First slide

Surface tension and surface energy

Question

Energy needed in breaking a drop of radius R into n drops of radii r is given by

Moderate

A

$4 πT ({nr}^{2} - R^{2})$
B

$\frac{4}{3} π (r^{3} n - R^{2})$
C

$4 πT (R^{2} - {nr}^{2})$
D

$4 πT ({nr}^{2} + R^{2})$

Solution

Energy needed = Increment in surface energy
= (surface energy of n small drops) – (surface energy of one big drop)
$= n 4 {πr}^{2} T - 4 {πR}^{2} T = 4 πT ({nr}^{2} - R^{2})$

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