Insulators and Dielectric Material
Insulators are materials with very poor conductivity. Insulators are not very good at conducting either heat or electricity owing to the absence of loosely bound or freely moving charges in the atoms of the insulator. When insulators are placed in an electric field, practically no current flows in them, unlike in metals. Instead in some insulators, electric polarization occurs. A dielectric is an insulator that undergoes electric polarization on the application of electric field. The charges in dielectric material does not move but only shifts slightly from the equilibrium position resulting in the dielectric polarization. Not every insulator is a dielectric material, the necessary condition for dielectric polarization is the presence of polar molecules. Let’s explore this concept.
Polar and Non Polar Molecules
Polarity of a molecule is a property of the bonds between the atoms making the molecule. Polarity refers to the separation of electric charge leading to a presence of an electric dipole in the molecule. In chemistry, polarity is attributed to the difference in electronegativity of atoms in a molecule. Electronegativity speaks to the magnitude of attraction that an atom has on the electrons shared in a bond. Nonpolar bonds occur when the difference in electronegativity between two atoms is less than 0.4, this is to say that the atoms of the molecule exert approximately the same pull on the charges, therefore no dipole. Polar bonds occur when the difference in the electronegativity between two atoms ranges from 0.4 to 1.7 which is to say that one atom exerts significantly more pull on the charges than the other.
In a Hydrogen Fluoride molecule, the electronegativity of Fluorine is much higher than Hydrogen, leading to an asymmetry in charge distribution. This asymmetry leads to the creation of a Dipole as seen in the image above.
In nonpolar molecules, the electric charges between the atoms in the molecule are equally distributed due to similar levels of attraction for the charges, as in the centres of their charges coincide. The lack of polarity in nonpolar molecules can be a result of a nonpolar bond as in the case of H2 or O2 or also due to the symmetry of its structure as is the case with Carbon Dioxide (CO2) and Methane, both of which comprise of polar bonds which are cancelled due to its symmetrical structure. In some cases if the difference in electronegativity is high, it leads to the formation of Ionic bond which consists not of sharing electrons in covalent bond but the complete transfer to electrons.
Carbon Dioxide molecule is Nonpolar due to the symmetry of its bonds and so is Boron Trifluoride in the second image.
Meanwhile in contrast to nonpolar molecules, in polar molecules the centers of the positive and the negative charges do not coincide and they are separated. A polar molecule has a net dipole as a result of opposing charges due to asymmetrical bonds. Water (H2O), Ammonia (NH3) and Ozone (O3) are examples of polar molecules. Dipole cannot be cancelled out because all the constituents are exerting a force on the charges. Water is an excellent solvent due to its polar nature. Due to its polar nature, a polar solvent can usually dissolve other polar molecules. In the absence of an electric field, the electric dipole moment of the molecules is oriented haphazardly throughout the material. Due to this random thermal agitation of molecules, there is no net direction and as a result no net polarity. In the presence of an electric field, the dipole moment attracts and affects the orientation of the polar bond. This dipole in polar molecule aligns itself in the direction of the electric field.
The water molecule is made up of oxygen and hydrogen, with respective electronegativities of 3.44 and 2.20. The dipoles from each of the two bonds (red arrows) add together to make the overall molecule polar. The same is true for the Ammonia molecule on the right as well. The red represents the partially negatively charged regions of the molecule.
The presence of dielectric material affects other electrical phenomena. The force between two electric charges in a dielectric medium is less than it would be in a vacuum, while the quantity of energy stored in an electric field per unit volume of a dielectric medium is greater. The capacitance of a capacitor filled with a dielectric is greater than it would be in a vacuum. The effects of the dielectric materials along with its characteristics such as dielectric constant, dielectric strength will be discussed in depth along with different types of polarization in the next article on the Dielectric phenomenon.
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