Question

In case of a linear isotropic dielectric, when placed in an external electric field, the induced dipole moment is
1.  in the direction of the field and is proportional to the field strength
2. in the opposite direction of the field and is proportional to the field strength
3. in the direction of the field and is inversely proportional to the field strength
4. in the opposite direction of the field and is inversely proportional to the field strength

Answer 1

Correct Answer - Option 1 :  in the direction of the field and is proportional to the field strength

CONCEPT:

• A dielectric develops a net dipole moment in the presence of an external field E. The dipole moment per unit volume is called polarization and is denoted by P.

\(⇒ P= ε_0 χ_e E\)

where χe is a constant. It is the characteristic of the dielectric and is known as the electric susceptibility of the dielectric medium.

Electric susceptibility

• Electric susceptibility can also be expressed in terms of dielectric constant K as

\(⇒ χ_e= K-1\)

EXPLANATION:

• When a dielectric is placed in an external electric field, charges are induced, and a net dipole moment is formed.
• The dipole moment per unit volume is defined as polarisation.
• ​Linear isotropic dielectrics are defined as dielectrics for which
  • The direction of polarisation is the same as that of the external electric field. 
  • The strength of polarization is proportional to the external electric field. 
• Therefore option 1 is correct.