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 $1.5\times {10}^{–2}N$ (which includes the small weight of the slider). The length of the slider is 30 cm. What is the surface tension of the film?

Answer 1

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