First slide

Surface tension

Question

Capillary rise of water in a vertical glass capillary tube of radius r is 1 m what will be the excess pressure in a buble of radius 4r formed in water in a lake ? Take density of water = 1 gm/C.C and $g = 10 m / s^{2} .$

Moderate

A

$2.5 \times 10^{5} N / m^{2}$
B

$10^{5} N / m^{2}$
C

$2500 N / m^{2}$
D

$5000 N / m^{2}$

Solution

(3)
$\begin{array}{l} C a p i l l a r y r i s e h = \frac{2 S}{ρ r g} w h e r e S = s u r f a c e t e n s i o n . \\ \Rightarrow \frac{2 S}{r} = ρ r g \end{array}$
Excess pressure in the bubble,
$\begin{array}{l} Δ P = \frac{2 S}{(4 r)} = \frac{1}{4} \times 10 \times 10 \times 1 N / m^{2} \\ = 2500 N / m^{2} \end{array}$

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