First slide

Surface tension and surface energy

Question

The surface tension of soap solution is $25 \times 10^{- 3} {Nm}^{- 1}$ . The excess pressure inside a soap bubble of diameter 1 cm is

Moderate

A

10 Pa
B

20 Pa
C

5 Pa
D

None of the above

Solution

Excess pressure $ΔP = \frac{4 T}{r}$
$= \frac{4 \times 2 \times 25 \times 10^{- 3}}{1 \times 10^{- 2}} = 20 N / m^{2} = 20 Pa (as r = d / 2)$

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