Question

Figure shows a capillary tube of radius r dipped into water. If the atmospheric pressure is P0, the pressure at point A is
(a) P0
(b) P0+2Sr
(c) P0-2Sr
(d) P0-4Sr
Figure

Answer 1

Answer:

(b) po+2sr... ..

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Answer 2

(c) P0-2Sr

Explanation:

• Radius of the hemispherical surface = r
• Net upward force due to surface tension = S cos θ x 2 π r  
• Net downward atmospheric pressure = P0
• Upward pressure =  S cos θ x 2 π r (divided by) π r2 = 2Scos θ (divided by) r
• Net pressure at point A = P0 - 2Scos θ (divided by) r
• Cos θ is approximately 1, since θ is small.
• Therefore, net pressure = 2S (divided by) r
• The atmospheric pressure is greater than the pressure inside a concacve meniscus.