The notation ρl for line charge densities will be used throughout this book. Figure 3.1a shows this distribution schematically. Line charge densities are typically used for thin conductors or, sometimes, as approximations of thick, charged conductors. (b) Now, the same amount of charge is distributed per unit length of a cylindrical conductor of radius d. Since the charge on the conductor can only reside on the surface, the charge distributes itself (in this case uniformly) over the surface of the cylinder. The total external surface of a 1 m length of the cylindrical conductor is (2πd)(1). Thus, the surface charge density on the conductor is