First slide

Surface tension and surface energy

Question

The amount of work done in blowing a soap bubble such that its diameter increases from d to D is (T= surface tension of the solution)

Moderate

A

$4 π (D^{2} - d^{2}) T$
B

$8 π (D^{2} - d^{2}) T$
C

$π (D^{2} - d^{2}) T$
D

$2 π (D^{2} - d^{2}) T$

Solution

$\begin{array}{l} W = T \times 8 π (r_{2}^{2} - r_{1}^{2}) = T \times 8 π (\frac{D^{2}}{4} - \frac{d^{2}}{4}) \\ = 2 π (D^{2} - d^{2}) T \end{array}$

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