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

## Electric field

`\( \class{blue}{\boldsymbol{E}} \)`Unit

`\( \frac{\mathrm V}{\mathrm m} \)`

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

`\( A \)`

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

`\( \class{red}{Q} \)`Unit

`\( \mathrm{C} \)`

This is the charge that is enclosed by the selected surface \( A \).

## Electric field constant

`\( \varepsilon_0 \)`Unit

`\( \frac{\mathrm{As}}{\mathrm{Vm}} \)`

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