Question

A film of water is formed between two straight parallel wires each 10 cm long and at a separation of 0.5 cm. Calculate the work required to increase the distance between them by 1 mm
Surface tension of water = $72\times {10}^{-30}N/m$

Answer 1

Here the increase in area is shown by shaded portion in the figure Since this is a water film it has two surfaces therefore increase in area $\Delta S=2\times 10\times 0.1=2c{m}^2$

$\therefore$ Work required to be done
$W=\Delta S\times T$
$=2\times {10}^{-4}\times 72\times {10}^{-3}$
$=144\times {10}^{-7}$ joule
$=1.44\times {10}^{-5}$ joule