 By definition, a perfect vacuum has a relative permittivity of exactly 1 whereas at STP, air has a relative permittivity of "κ"air = 1.0006.Relative permittivity is directly related to electric susceptibility ("χ") by

otherwise written as

The standard SI unit for permittivity is farad per meter (F/m or F·m−1).In electromagnetism, the electric displacement field represents how an electric field influences the organization of electric charges in a given medium, including charge migration and electric dipole reorientation.