# Formula 2. Maxwell equation (integral form)

## Magnetic field

Unit
Magnetic flux density determines the force on a moving electric charge.

The second Maxwell equation states that the magnetic flux (i.e., the integral of $$\boldsymbol{B}$$ over a closed surface) is always zero. In other words, there are no magnetic monopoles that would generate the flux through a surface.

## Surface

The surface over which the magnetic field $$\boldsymbol{B}$$ is integrated. This can be, for example, a spherical surface or a cylindrical surface.

Here $$\text{d}\boldsymbol{a}$$ is a small surface element. The direction of $$\text{d}\boldsymbol{a}$$ is perpendicular to the respective surface element by definition.