# Soap bubble question, surface tension and excess pressure [closed]

A soap bubble is blown slowly at the end of a tube by a pump supplying air at a constant rate. Draw a graph to represent the variation between the excess pressure inside the bubble with time.

## closed as off-topic by sammy gerbil, Jon Custer, Kyle Kanos, Yashas, John RennieMar 10 '17 at 10:38

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Excess pressure inside a soap bubble is 4T/R. Since air is blown at constant rate dV/dt is known. From rate change of volume calculate rate change of radius. Hence you get excess pressure in terms of time by putting R as f(t) in the formula.

• Will it be dv/Dt= 4 pi r^2 Dr/Dt , then I'll put the relation in p=4T/r is it correct – Shub Mar 9 '17 at 18:39
• @PhysiBoy I think you need to use a constant mass rate rather than a volume rate? – Farcher Mar 9 '17 at 21:03
• @Farcher Agreed, but it probably won't make a difference at these low pressures. – WetSavannaAnimal Mar 9 '17 at 23:52
• Correct @WetSavannaAnimal – user147979 Mar 10 '17 at 3:36
• Yes @Shub you are right – user147979 Mar 10 '17 at 3:41