# 16. Physics | Surface Tension | Force Between Two Parallel Plates Due to Liquid Between Them (GA)

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

### 3 Replies to “16. Physics | Surface Tension | Force Between Two Parallel Plates Due to Liquid Between Them (GA)”

1. Surakul Cuesat says:

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.

2. 최지원 says:

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

3. Egusi Brothers says:

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.