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Two glass plates are separated by water. If surface tension of water is 75dyn cm1 and the area of each plate wetted by water is 8cm2 and the distance between the plates is 0.12mm , then the force applied to separate the two plates is:
A. 102dyne
B. 104dyne
C. 105dyne
D. 106dyne

Answer
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Hint: Use the excess pressure formula to find the pressure lowered between the plates i.e.
P=TR
Here, T is the surface tension of water
R is the radius of the two surfaces due to which the pressure is lowered.
In simple words we will put the values in the equation to get the required force applied to separate the two plates.

Complete answer:
seo images

Here we can see the shape of the water layer between the two plates.
Thickness of the film, d=0.012cm
Pressure of the cylindrical face, R=d2
Now, pressure across the surface,
P=\fdracTR
P=2Td
Area of each plate wetted by water, A
And we know that force required to separate the two plate is,
F=pressure×area
F=2TdA
Now, substituting the values in above equation we get,
F=2TdA
F=2×75×80.012
F=105dyne
So, the force applied to separate the two plates is 105dyne .

Hence, the correct option is C.

Note:
Here the curved surface is a cylinder with radius R. Since this behaves as an air bubble in water that is why we have taken the radii to be equal. Otherwise, one should calculate the excess pressure using the radii of the given shape which is not a cylinder or spherical as an air bubble.