let us discuss the calculation of force between

two parallel plates, due to a liquid between these, lets first discuss a situation . here

you can see in this picture, there are two glass plates. say these are glass plates.

and between these two glass plates a liquid is trapped, and it is also given that the

circular area of contact of the liquid with the plates is. here given that . area of contact

is equal to ay. in this situation at all the edges we can say. an inward meniscus, will

be obtained, and the liquid will be having a cylindrical sort of shape. on which one

side the curvature radius is r and other side it is flat. so in this situation we can say

if, meniscus is having a radius of curvature r. then pressure difference at point ay and

b , inside and outside can be easily calculated. and in this situation if . the separation

between the plates is d. and if this contact angle is theta , we can directly write down

in this situation if , this is the curvature , radius and sphere of curvature for this

meniscus . then we can directly state in this situation this angle is theta , and the separation

d can be written as two r coz theta in this situation. and pressure difference at point

ay and b can be written as p-ay minus p-b is equal to . t by r because it is surface

having two curvatures at one side it is flat and other side it is having, a curvature radius

r, so will use multiple curvature expression . and in this situation at point ay pressure

is p-atmospheric so at point b pressure can be calculated directly as p-atmospheric minus

t by r . now in this situation when we talk about these two plates. due to surface tension

inside pressure is low but outside we are having pressure equal to p atmospheric, so

as inside pressure is , less then p-atmospheric this atmospheric pressure will be pushing

from the two sides the plates. that’s why we can say, the two plates will have a tendency

to attract actually these are not attracting it is the atmospheric pressure which is pushing

them close to each other. so if we wish to separate this two plates we need to apply

a force f onto these plates . and we can say. to separate. these plates. we need to apply.

a force, f. which should be equal to . in this situation we must write p-atmospheric.

minus, pressure at point b multiplied by the surface area where, the liquid is in contact

or the zone in which the pressure is less. so in this situation p-atmospheric minus pressure

b can be written as t by r, so it can be written as t-ay by r . and in this situation , the

radius of curvature can be given as d by two coz theta . so here we can write it. t-ay

divided by , d by two coz theta this is the expression for the force. and, like for glass

water. pair generally we take theta approximately zero , if we take theta approximately zero

this implies the value of force can be written as two t ay by b. so this also a quite a useful

expression which is used in, different numerical problems, so just keep the expression and

the analysis in your mind.

What is T? I am a Thai student. I am not good at English so sometimes I don't know what did you say. Could you tell me what T is. Thank you so much for this video.

I am Korean high school student. The force due to surface tension does not the considered? For example, 2 pi R T sin theta

When you plugged in the expression for the pressure at pt. B, for the force needed to separate the plate, why didn't you include the factor 2, because according to laplace eqn, change in pressure is equal to surface tension multiplied by 2 and curvature for sperical interfaces.