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MHT CET 2026 (17 Apr, Shift 1) · Surface Energy & Work Done

Surface Energy & Work Done question from MHT CET 2026 (17 Apr, Shift 1)

Q1MHT CET 2026 · 17 APR · SHIFT 1Easy

Surface Energy & Work Done

A soap bubble of radius 1π\frac{1}{\sqrt{\pi}} cm is expanded to radius 3π\frac{3}{\sqrt{\pi}} cm. Surface tension of soap is 25 dyne/cm. The work done during expansion in erg is

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Answer: (c)

  1. A soap bubble has two surfaces: W=2T×4π(R22−R12)=8πT(R22−R12)W = 2T \times 4\pi(R_2^2 - R_1^2) = 8\pi T(R_2^2 - R_1^2).
  2. R22−R12=9π−1π=8π cm2R_2^2 - R_1^2 = \frac{9}{\pi} - \frac{1}{\pi} = \frac{8}{\pi}\text{ cm}^2.
  3. W=8π×25×8π=1600 ergW = 8\pi \times 25 \times \frac{8}{\pi} = 1600\text{ erg}. So the answer is (c).
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