Water rises in a capillary tube of radius r. If the atmospheric pressure is P0, what is the pressure at point A located just below the curved (concave) liquid meniscus inside the tube?

P0
P0 + 2S/r
P0 - 2S/r
P0 - 4S/r

Correct answer: P0 - 2S/r

Solution

Across a curved liquid surface the pressure on the concave side is lower. Just above the meniscus pressure is atmospheric (P0). Point A, just below the meniscus inside the liquid (concave side), is therefore at P0 - 2S/r.

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