Electric Flux Density or Electric Displacement
Electric Flux Density
It is given by the normal flux per unit area.
If a flux of Ψ coulombs passes normally through an area of A m2, then flux density is
It is related to electric field intensity by the relation
D = ε0εr E ………………………………..in a medium
D = ε0 E ………………………………..in free space
In other words, the product of electric intensity E at any point within a dielectric medium and the absolute permittivity ε (= ε0, εr) at the same point is called the displacement at that point. Like electric intensity E, electric displacement D is also a vector quantity whose direction at every point is the same as that of E but whose magnitude is ε0 εr times E. As E is represented by lines of force, similarly D may also be represented by lines called lines of electric displacement.
The tangent to these lines at any point gives the direction of D at that point and the number of lines per unit area perpendicular to their direction is numerically equal to the electric displacement at that point. Hence, the number of lines of electric displacement per unit area (D) is ε0, εr times the number of lines of force per unit area at that point.
It should be noted that whereas the value of E depends on the permittivity of the surrounding medium, that of D is independent of it. One useful property of D is that its surface integral over any closed surface equals the enclosed charge (Gauss Law).
Let us find the value of D at a point distant r meters from a point charge of Q coulombs. Imagine a sphere of radius r meters surrounding the charge. Total flux = Q coulombs and it falls normally on a surface area of 4 π r2 meters. Hence, electric flux density
Read article – Units of Resistivity
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