First slide

Surface tension

Question

The excess of pressure across a soap bubble of radius r is $p = 4 T / r$ . where T is surface tension of soap solution. What is the excess of pressure across an air bubble of the same radius r formed inside a container of soap solution ?

Easy

A

T/r
B

$2 T / r$
C

$4 T / r$
D

$8 T / r$

Solution

For an air bubble formed inside the soap solution, there is only one surface in contact with soap solution. Hence the excess of pressure is half that of a soap bubble. $∴ Excess of pressure = \frac{1}{2} \times \frac{4 T}{r} = \frac{2 T}{r}$

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