Question

Due to capillary action a liquid will rise in a tube if angle of contact is

• A. acute
• B. obtuse
• C. ${90}^o$
• D. ${180}^o$

Answer 1

The correct option is A acute

The rise of liquid in a capillary tube when placed in a vessel full of liquid is known as capillary action. The height of the rise of liquid is given by the formula $h=\frac{2Tcos\theta }{r\rho g}$. Here $\theta$ is the angle of contact, T is surface tension, and other symbols have their usual meaning.

Only if $\theta$ is acute, $cos\theta$ will be positive, meaning the height will be positive and liquid will rise.

If $\theta$ is obtuse, $cos\theta$ will be negative, meaning the height will be negative and liquid will fall inside the capillary.

This means the level of liquid in the capillary will be lesser than the vessel. This happens in case of mercury.

Only if $\theta$ is 90, $cos\theta$ will be zero, meaning the height will be zero and liquid will not rise.

$\theta ={180}^o$ is not possible.