A water film is formed between the two straight parallel wires, each of length 10 $\mathrm{cm}$, kept at a…
A water film is formed between the two straight parallel wires, each of length 10
$\mathrm{cm}$, kept at a separation of $0 \cdot 5 \mathrm{~cm}$. Now, the separation between them is
increased by $1 \mathrm{~mm}$ without breaking the water film.
The work done for this is
(surface tension of water $=7 \cdot 2 \times 10^{-2} \mathrm{Nm}^{-1}$ )
$7 \cdot 22 \times 10^{-6} \mathrm{~J}$
$5 \cdot 76 \times 10^{-5} \mathrm{~J}$
$1 \cdot 44 \times 10^{-5} \mathrm{~J}$
$2 \cdot 88 \times 10^{-5} \mathrm{~J}$
Solution
Increase in the area of the film is
$\Delta \mathrm{A}=10 \mathrm{~cm} \times 0.1 \mathrm{~cm}=1 \mathrm{~cm}^{2}=10^{-4} \mathrm{~m}^{2}$
Work done $W=2 T . \Delta A$
$\begin{array}{l}
=2 \times 7.2 \times 10^{-2} \times 10^{-4} \mathrm{~J} \\
=1.44 \times 10^{-5} \mathrm{~J}
\end{array}$