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A capillary tube dipped in a wetting liquid causes the liquid to rise to a height h inside it, the meniscus being hemispherical. The tube is now pushed further down so that the length of tube projecting above the liquid surface outside is less than h. What happens then?
- Water spurts out of the top like a tiny fountain
- Water slowly trickles out over the top edge
- The liquid rises to the top of the tube but does not overflow
- The meniscus inside the tube ceases to be hemispherical
Correct answer: The meniscus inside the tube ceases to be hemispherical
Solution
Capillary rise h corresponds to a meniscus of radius equal to the tube radius (hemispherical). If the tube above the liquid is shorter than h, the liquid reaches the top but cannot overflow; instead the meniscus flattens (larger radius of curvature) so the reduced pressure deficit matches the smaller available height. The liquid does not flow out.
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