Question

The radius of a soap bubble is increased from $\frac{1}{\sqrt{\pi }}$ cm to $\frac{2}{\sqrt{\pi }}$cm If the surface tension of water is 30 dynes per cm, then the work done will be

• A. 180 ergs
• B. 360 ergs
• C. 720 ergs
• D. 960 ergs

Answer 1

The correct option is C

720 ergs

$W=8\pi T(r_2^2-r_1^2)=8\pi T[⟮\frac{2}{\sqrt{\pi }}{⟯}^2-⟮\frac{1}{\sqrt{\pi }}{⟯}^2]$

$\therefore W=8\times \pi \times 30\times \frac{3}{\pi }=720erg$