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A dielectric with non-polar molecules is kept in an external electric field

A dielectric with non-polar molecules is kept in an external electric field Correct Answer The induced electric field and the net induced dipole moment of the dielectric are in the same direction.

CONCEPT:

Conductors and dielectrics

  • A conductor has free electrons.
    • When a conductor is placed in an external electric field, these free electrons move and align themselves to create an induced charged density σ which produces an induced electric field such that the net field inside the conductor is zero. 
  • Dielectrics are non-conducting substances. They do not contain free electrons.
    • When a dielectric is placed in an external electric field, its molecules align themselves in such a way that charges are induced on the surfaces of the dielectric with having charge density σp, and thus an electric field is created inside the dielectric.
    • This induced electric field Ein has a value less than that of the external electric field.
  • Both these fields have opposite directions.
  • Therefore the magnitude of the net electric field inside a dielectric is less than that of the external electric field.
  • The direction of the net electric field inside the dielectric is the same as that of the external electric field.

EXPLANATION:

Polar and non-polar molecules

  • In a non-polar molecule, the centers of positive and negative charges coincide.
    • The molecule then has no permanent dipole moment. 
  • In a polar molecule, the centers of positive and negative charges are separated.
    • Such molecules have a permanent dipole moment.

In an external electric field

  • In a nonpolar molecule,
    • The positive and negative charges are displaced in opposite directions.
    • The displacement stops when the external force on the constituent charges of the molecule is balanced by the restoring force
    • The non-polar molecule thus develops an induced dipole moment.
    • This dipole moment is in the direction that of the external electric field. 
    • The dielectric is said to be polarised by the external field.
  • Therefore option 3 is correct.

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