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  4. 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 gives rise to lower pressure in the water in comparison to 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 :

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Q. 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 gives rise to lower pressure in the water in comparison to 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 :Physics Question Image

JEE MainJEE Main 2015Mechanical Properties of Fluids

Solution:

The pressure inside the film is less than the outside atmospheric pressure by
$\frac{2 T}{d}=\frac{T}{d / 2}=\frac{I}{R}$
where $d$ is the thickness of the water layer between the plates and $R$ is the radius of curvature of the two concave surfaces $R \approx \frac{d}{2}$
To an approximation excess pressure $=\frac{2 T}{R}$