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

Weight $mg=1.5\times {10}^{-2}N$

Length of slider $l=30\times {10}^{-2}m$

Force due to surface tension = weight

We know that $\sigma \times 2l=mg$

Therefore surface tension $\sigma =\frac{mg}{2l}$

$=\frac{1.5\times {10}^{-2}}{2\times 30\times {10}^{-2}}$

$=2.5\times {10}^{-2}N{m}^{-1}$.