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Question

A thin liquid film formed between a $$\mathrm{U}$$-shaped wire and light slider supports a weight of $$1.5\times 10^{-2}N$$ (see figure). If the length of the slider $$30 cm$$ and its weight negligible, then the surface tension of the liquid film is:

40199_a6aa031f23ef43eebffb327cc518c363.png


A
0.0125Nm1
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B
0.1Nm1
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C
0.05Nm1
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D
0.025Nm1
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Solution

The correct option is B $$0.025Nm^{-1}$$
Here, $$W = 1.5 \times 10^{-2} N,$$

$$ l = 30 cm = 30 \times 10^{-2} m$$

The force due to surface tension will balance the weight. i.e., $$F = W$$

A liquid film has two free surfaces.

$$2 TL = W \ or \ T =$$ $$\displaystyle \frac{W}{2L}$$

$$T = \displaystyle \frac{1.5 \times 10^{-2}}{ 2 \times 30 \times 10^{-2}} = 0.025 N m^{-1}$$

A slider will support the weight when the force of surface tension acting upwards on the slider $$( 2Sl)$$ balances the downward force due to weight $$(W).$$

1501173_40199_ans_fd8de38c73944d06befbc58a07b12e9c.jpg

Physics

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