Find the free energy of the surface layer of
a mercury droplet of diameter `d = 1.4 mm` ,
(b) a soap bubble of diameter `d = 6.0 mm` if the surface tension of the soap water solution is equal to `alpha = 45 mN//m`.

Explanation

(a) The free energy per unit area being `alpha`,
`F = pi alpha d^2 = 3 mu J`
(b) `F = 2 pi alpha d^2` because the soap bubble has two surfaces. Substitution gives `F = 10 mu J`.