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 and the height h upto which water rises change. These changes are: (a) θ increases and h also increases (b) θ decreases and h also decreases (c) θ increases and h decreases (d) θ decreases and h increases

Answer: (c). Water does not wet wax, so the contact angle increases (becomes more than 90°). h = (2Tcos θ)/(rρ g): larger θ gives smaller cos θ, so h decreases (water may even be depressed). So the answer is (c) θ increases and h decreases.

Wax is coated on the inner wall of a capillary tube and the tube is… | JEE Main 2013 (23 Apr, Shift 1)

Q67JEE Main 2013 · 23 APR · SHIFT 1Easy

Angle of Contact & Capillarity

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 $\theta$ and the height $h$ upto which water rises change. These changes are:

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Answer: (c)

Water does not wet wax, so the contact angle increases (becomes more than $90^\circ$ ).
$h = \frac{2T\cos\theta}{r\rho g}$ : larger $\theta$ gives smaller $\cos\theta$ , so $h$ decreases (water may even be depressed).
So the answer is (c) $\theta$ increases and $h$ decreases.

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