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Homework Help: Pressure of a soap bubble

  1. Aug 28, 2010 #1
    1. The problem statement, all variables and given/known data
    Two spherical soap bubbles of radii a[1] and a[2] are made to coalesce. Show that when the temperature of the gas in the resulting soap bubble has returned to its initial values, r of the bubble is given by p*r^3+4*[tex]\gamma[/tex]*r^2=p(a[1]^3+a[2]^3)+4[tex]\gamma[/tex](a[1]^2+a[2]^2) where p is the ambient pressure and [tex]\gamma[/tex] is the surface tension.

    2. Relevant equations

    3. The attempt at a solution

    I cut the bubble in half and found that the surface tension along the circumference is 2[tex]\gamma[/tex] therefore the total force is 2[tex]\gamma[/tex]*circumference (2[tex]\pi[/tex]*r) we then have the pressure inside the bubble acting over the entire cross section which gives total force of p*[tex]\pi[/tex]*r^2. adding the two together, my final equation is p*r^2+4*[tex]\gamma[/tex]*r=p(a[1]^2+a[2]^2)+4[tex]\gamma[/tex](a[1]+a[2]). I have no idea how I did it but my entire equation has been divided by r somehow. Please help. Thanks.
  2. jcsd
  3. Aug 30, 2010 #2


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