Question

The work done in increasing the size of a soap film from $10cm\times 6cm$ to $10cm\times 11cm$ is $3\times {10}^{-4}J$. The surface tension of the film is:

• A. $3.0\times {10}^{-2}N/m$
• B. $6.0\times {10}^{-2}N/m$
• C. $1.5\times {10}^{-2}N/m$
• D. $11.0\times {10}^{-2}N/m$

Answer 1

The correct option is A $3.0\times {10}^{-2}N/m$

Since the film is formed on both the surfaces
$\Delta U=2T\times \Delta A$ where $\Delta U$ is change in surface energy, $\Delta A$ is change in area and T is surface tension.
Substituting,
$(10\times 11-6\times 10)\times {10}^{-4}\times 2T=3\times {10}^{-4}$
$\Rightarrow T=3\times {10}^{-2}N{m}^{-1}$