Question

In a water drop of radius 1 mm all the molecular dipole points are in the same direction. If the dipole moment of a water molecule is 6 X 10^-30 m, the polarization in the water drop is ___________

(a) 6.4 X 10^-13 m^-2

(b) 7.4 X 10^-13 m^-2

(c) 8.4 X 10^-13 m^-2

(d) 9.4 X 10^-13 m^-2

The question was posed to me in an internship interview.

I would like to ask this question from Dielectric Polarisation in chapter Dielectrics of Engineering Physics II

Answer 1

The correct choice is (c) 8.4 X 10^-13 m^-2

Best explanation: Molecular mass of water = 18 gm

18 gm of water contains 6.023 X 10^23 molecules

18/10^3 m^3 of water contains 6.023 X 10^26/18 molecules

Volume of water drop = 4π/3 X (10^-3)^3 m^3

No of molecules in the drop, N = 6.023 X 10^26 X 4π X 10^-9/18 X 3

= 1.4 X 10^17 m^-3

Polarization, P = Np

= 1.4 X 10^17 m^-3 X 6 X 10^-30 m

= 8.4 X 10^-13 m^-2.