Total number of field lines passing through a certain element of area is called electric flux.It is a scalar quantity.SI Units for electric flux is Nm²/c. Electric flux is positive when the area is held perpendicular to the field lines and negative when the area is held parallel to the field lines.It may also be defined as “The scalar product of electric field intensity E and vector area A“.
Electric flux formula
It is denoted by Greek letter Φ.
Where θ is the angle between E and A.It is a scalar quantity.
SI units for electric flux
Its SI unit Nm²/c.Below figure showing electric flux through a surface normal to E.
When we place an element of an area in an electric field ,some of the lines of force pass through it.In order to give a quantitative meaning to flux ,the field lines are drawn such that the number of field lines passing through a unit area held perpendicular to field lines at a point represent the intensity E of the field at that point.Suppose at a given point the value of E is 4N/C .This means that if 1m² area is held perpendicular to the field lines at this point,4 field lines will pass through it.
Consider a certain area A is held perpendicular to the field lines E but direction of A is parallel to E as shown in figure below.
In this case ,angle between E and A is O°
If area is held parallel to the field lines E ,shown in figure below
In this case angle between E and A is 90°.
We know cos90°=0
Now we consider the case in which A is neither perpendicular nor parallel to field lines but is inclined at angle θ with the lines as shown in figure
In this case we have to find the projection of the area A which is perpendicular to the field lines.The area of this projection is
Acosθ.The flux Φ in this case is
Where θ is the angle between the field lines and normal to the area.