What is electric flux?

Flux of any field through closed surfaces tells you how much the volume acts as a source. Flux of zero means that there is no source (or net source) in the volume.

A charge is the source of electric fields. The electric flux that passes through a closed surface should equal the charge within. This is Gauss’ Law.

If a volume is acting as an electric field source, then the electric fields lines must emerge out (or in) according to the definition of electric line. It is obvious that the charge magnitude must determine the number of electric fields lines we see emerging from or converging in. We can also say that electric flux is proportional the number of electric fields lines passing through the surface.

If the same amount of charge produces the same number of field line, then the electric flux through any closed surfaces (that is, the charge) must be the same. It doesn’t matter how you store the charge within the closed surface, the number of field lines that pass through the surface would also be the same.

If a charge is not within the surface (closed), then any field line entering the surface must also exit the surface, since field lines are continuous. There is equal amount of inward and outer flux, so the total flux is zero if there is no charge inside the surface.


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