Question

Work of $3.0\times {10}^{-4}$ joule is required to be done in increasing the size of a soap film from $10cm\times 6cm$ to $10cm\times 11cm$. The surface tension of the film is

• A. $5\times {10}^{-2}N/m$
• B. $3\times {10}^{-2}N/m$
• C. $1.5\times {10}^{-2}N/m$
• D. $1.2\times {10}^{-2}N/m$

Answer 1

The correct option is B $3\times {10}^{-2}N/m$

Surface tension, $S=\frac{\text{Work done}}{\text{Increase in area}}$
As the soap film has two surfaces.
$S=\frac{3.0\times {10}^{-4}J}{2\times (10\times 11-10\times 6)\times {10}^{-4}{m}^2}=3\times {10}^{-2}N/m$.
Hence, the correct answer is option (b).