Page 13 - 12-phy-12 ELECTROSTATICS

12. Electrostatics eLearn.Punjab

12.5 ELECTRIC FLUX

When we place an element of area in an electric field,

some of the lines of force pass through it (Fig. 12.13 a).

The number of the field lines passing through a certain

element of area is known as electric flux through that

area. It is usually denoted by Greek letter z. For example
the flux z through the area A in Fig. 12.13 (a) is 4 while

the flux through B is 2.

In order to give a quantitative meaning to flux, the field

lines are drawn such that the number of field lines passing Animation 12.6 : Electric flux
through a unit area held perpendicular to field lines at a Source & Credit : adelaide
point represent the intensity E of the field at that point.

Suppose at a given point the value of E is 4NC-1. This means

that if 1m2 area is held

perpendicular to field lines at this point, 4 field lines will pass

through it. In order to establish relation between electric flux z,

electric intensity E and area A we consider the Fig. 12.13(b,c,d)

which shows the three dimensional representation of the electric

field lines due to a uniform electric field of intensity E.

In Fig.12.13 (b), area is held perpendicular to the field lines, then

EAb lines pass through it. The flux z in this case is
e

ze = EAb ......... (12.10)

where Ab denotes

that the area is held

Fig. 12.13 (a) Electric flux through a perpendicular to field
surface normal to E.
lines. In Fig. 12.13 (c),

area A is held parallel to

field lines and, as is obvious no lines cross this area, so that

flux ze in this case is
ze = EA|| = 0 ......... (12.11)

Fig. 12.13 (b) Maximum

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