# Formula 4. Maxwell equation (integral form) Magnetic field    Electric current    Electric field

## Magnetic field

Unit
The magnetic flux density indicates how strong the magnetic field is at a certain location $$(x,y,z)$$ and in which direction it points.

## Closed line

Line (e.g. a current-carrying conductor) over which you integrate. It is the edge of the surface $$A$$ (for example the edge of a circle). The line must be closed, i.e. its beginning and its end must be connected.

## Surface

This surface is enclosed by the closed loop $$L$$. This can be, for example, the area of a circle.

## Electric current

Unit
Electric current running along the line $$L$$.

## Electric field

Unit
Electric field tells what would be the electric force on a sample charge at a given location $$(x,y,z)$$ if that sample charge is placed at that location.

Here the E-field is differentiated with respect to time: $$\frac{\partial \boldsymbol{E}}{\partial t}$$.

## Magnetic field constant

Unit
Magnetic field constant is a natural constant and occurs in the description of electromagnetic phenomena. It is: $$\mu_0 ~=~ 4\pi \cdot 10^{-7} \, \frac{\text N}{\text{A}^2}$$.