Ncert Exercises & Solutions

10.18 Figure 10.24 (a) shows a thin liquid film supporting a small weight = 4.5 × 10^–2 N. What is the weight supported by a film of the same liquid at the same temperature in Fig. (b) and (c)? Explain your answer physically.

Fig. 10.24

In the case (a):

The length of the liquid film supporting the weight, l = 40 cm = 0.4 cm

The weight supported by the film, W = 4.5 × 10^–2 N

A liquid film has two free surfaces.

$∴ Surface tension = \frac{Weight}{Total length} = \frac{4.5 \times 10^{- 2}}{2 \times 0.4} = 5.625 \times 10^{- 2} N m^{- 1}$

The surface tension in all the cases is the same as the liquid and temperature of the liquid is the same in all the cases.

Since the length of the film in all three cases is 40 cm, the weight supported in each case is 4.5 × 10^–2 N.

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