Q. A film of water is found between two straight parallel wires of length 10 cm each separated by 0.2 cm. If their separation is increased by 1 mm, while still maintaining their parallelism, how much work will have to be done? (surface tension of water is $7.2 \times 10^{- 2}\ Nm^{- 1}$)
A) $7.22 \times 10^{- 6}$ J
B) $1.44 \times 10^{- 5}$ J
C) $2.88 \times 10^{- 5}$ J
D) $5.76 \times 10^{- 5}$ J
Explanation
(b) Work done = surface tension $\times$ increase in area
$W =$ surface tension $\times \lbrack 0.10 \times 0.006 - 0.10 \times 0.005\rbrack \times 2$
$= 7.2 \times 10^{- 2} \times 0.10 \times 0.001 \times 2$
$= 1.44 \times 10^{- 5}$ J
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.