JEE Advance - Physics (2018 - Paper 1 Offline - No. 11)

A uniform capillary tube of inner radius $$r$$ is dipped vertically into a beaker filled with water. The water rises to a height $$h$$ in the capillary tube above the water surface in the beaker. The surface tension of water is $$\sigma .$$ The angle of contact between water and the wall of the capillary tube is $$\theta .$$ Ignore the mass of water in the meniscus. Which of the following statements is (are) true?
For a given material of the capillary tube, $$h$$ decreases with increase in $$r$$
For a given material of the capillary tube, $$h$$ is independent of $$\sigma $$
If this experiment is performed in a lift going up with a constant acceleration, then $$h$$ decreases
$$h$$ is proportional to contact angle $$\theta $$

Explanation

We know that,

$$h = {{2\sigma \cos \theta } \over {r\,\rho g}}$$..... (1)

where r is inner radius of capillary tube; $$\sigma$$ is surface tension; $$\theta$$ is angle between water and wall of capillary tube; h is height of water in capillary tube and g is acceleration due to gravity. From Eq. (1),

$$h \propto {1 \over r}$$

Therefore, for a given material of the capillary tube, h decreases with increase in r.

Hence, option (A) is correct.

h depends on $$\sigma$$, therefore option (B) is incorrect.

In a lift going up with a constant acceleration a, the force due to surface tension remains same but weight of the water increases i.e., Wlift = $$\pi$$r2h$$\rho$$(g + a). Thus, the height of the water rise in a capillary becomes

$$h = {{2\sigma \cos \theta } \over {r\,\rho (g + a)}} = {{2\sigma \cos \theta } \over {r\, \rho{g_{eff}}}}$$,

where geff is the effective g in the lift. Hence, h decrease if the experiment is performed in a lift going up with a constant acceleration.

From (1), h $$\propto$$ cos$$\theta$$ and not $$\theta$$, therefore, option (D) is incorrect.

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