In SI units, permittivity is measured in farads per meter (F/m or A ·s ·kg ·m). The displacement field D is measured in units of coulombs per square meter (C/m), while the electric field E is measured in volts per meter (V/m). D and E describe the interaction between charged objects. D is related to the charge densities associated with this interaction, while E is related to the forces and potential differences.