6.18 An electromagnetic wave propagating in seawater has an electric field with a time variation given by E = zE0 cos wt.If the permittivity of water is 81 ɛo and

its conductivity is 4 (S/m), find the ratio of the magnitudes of the conduction current density to displacement current density at each of the following frequencies: (a) 1 kHz (b) 1 MHz (c) 1 GHz (d) 100 GHz