Question

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

• A. $1.5\times {10}^{-2}N/m$
• B. $3\times {10}^{-2}N/m$
• C. $6\times {10}^{-2}N/m$
• D. $11\times {10}^{-2}N/m$

Answer 1

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

$\Delta A=[11\times 10-10\times 6]$
$=5\times {10}^{-3}{m}^2$
$\Delta U=3\times {10}^{-4}J$
Using
$\Delta U=2\Delta AT$
$3\times {10}^{-4}=2\times 5\times {10}^{-3}\times T$
Thus
$T=3\times {10}^{-2}N/m$