From Equation 2.14, the surface resistance R_S is written as

The surface resistance is the AC resistance of a rectangular, good conductor material of 1 m long and 1 m wide, at high frequencies. In Figure 2.2, if l = 1 and b = 1, from Equation 2.38, R_AC = 1/σδ. R_S is given in the unit of Ω per square, but it increases with l and decreases as b increases. As l increases, the voltage increases ($\tilde{E}$ is in the z-direction). As b increases, the current $\tilde{I}$ increases. In computing fields in the dielectric medium at the interface of a dielectric–conductor medium, if we make the idealizing assumption that the conductor is a PEC, the current in the conductor is idealized as a surface current $\tilde{K}$ on the interface. Thus, if b = 1, the current is K and the power loss per meter square is

From the boundary condition at the PEC we obtain

Thus, the power loss per meter square is

The total power loss over a surface S is given by

This is the basis of Equation 4.14.