First slide

Surface tension

Question

If T is surface tension of soap solution, the amount of work done in blowing a soap bubble from diameter D to a diameter 2 D is

Easy

A

$2 {πD}^{2} T$
B

$4 {πD}^{2} T$
C

$6 {πD}^{2} T$
D

$8 {πD}^{2} T$

Solution

Work done
$= 4 π [{(2 \times \frac{D}{2})}^{2} - {(\frac{D}{2})}^{2}] T = 4 π \frac{D^{2}}{4} [4 - 1] T = 3 {πD}^{2} T Ihere are two surfaces of soap bubble ∴ W = 2 \times 3 {πD}^{2} T = 6 {πD}^{2} T$

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