Explain electric flux.

(N/A) If a small planar element of area $\overrightarrow{\Delta S}$ is placed in an electric field $\vec{E}$, the number of field lines crossing it is proportional to $\vec{E} \cdot \overrightarrow{\Delta S}$.
Suppose we tilt the area element by an angle $\theta$ relative to the normal, the number of field lines crossing $\Delta S$ is proportional to $E \Delta S \cos \theta$.
When $\theta = 90^{\circ}$, the field lines are parallel to the surface and do not cross it at all.
When $\theta = 0^{\circ}$, the field lines are normal to the surface.
The vector associated with every area element of a closed surface is taken to be in the direction of the outward normal. Thus, the area element vector $\overrightarrow{\Delta S}$ at a point on a closed surface equals $\Delta S \hat{n}$, where $\Delta S$ is the magnitude of the area element and $\hat{n}$ is a unit vector in the direction of the outward normal at that point.
Electric flux is the number of electric field lines passing through or associated with a surface placed in an electric field.
Therefore, the electric flux $\Delta \phi$ through an area element $\Delta \overrightarrow{S}$ is $\Delta \phi = \vec{E} \cdot \Delta \overrightarrow{S} = E \Delta S \cos \theta$, where $\theta$ is the angle between $\vec{E}$ and $\overrightarrow{\Delta S}$.
The total flux $\phi$ is given by $\phi = \int \vec{E} \cdot d\overrightarrow{S} = E \Delta S \cos \theta$.
The $SI$ unit of electric flux is $N \cdot m^{2} \cdot C^{-1}$ or $V \cdot m$, and it is a scalar quantity.
Definition of electric flux: "Electric flux associated with any area is the surface integral of the electric field vector over that area."