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

MCQ

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?

A
$6.32 \times 10^{-3}\; N m ^{-1}$
B
$5.25 \times 10^{-4}\; N m ^{-1}$
C
$6.8 \times 10^{-3}\; N m ^{-1}$
✓
$2.5 \times 10^{-2}\; N m ^{-1}$

✓

Answer

Correct option: D.

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

d
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 $=2 l=2 \times 0.3=0.6 m$

Surface tension, $S=\frac{\text { Force o-Weight }}{2 l}$ $=\frac{1.5 \times 10^{-2}}{0.6}$$=2.5 \times 10^{-2} N / m$

Therefore, the surface tension of the film is $2.5 \times 10^{-2}\; N m ^{-1}$

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