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Question:

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

T2R

4TR

2TR

T4R

Solution:

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