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Why does air escape through a pinched ballon?

  1. Apr 20, 2010 #1
    Hello guys,

    This is simple question but I have gone completely blank. Ok let's say there was a balloon outside that was inflated. This means that it has expandes so it's pressure is equivalent to atmpospheric pressure right?

    So when I pinch it why does air escape, how does balloon suddenly have higher pressure to make air move out? Is pinching the balloon equivalent to making the volume of the ballon smaller. Thanks :smile:
  2. jcsd
  3. Apr 20, 2010 #2
    I believe the answer to this question isn't about the atmospheric pressure, but about the force that the elasticity of the rubber of the balloon has on the air. What happens when you stretch the rubber and let it go? It returns back to its original shape right? The same thing in this case. The expansion of the rubber is just stretching it, so when it has the opportunity, the balloon will go back to its original shape and size, thus pushing out the air.
  4. Apr 20, 2010 #3

    D H

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    The answer to this question is all about pressure. The pressure inside a balloon is not equal to the atmospheric pressure. In particular,
    This is not correct.
  5. Apr 21, 2010 #4
    I have heard the same thing before (pressure inside a balloon= pressure outside), from a physics teacher or something like this...so I'm not surprised that OP is asking this question. I was wondering the same thing.

    I think I resolved it to myself by dismissing what the physics teacher said as BS, though. Sure the balloon expanded, but the expanded balloon is nevertheless pressing on the air inside of it, so surely the pressure must be higher.
  6. Apr 21, 2010 #5


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    Yup .. the internal pressure must be higher to balance both the external pressure + the "surface pressure" from the elastic material of the balloon. This latter term is directly proportional to the surface tension, and inversely proportional to the radius (assuming a spherical shape).
  7. Apr 21, 2010 #6


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    Note, it does depend on the type of balloon.
  8. Apr 22, 2010 #7


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    If you took a deflated balloon up into space (assume ambient pressure is zero). Let a small amount of air into the balloon. It will inflate until the pressure balances. It could well burst if you put 'too much' air in but, if it settles down, the pressure inside will be finite and the pressure outside will be zero. The pressure inside is due, entirely, to the forces of tension in the balloon membrane.
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