First slide

Surface tension

Question

Two soap bubbles A and B of radii $r$ and 2r are formed at the ends of two capillary tubes. If excess pressure in A is 40 $N / m^{2}$ , what is the excess pressure in B?

Easy

A

40 $N / m^{2}$
B

80 $N / m^{2}$
C

20 $N / m^{2}$
D

60 $N / m^{2}$

Solution

Excess pressure $Δ p \propto \frac{1}{r}$
$∴ \frac{Δ p_{B}}{Δ p_{A}} = \frac{r_{A}}{r_{B}} \Rightarrow \frac{Δ p_{B}}{40} = \frac{r}{2 r} \Rightarrow Δ p_{B} = 20 N / m^{2}$

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