We consider a spherical liquid drop of radius R. Let S be the surface tension of the liquid. Due to its spherical shape, there is an access pressure P inside the drop over that on outside. This access pressure acts normally outwards.
Let the radius of the drop increase from R to R+dR, under the access pressure P.
Excess Pressure Inside A Soap Bubble
Note: In liquid drop there is only one surfaces in contact with air because in liquid outside the drop is air and inside is water while in case of soap bubble there is a two surface of soap bubble in contact with air inside and outside.
From previous topic-