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 × 10–2 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 × 10 - 2 N
Length of the slider, l = 30 cm = 0.3 m
A soap film has two free surfaces.
∴Total length = 2l = 2 × 0.3 = 0.6 m

Surface tension, $S = \frac{\text{Force or Weight} }{ 2l}$

$= \frac{1.5 × 10 ^{- 2} }{ 0.6}$

= 2.5 × 10 - 2N / m

Therefore, the surface tension of the film is 2.5 × 10 - 2 Nm-1.