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# A Question from Electric Flux and Gauss Theorem

How can you prove that dS in φ =E.dS is dS Cos θ if the area is tilted at an angle ? I need the mathematical Steps.

Ans;

Dear Zeenath,
Electric Flux is defined as the total no of field lines passing normal to the surface. So while calculating, we need to consider the area of the surface normal to the field lines only. That is why we take the dot product of E and ds, where ds is the area vector (not just the area: Area vector is a vector whose magnitude is equal to the area and dirtected normal to the surface.)
Then by definition of dot product,
dφ =E.dS = EdS cos θ
which gives the component of E and the component of area vector in the direction of E (When area vector is in the direction of E, the actual component of area is perpendicular to E)
Hope the matter is clear.
Mathew Abraham

1. as u said that flux is calculation of lines then why units of flux r given .it should be given only in mathmatical value for e.g 5.12,87 11 etc pl explain?

2. ashis satapathy says:

i am talking about only electric field

electric flux is the number of electric field lines crossing through a surface which is held perpendicular to the field lines. more over the field lines are not real lines they just represent the existence of the electric field. that is why the unit cannot be a number

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