10. Class 11th Physics | Surface Tension | Excess Pressure Inside a Soap Bubble | Ashish Arora (GA)

10. Class 11th Physics | Surface Tension | Excess Pressure Inside a Soap Bubble | Ashish Arora (GA)

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lets discuss the , concept of excess pressure
inside a bubble. if we talk about the soap bubble or a bubble made up of a liquid film.
and say we consider the wall thickness of the bubble to be very small compare to its
radius. in this situation again due to surface tension . the. liquid surface . inside as
well as outside on the, soap bubble or the liquid bubble will have a tendency to contract.
and due to which inside, say if there is a point ay and outside there is a point b, obviously
due to this contracting tendency pressure at ay will be more then pressure at b. and
at some point when the bubble will be in equilibrium we can say . the excess pressure inside is
balance by the force of the surface-tension. like what we have discussed in the case of
excess pressure inside a liquid drop. here also to analyze this excess pressure we divide
the. bubble in two hemispherical sections. and here you can see . if i just draw hemispherical
section it’ll look like this. this is the hemispherical. film of the , soap bubble or
the liquid bubble. and in this situation we can say . the right part will exert a force
. on it in outward direction due to excess pressure. and this outward force can be simply
written as excess pressure multiplied by the surface area which is pie r square that is
the . area of this . hemispherical , base. which is pie r square. and on the two sircumferences
in. inward direction surface tension will exert a force that is t into two pie r. and
on both the surfaces it’ll be t into two pie r and we can take the radius of. the two
surfaces to be r as. the film thickness or wall thickness of this bubble can be considered
to be very small. so in this situation we can say . for, iquilibrium of. hemispherical
. section. of the bubble. we can use. in this situation it is excess pressure multiplied
by pie r square that is outward force must be balance by twice of . t into two pie r
. in this situation pie and r gets cancelled out ,and. excess pressure can be written as
. four t by r . this exactly double that of the excess pressure inside a liquid drop because
here there are two , liquid surfaces where as in case of drop only outer surface exist
. that’s why the excess pressure, in a soap bubble or a liquid bubble, is double then
that of. the liquid drop of same radius , made up of same, solution. so just be careful about
this expression of excess pressure inside a bubble.

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