Question

The lower end of a glass capillary tube is dipped in water. Water rises to a height of $8cm.$ The tube is then broken at a height of $4cm.$ The height of water column and angle of contact will be.

• A. $2cm,60^{\circ }$
• B. $4cm,120^{\circ }$
• C. $4cm,60^{\circ }$
• D. $2cm,30^{\circ }$

Answer 1

Answer: (C)

Water rises in capillary tube due to force of adhesion between solid and liquid molecules. Tube is broken at height of $4cm$, so force of adhesion is present till $4cm$.
So height of water column is $4cm$.
$\therefore \frac{h}{\cos \theta }=\frac{h_1}{\cos {\theta }^{\prime }}\left(\frac{h}{\cos \theta }=\frac{2S}{\rho rg}\right)$
$\therefore \frac{8}{\cos 0^0}=\frac{4}{\cos \theta }$
$\Rightarrow \cos \theta =\frac{1}{2}$
$\Rightarrow \theta =60^{\circ }$