The excess pressure inside a **soap bubble** of radius $r$ and surface tension $T$ is:
A$T/r$
B$2T/r$
C$4T/r$
D$8T/r$
Answer & Solution
Correct answer: C. $4T/r$
Soap bubble has TWO air-soap interfaces (inside and outside surface of the film). So excess pressure doubles: $\Delta P = 4T/r$. (Liquid drop has one interface ⇒ $2T/r$.)
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