Question

Explain the Maxwell first equation.

Answer 1

The Maxwell first equation (integral) has the form:

$$\oiint_S\mathbf{D}\cdot d\mathbf{s} = Q$$

where the right hand side of this equation is the flux of a D-field through a closed surface $S$ and $Q$ is a net free charge inside $S$.

This equation actually is the Gauss's law and it tells, that the flux of the D-field field vector through any arbitrarily chosen closed surface is proportional to the electric charge contained within this surface.