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U-shaped wire is dipped in a soap solution,and removed.The thin film formed between the wire and the light slider,supports a weight of $1.5\times 10^{-2}N$(which includes the small weight of the slider).The length of slider is 30cm.What is the surface tension of the film?

$\begin{array}{1 1}(A)\;2.5\times 10^{-2}N/m\\(B)\;3.5\times 10^{-2}N/m\\(C)\;3\times 10^{-2}N/m\\(D)\;2.5\times 10^{-3}N/m\end{array} $

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The weight that the soap film supports $\omega=1.5\times 10^{-2}N$
A soap film has two free surfaces
$\therefore$ Total length=2l=0.6m$
Surface tension $S=\large\frac{\text{weight}}{2l}$
$\Rightarrow \large\frac{1.5\times 10^{-2}}{0.6}$
$\Rightarrow 2.5\times 10^{-2}N/m$
Hence (A) is the correct answer.
answered May 9, 2014 by sreemathi.v

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