Question

A small drop of water of surface tension T is squeezed between two clean glass plates so that a thin layer of thickness d and area A is formed between them. If the angle of contact is zero, the force required to pull the apart is :

• A. $\frac{TA}{d}$
• B. $\frac{2TA}{d}$
• C. $\frac{4TA}{d}$
• D. $\frac{8TA}{d}$

Answer 1

The correct option is B $\frac{2TA}{d}$

$F=pA$

$P=T(\frac{1}{R_1}+\frac{1}{R_2})$

where

P - excess pressure inside

In general force entered by a liquid due to surface tension is given by

$F=t(\frac{1}{R_1}+\frac{1}{R_2})\times A$

Here, $R_1$ and $R_2$

is radius of liquid profile in two dimensions.

Here, $r_1=\frac{d}{2}$

$r_2=\infty$ (surface along it's breadth is flat)

$F=T(\frac{2}{d}+0)AP=\frac{2TA}{d}$