Question

The lower end of a capillary tube is at a depth of $12cm$ and water rises $3cm$ in it. The mouth pressure required to blow an air bubble at the lower end will be $xcm$ of water column, where $x$ is

• A. $12$
• B. $15$
• C. $3$
• D. $9$

Answer 1

The correct option is B $15$

The radius of air bubble is same as radius of capillary tube = R
Mouth Pressure be $P$ then pressure just below meniscus at the top of capillary = $\frac{2T}{R}-P$
Pressure equations at the top of capillary and bubble gives,
$\frac{2T}{R}-P+\rho gh=\frac{2T}{R}$
$h=12\text{cm}+3\text{cm}$
$P=15\text{cm of water column}$