What is Electric Flux?
“Total number of field lines passing through a certain element of area is called flux”.
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 unit of 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.