First slide

Surface tension

Question

A large number of water drops each of radius r combine to have a drop of radius R. If the surface tension is T and the mechanical equivalent of heat is J, then the rise in temperature will be

Moderate

A

$\frac{2 T}{rJ}$
B

$\frac{3 T}{RJ}$
C

$\frac{3 T}{J} (\frac{1}{r} - \frac{1}{R})$
D

$\frac{2 T}{J} (\frac{1}{r} - \frac{1}{R})$

Solution

Rise in temperature, $∆ θ = \frac{3 T}{JSd} (\frac{1}{r} - \frac{1}{R})$
$∴ ∆ θ = \frac{3 T}{J} (\frac{1}{r} - \frac{1}{R}) (For water S = 1 and d = 1)$

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