Question

Two vertical parallel glass plates are partially submerged in water. The distance between the plates is $d$ and the length $l$. Assume that the water between the plates does not reach the upper edges of the plates and that the wetting is complete. If water is replaced by another liquid of surface tension $2\sigma$, then the force $F$ of attraction between two plates would become($\rho =$density of water and $\sigma =$ surface tension water)

• A. $2F$
• B. $4F$
• C. $8F$
• D. $16F$

Answer 1

The correct option is B $4F$

Since,
$F=l\rho g{h}^2/2$
Also,
$h=2\sigma /\rho gd$
$\therefore F\propto {\sigma }^2$
So when $\sigma$ is doubled, $F$ changes to $4F$