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A glass capillary tube of internal diameter 0.28 mm is dipped vertically into water. What gauge pressure (in N/m²) must be applied to the water inside the tube so that the water level in the tube becomes the same as that in the vessel? (Surface tension of water = 0.07 N/m, atmospheric pressure = 10⁵ N/m².)
- 10³
- 99 * 10³
- 100 * 10³
- 101 * 10³
Correct answer: 99 * 10³
Solution
Capillarity normally raises water by creating a pressure just below the meniscus that is 2T/r less than atmospheric. To keep the level equal to the vessel, the pressure inside must equal atmospheric minus 2T/r.
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