Question

If two glass plates have water between them and are separated by very small distance (see figure), it is very difficult to pull them apart. It is because the water in between forms cylindrical surface on the side that give rise to lower pressure in the water in comparison to the atmosphere. If the radius of the cylindrical surface is $R$ and surface tension of water is $T$ then the pressure in water between the plates is lower by :

• A. $\frac{T}{4R}$
• B. $\frac{T}{2R}$
• C. $\frac{4T}{R}$
• D. $\frac{2T}{R}$

Answer 1

The correct option is D $\frac{2T}{R}$

Excess pressure inside a drop or an air bubble is given by $\Delta =\frac{2T}{R}$
where $T$ and $R$ are the surface tension and radius of drop.
Here, the figure show that the water between the plates forms a curved surface, similar to that formed by air bubble inside the water.
So, pressure in water between the plates is lowered by $\frac{2T}{R}$.