Nature of Electricity
Drift Velocity & Electron Mobility
Heating Effect of Electric Current
Magnetic field of current carrying conductor
Magnetic Flux Density
Resistance Variation with Temperature
Temperature Coefficient of Resistance
Theory of Electrical Potential
Capacitor and Capacitance
What is Capacitor?
Single Phase Power
Single Phase Power Equations
Three Phase Power
We know that there is an electric field around a point charge, and whenever there is an electric field there is an electric flux around it , this field though theoretically assumed to be spread up to infinity but practically they are taken to be composed of small closed areas. But what is this field made of? Or what is the component of this electric field? The field is nothing but an energy field, i.e. to go through the field work is done by or done upon a point charge. So, there must be some kind of energy present in the electric field. Now from each point of this electric field tube force emerges, which radiates through the surrounding. Now this total number of force tube is called electric flux.
Electric Field due to a Point Charge
Let a point charge Q is placed and an imaginary spherical surface of radius r is kept around it such that the point charge Q rests at the center of the sphere. Now the total number of flux radiating from the charge (Q Coulombs) is equal to Q coulombs. We can prove it also
Now, the electric field intensity at any point on the surface is
The field intensity is normal to the surface of the sphere at that point.
Theoretically the number of electric flux crossing the sphere at that point is infinite but practically the flux at that point is εoεr times the intensity of the field. Which is equal to
Surface area of the sphere = 4πr2
∴ the total number of flux radiating from the charge is Q
So, electric field due to a point charge is equal to the charge in coulombs.
Electric flux density
We have seen that the tube forces which are termed as electric flux, radiates normally from the surface and is assumed to be infinite in no. the amount of radiating this flux through unit surface area is known as electric flux density. The unit of this is coulombs/m2.
Let's take a point charge of Q coulomb and place it at the center of a sphere of radius ’r’ then the electric flux density is
From the above relation we can see that the electric flux density does not depend on the medium, i.e. the absolute permittivity and relative permittivity, and it is inversely proportional with the square of the distance from the charge.
The relation between electric flux intensity and density is given by the equation
Electric dipole moment
The most basic definition of an electric dipole moment can be given as the system consisting of two equal in magnitude but opposite charges separated by certain distance, it can be infinite or less than that. Let us assume two point charges +Q and –Q, both have magnitude same i.e.Q coulomb, but one of them has positive charge and the other one is negatively charged, now the dipole moment is the vector at the mid point between the two charges and denoted by P and represented as
P = Q.d
Where, Q is the charge and d is the distance between the two charges.
In a polarized material the molecules reside as electrical dipole. When that substance enters inside an electric field the field causes the dipoles to orient parallel inside the substance. The net electric dipole moment of the substance at that point of time acts at the same direction and the sum of the dipole moments per unit volume in a material is called dielectric polarization.
Electric displacement vector
The electric field intensity is given by
We can see from the above equation that the intensity depends on the surrounding medium (here t which permittivity of the medium is different for various mediums), but there is another term electric displacement vector which only express the magnitude of the field and does not depend on the surrounding medium. It basically gives the amount of electric flux radiated normally through per unit surface area and it is unaltered by the change of medium. It is denoted by D and given as
The unit of electric displacement vector is coulombs/m2
From Gauss’s theorem the relationship between electric displace vector, field intensity and electric dipole moment E, D & P can be given by
D = εoE + P