The charge per unit area of solid surface expressed in Coulomb/square meter. For a unidirectional, uniform field in air the maximum surface charge density before air breakdown occurs is 2.65 x10-5 C/m2. Only a fraction of this value is achieved in most practical situations. However, for non-uniform fields around a charged particle much greater charge densities can be supported in air, depending on the radius of the particle. Also, much greater surface charge densities can be supported when nonconductive charged surfaces have a metal substrate so as to form a capacitor. Since the electric field is primarily exerted through the nonconductor to the substrate the maximum charge density may be limited by the breakdown strength of the nonconductor.
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