Question

The excess pressure across a soap bubble of radius $r$ is $p=\frac{4\sigma }{r},$ where $\sigma$ is the surface tension of soap solution. What is the excess pressure across an air bubble of the same radius $r$ formed inside a container of soap solution?

• A. $\frac{\sigma }{r}$
• B. $\frac{2\sigma }{r}$
• C. $\frac{4\sigma }{r}$
• D. $noneofthese$

Answer 1

The correct option is B $\frac{2\sigma }{r}$

Soap bubble in air has two surfaces whereas air bubble in soap solution has only one surface. So air bubble in soap has half of the excess pressure of soap bubble in air. i.e. $\frac{1}{2}\times \frac{4\sigma }{r}=\frac{2\sigma }{r}$