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The lower end of a capillary tube is held at a depth of 12 cm below the surface of water, and water rises 3 cm inside the capillary. Taking the contact angle as 0 deg, find the mouth pressure (in cm of water column) required to blow an air bubble at the lower end of the tube.
- 12
- 15
- 3
- 9
Correct answer: 15
Solution
At the lower end, depth = 12 cm of water column hydrostatic pressure must be overcome. The capillary rise of 3 cm corresponds to the excess pressure 2T/r = (rise height) of water column needed to form the bubble at the tube mouth. So required mouth pressure = 12 + 3 = 15 cm of water column.
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