Surface Energy & Work Done question from MHT CET 2026 (19 Apr, Shift 1)

Answer: (c)

1. Surface energy of a bubble $= 8\pi r^2 T$. Big bubble radius $R = D/2$: $E_1 = 8\pi (D/2)^2 T = 2\pi T D^2$.
2. Volume conserved: $64 r^3 = R^3 \Rightarrow r = R/4 = D/8$.
3. $E_2 = 64 \times 8\pi (D/8)^2 T = 8\pi T D^2$
4. $\Delta E = 8\pi TD^2 - 2\pi TD^2 = 6\pi TD^2$
5. So the answer is (c) $6\pi TD^2$.