Q. The work done in increasing the size of a rectangular soap film with dimensions $8cm \times 3.75$ cm to $10cm \times 6cm$ is $2 \times 10^{- 4}$ J. The surface tension of the film in $Nm^{- 1}$ is
A) $1.65 \times 10^{- 2}$
B) $3.3 \times 10^{- 2}$
C) $6.6 \times 10^{- 2}$
D) $8.25 \times 10^{- 2}$
Explanation
(b) Change in surface energy $= 2 \times 10^{- 4}$ J
$\mathrm{\Delta}A = 10 \times 6 - 8 \times 3.75$
$= 30\ cm^{2}$
$= 30 \times 10^{- 4}m^{2}$
Work done $W = T \times 2 \times$ (change in area)
Now, change in surface energy $=$ Work done
$2 \times 10^{- 4} = T \times 2 \times 30 \times 10^{- 4}$
$T = 3.3 \times 10^{- 2}\ Nm^{- 1}$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.