InterviewSolution
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InterviewSolution
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A vertical capillary is brought in contact with the water surface. What amount of heat is liberated while the water rises along the capillary ? The wetting is assumed to be complete, the surface tension equals `alpha`. |
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Answer» If the liquid rises to a height `h`, the energy of the liquid column becomes `E = rho g pi r^2 h. (h)/(2) - 2pi rh alpha = (1)/(2) rho g pi (r h - 2(alpha)/(rho g))^2 - (2 pi alpha^2)/(rho g)` This is minimum when `rh = (2 alpha)/(rho g)` and that is relevant height to which water must rise. At this point, `E_(min) = - (2 pi alpha^2)/(rho g)` Since `E = 0` in the absence of surface tension a heat `Q = (2 pi alpha^2)/(rho g)` must have been liberated. |
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