When the algebraic sum of electric quantities in Gaussian plane S is Q=0, the electric flux is zero, q =Q/e, and e is the dielectric constant of the medium; D =EcosQ*dS, and q is the included angle between the electric field line and the outer normal direction of the bin.

Q > in Gaussian plane s; At 0, Q0, the electric field line penetrates from the back of the box.

Q/2 in gauss plane s.

Electrostatic and magnetic fields.

There is an essential difference between the two. In the electrostatic field, due to the existence of independent charges in nature, electric field lines have a starting point and an ending point. As long as there is a net positive charge (or negative charge) on the closed surface, the electric flux passing through the closed surface is not equal to zero, that is, the electrostatic field is the active area.

In the magnetic field, because there is no magnetic monopole in nature, the N pole and the S pole cannot be separated, and the magnetic induction line is a closed line with no head and no tail, so the magnetic flux passing through any closed surface must be equal to zero.