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JEE Main 2018 (16 Apr, Shift 1) · Excess Pressure: Drops & Bubbles

Excess Pressure: Drops & Bubbles question from JEE Main 2018 (16 Apr, Shift 1)

Q57JEE Main 2018 · 16 APR · SHIFT 1Medium

Excess Pressure: Drops & Bubbles

A small soap bubble of radius 4 cm is trapped inside another bubble of radius 6 cm without any contact. Let P2 be the pressure inside the inner bubble and P0, the pressure outside the outer bubble. Radius of another bubble with pressure difference P2 ______ P0 between its inside and outside would be:

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Answer: (b)

P2P1P0r = 4 cm, R = 6 cm
  1. Outer bubble: P1−P0=4T6P_1 - P_0 = \frac{4T}{6}; inner bubble: P2−P1=4T4P_2 - P_1 = \frac{4T}{4}.
  2. Adding: P2−P0=4T(14+16)=4T⋅512P_2 - P_0 = 4T\left(\frac{1}{4} + \frac{1}{6}\right) = 4T \cdot \frac{5}{12}
  3. A single bubble with the same difference: 4Tr=4T12/5⇒r=2.4 cm\frac{4T}{r} = \frac{4T}{12/5} \Rightarrow r = 2.4\text{ cm}
  4. So the answer is (b) 2.4 cm2.4\text{ cm}.
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