Question

1. One drop of soap bubble of diameter D breakers into 27 drops having surface tension T. The change in surface energy is

Answer 1

One drop of diameter is D breaks into 27 drops of diameter d
∴ volume of big drop = 27 × volume of small drop
⇒1/6πD³ = 27/6 πd³
⇒D³ = 27d³
⇒D³ = (3d)³
⇒D = 3d -------(1)

∵ change in surface energy = Surface tension × change in area
= T [27area of small drop - area of big drop]
= T[ 27 × πd² - πD²]
= Tπ[27(D/3)² - D²]
= πT[3D² - D²]
= 2πD²T

Hence, change in surface energy = 2πD²T