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Homework Help: Different Pressure Equations

  1. Jun 11, 2013 #1
    I am going through old exam questions about pressure and fluid dynamics.
    For one particular question I had to calculate the change in pressure in a system. These two equations were given on the formula sheet:




    I've never seen the equation with the '4' in it. I've only every used the second equation to calculate pressure change (and always got answers correct). I can't find the first equation anywhere in my textbook or study guide.
    I was wondering if someone could briefly explain when you would use that equation?

    I am assuming that it's got something to do with where the pressure difference is only half (e.g. half a bubble sitting on top of water)?
  2. jcsd
  3. Jun 11, 2013 #2
    The first equation applies to a thin film bubble, like a soap bubble. The second equation applies to a bubble of gas within a body of surrounding liquid.
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