Question

Debye is a unit of
A) RMS velocity
B) force
C) specific gravity
D) electric dipole moment

Answer 1

Hint : Debye is a unit associated with the charge and the distance associated with two charges of an equal magnitude but an opposite sign located some distance apart. We will use this fact to determine the quantity defined by Debye.

Complete step by step answer
Debye is a unit used in the CGS system of electric dipole moments. The electric dipole moment is defined as the charge of the two charges present in an electric dipole which have an equal magnitude of charge but an opposite sign and are present close to each other.
Hence the correct choice is option (D).

Additional Information
Historically, the Debye was defined as the dipole moment resulting from two charges that have the opposite signs but the same magnitude of roughly $ {10^{ - 10}}\, $ stat coulomb (generally known as e.s.u. (electrostatic unit) in the CGS unit), which were separated by $ 10^{-10} \,m $ . This was a convenient way to measure the dipole moment of different molecules that would have covalent bonds between them. However, the SI unit involves measuring the dipole moment as the product of charge magnitude in Coulombs and the distance between them in meters. Approximately
 $\Rightarrow 1\,{\text{Debye = 3}}{\text{.335}} \times {\text{1}}{{\text{0}}^{ - 30}}\,Cm $
Typical dipole moments for simple diatomic molecules are in the range of 0 to 11 D. While symmetric diatomic molecules such as chlorine or oxygen molecules have zero dipole moment, covalent compounds have a higher dipole moment.

Note
To answer this question, we must be aware of the units of measurement of different physical quantities. Since velocity and force don’t have any other units of measurement, we can discard options (A) and (B) as the correct choice. While specific gravity is dimensionless so it won’t have any units so we can discard option (C) as the correct choice too.