Here formula_10 is the electric field again (in units of V/m), formula_16 is the total charge density (including the bound charges), and formula_13 (approximately 8.854 pF/m) is the permittivity of free space. One can also write formula_11 as formula_19. Here, formula_20 is the permittivity of the material when compared to the permittivity of free space. This is called the relative permittivity or "dielectric constant".