Question

A film of water is formed between two straight parallel wires of length $10cm$ each separated by $0.5cm$. If their separation is increased by $1mm$ while still maintaining their parallelism how much work will have to be done of water (Surface tension of water $=7.2\times {10}^{-2}N/m$ ):

• A. $7.22\times 10^{-6}J$
• B. $1.44\times 10^{-5}J$
• C. $2.88\times 10^{-5}J$
• D. $5.76\times 10^{-5}J$

Answer 1

Answer: (B)

$1.44\times 10^{-5}J$

Work done $=2\times \Delta A\times T$

Change in area $\Delta A=\left(lengthofwire\right)\times \left(changeindistancebetweenwires\right)$

$\Delta A=l\times \Delta d$

$\Delta d=1mm=1\times {10}^{-3}m$

$\Delta A=l\times \Delta d=\frac{10}{100}\times 1\times {10}^{-3}={10}^{-4}{m}^2$

Work done $=2\times \frac{10}{100}\times \frac{1}{1000}\times 7.2\times {10}^{-2}$$=1.44\times$ ${10}^{-5}J$