## Thursday, October 25

### Electric Flux Density

Introduction to electric flux density:

The electric field creates a force on a charge and hence the charge moves along a certain path called the electric flux line. Also the force between two charges acts along a certain path. This path is also called the electric flux line. The electric flux through a surface held inside an electric field represents the total number of electric lines of force crossing the surface in a direction normal to the surface. it is denoted by  ?.

Magnitude of flux depends only on the charge from which it originates. The flux lines are equal to the charge in Coulombs. It is only an imaginary line.  Its direction is same as that of the electric field. It is a scalar quantity.

Electric Flux Lines

Electric flux lines from a point charge:

Electric flux lines between a positive charge and a negative charge:

Electric Flux Density:

1. Electric flux density is defined as the electric flux crossing the surface area.

Mathematically, Electric flux density,  D is defined as

D = ??/ ?S  ,   C/m2

Where, ? is electric flux crossing the differential area, ?S.

The direction of ?S is always outward, normal to ?S, that is, ?S = ?S an .

2. Electric flux density is also defined as (in a general medium)

D = eE , C/m2

Where e is permittivity, F/m

E is electric field strength, V/m

3. Electric flux density in a dielectric medium is given by

D = e0E + P

Where P is polarization of medium

e0 is permittivity of free space.

4. Electric flux density is a vector quantity.

5. The unit of electric flux density is C/m2.

6. In free space, D is in the direction of E.

7. D, in a Gaussian surface, is determined from gauss’s law.

8. D is independent of the medium.