First slide

Surface tension

Question

Excess pressure in a soap bubble is 10 N/m². If radius of the soap bubble is 5 mm, the total surface energy stored in the bubble is

Moderate

A

12.45 $μJ$
B

6.75 $μJ$
C

5.25 $μJ$
D

7.85 $μJ$

Solution

${ΔP}_{} = \frac{4 T}{r} \Rightarrow 4 T = r . Δ P$
Total surface energy, $U = (4 {πr}^{2} T) \times 2 = 2 {πr}^{3} . Δ P$
$\Rightarrow U = 2 \times 3 .14 \times {(5 \times 10^{- 3})}^{3} \times 10 J = 7 .85 μJ$

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