Question

The work done in blowing a soap bubble of radius $r$ and surface tension $T$ is:

• A. $4\pi r^{2} T$
• B. $8\pi r^{2} T$
• C. $2\pi r^{2} T$
• D. $16\pi r^{2} T$

Hint:

Liquid drop = 1 surface; Soap bubble = 2 surfaces.

Answer 1

The Correct Option is B

Step 1: Concept
Work done $W = T \times \Delta A$.
Step 2: Analysis
A soap bubble has two free surfaces (inner and outer). Surface area of one side $= 4\pi r^{2}$. Total area change $= 2 \times 4\pi r^{2} = 8\pi r^{2}$.
Step 3: Conclusion
$W = 8\pi r^{2} T$.
Final Answer: (B)