First slide

Surface tension

Question

Water rises is a vertical capillary tube of radius 1 mm. Surface tension of water is 0.07 N/m. A and B are two points just above and just below the free surface of water in the capillary tube. The difference in pressure at A and B is

Moderate

A

1400 $N / m^{2}$
B

140 $N / m^{2}$
C

720 $N / m^{2}$
D

1100 $N / m^{2}$

Solution

Glass-water angle of contact is $0^{o}$ . The free surface of water inside the capillary tube is part of an air bubble of radius $r = 1 m m$ .
$∴ p_{A} - p_{B} = \frac{2 S}{r} = \frac{2 \times 0.07}{10^{- 3}} N / m^{2} = 140 N / m^{2}$

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