# Formula 1. Maxwell equation (integral form) Electric field    Electric charge ## Electric field

Unit
This quantity is a vector field (it assigns a field vector to each point in space) and tells how large the electric force on a test charge would be if it were placed in a particular location.

## Surface

The (imaginary) surface over which the electric field $$\class{blue}{\boldsymbol{E}}$$ is integrated. This can be, for example, a spherical surface or a cylindrical surface. For example, to calculate the $$\class{blue}{\boldsymbol{E}}$$ field inside a charged sphere, this imaginary surface is placed inside the charged sphere.

Here $$\text{d}\boldsymbol{a}$$ is a infinitesimal piece of the surface. By definition the direction of $$\text{d}\boldsymbol{a}$$ is perpendicular on the surface.

## Electric charge

Unit
This is the charge that is enclosed by the selected surface $$A$$.

## Electric field constant

Unit
This quantity always occurs in electromagnetic phenomena and is a natural constant with the value $$\varepsilon_0 ~=~ 8.854 \,\cdot\, 10^{-12} \, \frac{\text{As}}{\text{Vm}}$$.