Question

If excess of pressure inside a soap bubble of radius $2 cm$ is balanced by that due to column of oil of (specific gravity $=0.8$ ) $2 mm$ high. What is the surface tension of soap bubble?

• A. $12 \times 10^{2} N / m$
• B. $6 \times 10^{-2} N / m$
• C. $8.12 \times 10^{-2} N / m$
• D. $7.84 \times 10^{-2} N / m$

Answer 1

Answer: (D)

$P = h \rho g $
$=2 \times 10^{-3} \times 0.8 \times 10^{3} \times 9.8$ (sp. gravity $\left.=\frac{\rho_{\ell}}{ P _{ w }}\right)$
$P=15.68 Pa$
For soap bubble. $P =\frac{4 S }{ R } $
$\Rightarrow 15.68=\frac{4 \times S }{2 \times 10^{-2}}$
$\therefore S =7.84 \times 10^{-2} N / m$