Question

A soap bubble is formed from $5\times {10}^{-3}g$ of soap solution. When filled with hydrogen of density $0.0009g/c{m}^3$, it floats in air of density of $0.00129g/c{m}^3$. If surface tension of soap solutions is $30dyne/cm$ the excess pressure in $dyne/c{m}^2$ is:

• A. $30$
• B. $60$
• C. $120$
• D. $240$

Answer 1

The correct option is C $120$

Let the volume of bubble be $V$

Balancing the weight:

$5\times {10}^{-3}g+0.00009\times V\times g=0.00129\times V\times g$

$V=4.16c{m}^2=4\pi r^3/3$

We get, $r=0.998cmor1cm$

Now, excess pressure, $P=\frac{4T}{r}=120/1=120dyne-c{m}^{-2}$