A soap bubble of radius 3 cm is formed inside another soap bubble of radius 6 cm. Find the radius, in cm, of a single soap bubble having the same excess pressure as the pressure inside the smaller bubble relative to atmosphere.
Physics · Properties of solid and fluid
JEE Mains 2021

Question

A soap bubble of radius 3 cm is formed inside another soap bubble of radius 6 cm. Find the radius, in cm, of a single soap bubble having the same excess pressure as the pressure inside the smaller bubble relative to atmosphere.

Physics · Properties of solid and fluid

JEE Mains 2021

G1 Physics · Accepted Answer

Step 1: Pressure inside the inner bubble relative to atmosphere equals 4T/3 + 4T/6 (with radii in cm), because it includes the excess across both soap films. Step 2: Set 4T/Req equal to that total excess pressure. This gives Req = 2 cm. Final answer: 2.