A U-shaped wire is dipped in a soap solution, and removed. The th… - Vidyadip

Question

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

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Answer

The weight that the soap film supports, $\mathrm{W}=1.5 \times 10^{-2} \mathrm{~N}$ Length of the slider, $\mathrm{I}=30 \mathrm{~cm}=0.3 \mathrm{~mA}$ soap film has two free surfaces.
$\therefore$ Total length $=2 \mathrm{l}=2 \times 0.3=0.6 \mathrm{~m}$
Surface tension, $S=\frac{\text { Force of Weight }}{21}=1.5 \times \frac{10^{-2}}{0.6}=2.5 \times 10-2 \mathrm{~N} / \mathrm{m}$
Therefore, the surface tension of the film is $2.5 \times 10-2 \mathrm{~N} \mathrm{~m}^{-1}$.

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