Fluid Mechanics

JEE Main level, single correct, medium

150 of 192

Question 150 of 192

Wax is coated on the inner wall of a capillary tube and the tube is then dipped in water. Then, compared to the unwaxed capillary, the angle of contact θ\theta and the height hh upto which water rises change. These changes are :

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Answer: (C) θ\theta increases and hh decreases

Angle of contact⁡θ\operatorname{contact} \theta
cos⁡θ=TSA−TSLTLA\cos \theta=\frac{\mathrm{T}_{\mathrm{SA}}-\mathrm{T}_{\mathrm{SL}}}{\mathrm{T}_{\mathrm{LA}}}
when water is on a waxy or oily surface
TSA<TSLcos⁡θ\mathrm{T}_{\mathrm{SA}} \lt \mathrm{T}_{\mathrm{SL}} \cos \theta is negative i.e.,
90∘<θ<180∘90^{\circ} < \theta < 180^{\circ}
i.e., angle of contact θ\theta increases
And for θ>90∘\theta\gt 90^{\circ} liquid level in capillary
tube fall. i.e., h decreases

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