Consider a dielectric placed in an external electric field E0 (say between the plates of a charged capacitor). Due to external field, there will be shifting of the centres of gravity of positive and negative charges. The positive charges are displaced in the direction of the applied field whereas the negative charges will get displaced in a direction opposite to the applied field E0 ( try to make the figure). Therefore, the positive charge is induced on one surface while negative charge on other.
This phenomenon of appearance of the charge on the surfaces of dielectric induced by the external electric field is called polarization or dielectric polarization.
The charge on the plates of the capacitor is called free charge and charge induced on the surfaces of the dielectric is called bound charge.
We can also say that when the atoms are placed in an electric field, they acquire an induced electric dipole moment in the direction of the field. This process is called the dielectric polarization and atoms are said to be polarized. Continue reading “POLARIZATION or DIELECTRIC POLARIZATION”
Definition: The ratio of the capacitance of a given capacitor (with the material filling the entire space between its plates) to the capacitance of the same capacitor in vacuum is called static dielectric constant or relative permittivity of the material.
A dielectric is characterized by its dielectric constant which is also called relative permittivity.
Electric field between two oppositely charged parallel plates:
In the last article, I have explained and derived the expression for the capacitance of parallel plate capacitor with dielectric and without dielectric. Today we will discuss and derive the relation for electric field between two oppositely charged parallel plates.
Let +q and –q be the charges on the plates of a parallel plate air capacitor. Let A be the area of the plate.
The magnitude of the electric field, E, between the parallel plates
is given by
E1 = q/ ε0A
If σ is the charge per unit area, then q = Aσ and thus
E1 = σ/ ε0
This is the expression for the electric field between two oppositely charged parallel plates.