# Formula 4. Maxwell Equation of Electrostatics in Integral Form Magnetic field    Electric current    Vacuum permeability

## Magnetic field

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

Maxwell's fourth equation of electrostatics states that an electric current $$I$$ causes a rotating magnetic field $$B$$ and vice versa.

## Closed loop

Unit
A closed loop (e.g. a circle) along which the magnetic field $$B$$ is summed up by means of the line integral.

Here $$\text{d}\boldsymbol{l}$$ is a small line element of the loop. The direction of $$\text{d}\boldsymbol{l}$$ points tangential to the loop at each point on the loop.

## Electric current

Unit
Electric current enclosed by the loop $$L$$.

## Vacuum permeability

Unit
The vacuum permeability is a physical constant and has the following experimentally determined value:$$\mu_0 ~=~ 1.256 \, 637 \, 062 \, 12 ~\cdot~ 10^{-6} \, \frac{\mathrm{Vs}}{\mathrm{Am}}$$