneously, it's exerting a negative stress on the air surrounding the balloon. This is why when you untie a blown up balloon it goes crazy and flys around the room. The higher pressure within the balloon escaps to the the lower Pascal rateable surroundings. The balloon is a device designed to support differential pressures in a static equilibrium.

Naturally, the same applies to a bubble--since they can POP.

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