THEORY OF EXCESS PRESSURE IN LIQUID DROPS
(i) There is no pressure difference across a plane liquid surface as the resultant force of surface tension on a molecule on its surface is zero.
(ii) In case of a curved liquid surface, the resultant force of surface tension on a molecule on the liquid surface acts inwards towards the center of curvature. Hence, pressure is greater on the concave side of a curved liquid surface.
Excess pressure inside a liquid drop: P = 2S/R
Excess pressure inside a soap bubble: P = 4S/R
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