Question

If the difference between pressure inside and outside of a soap bubble is 6 mm of water above atmospheric pressure and its radius is 8 mm. What is the surface tension in dynes per cm :

• A. 116
• B. 256
• C. 378
• D. 450

Answer 1

The correct option is A 116

The excess pressure inside a bubble is given by the expression,

P = $\frac{4T}{r}$

Since, the value of this expression is given as 6mm of water

$P_{excess}={10}^3\times 9.8\times 0.006$ = 58.8 N

This should be equal to the excess pressure developed.

$58.8=\frac{4T}{0.008}$

Solving, T = 0.116 N/m or 116 dyne/cm