Polar and Nonpolar molecules

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A) Polar molecules:

Polar means having electrical poles (i.e. electrical polarity). The molecules in which the arrangement or geometry of the atoms is such that one end of the molecule has a positive electrical charge and the other side has a negative charge are called as polar molecules. Examples of polar molecules are Water (H2O) (Fig. 2.1a), Ammonia (NH3), Hydrochloric acid (HCl), Sulfur Dioxide (SO2), Hydrogen Sulfide (H2S), Carbon Monoxide (CO) etc.

B) Nonpolar molecules:

A non-polar molecule is that in which the electrons are distributed more symmetrically and thus does not have an excess /abundance of charges at the opposite sides. The charges all cancel out each other. e.g. CO2,H2,N2,O2,CH4,CCl4 etc.

A Slab in an Electric Field:

Due to difference in their atomic structures, the materials like conductors and insulators exhibit different response when placed in an electric field.

A) Conductor

Suppose a slab of conducting material is placed in an external electric field E0.

  • Net charge resides on the surface: Electrons move to the positive side of the field. As they cannot leave the material, they accumulate on the surface i.e. charges are induced on the surface.
  • E = 0 inside the conductor: The induced charges produce a field of their own Ei which is directed opposite to the applied field E0 and tends to cancel it. Charge flow continues till cancellation of E0byEi is complete. Hence Einside = 0
  • Conductor is an equipotential surface: As E = - \triangledown \phi = 0. Potential φ is constant everywhere inside the conductor.
  • ρ = 0: According to Gauss law \triangledown . E = \frac {\rho}{\epsilon_0}. If Einside = 0 = = > ρ = 0
  • E is normal to the surface: The electric lines of force are normal to the surface.

B) Dielectrics

In nonpolar dielectric molecules, permanent electric dipoles are not present. However, after applying an external electric field to an insulator or dielectric, positive and negative electric charges within it undergo slight relative shift on opposite directions. Electric field distorts the negative cloud of electrons around positive atomic nuclei in a direction opposite to the field. This slight separation (stretching or shift of a fraction of an atomic diameter) of charge makes one side of the atom somewhat positive and the opposite side somewhat negative creating dipoles. These induced dipoles give rise to Polarization.

Fig 2.2(a1) Polar Molecule-Unpolarized (a2) Polarization in an applied electric Field

In polar dielectric molecules, permanent dipoles are present however, they are generally in random orientations due to thermal agitation, creating unpolarization condition (Fig. 2.2a). When an electric field is applied, the dipoles are oriented by rotation and aligned in the direction of the electric field so that one type of bound charges (+ve or -ve) appear on one surface and the opposite type on opposite surface. Thus electric field polarizes the material of polar molecules (Fig 2.2b).

  • Thus application of an electric field creates polarization either by inducing dipoles (in nonpolar) or reorienting and aligning them in external electric field (in polar) in both types of molecules.
  • Polarization reduces the electric field intensity within the dielectric material by a factor known as the dielectric constant (or relative permittivity) of the material.
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