Question

A film of water is formed between two straight parallel wires each $10cm$ long and at separation $0.5cm.$ Calculate the work required to increase $1mm$ distance between the wires. Surface tension of water $=72\times {10}^{-3}N/m.$

• A. $1.44\times {10}^{-3}$
• B. $1.44\times {10}^{-7}$
• C. $1.44\times {10}^{-5}$
• D. $1.44\times {10}^{-4}$

Answer 1

The correct option is C $1.44\times {10}^{-5}$

Given:

Length $L=10$ cm

Width $W=0.5$ cm

Surface tension $S=72\times {10}^{-3}$

Increase distance by $\Delta l=1$ mm

change in area $\Delta A=(L\times W)=10\left(.1\right)\times {10}^{-4}$

$\begin{array}{c}E=S\Delta A\\ =72\times {10}^{-3}\times 2\times 1\times {10}^{-4}\\ =1.44\times {10}^{-5}J\\ Ans.\left(C\right)\end{array}$