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 $1.5 \times 10^{-2} N$ (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

We know that the soap membrane has two independent surfaces length that supports the membrane
$
\begin{aligned}
l & =2 \times 30 cm=60 cm=60 \times 10^{-2} m \\
l & =6 \times 10^{-1} m \\
W & =1.5 \times 10^{-2} N \\
F & =T \times 2 l=T \times 6 \times 10^{-1} N
\end{aligned}
$
In the equilibrium condition the force F is on sliding due to surface tension should be equal to the weight W supporting the sliding
$
\begin{aligned}
F & =W=mg \\
T \times 0.6 & =1.5 \times 10^{-2} \\
T & =\frac{1.5 \times 10^{-2}}{0.6}=2.5 \times 10^{-2} N / m
\end{aligned}
$

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