 emf supply is 20 V. Calculate its impedance, and then calculate the current in the circuit. Use formula (A.37): Z = [10,000 + X 3.14159 X 1020 1,000,000/2 X 3.14159 X 1020 2.16)211/2 = [10,000 + (2563.53 - .000,000/13843.1) - 1/2 = 2493.3 S. To calculate the current in the circuit, again use the variant of Ohm's law: I = E/2493.3 = 20/2493.3 =0.00802 A or 8.02 mA. A.9 RESISTANCE IN ac CIRCUITS In certain situations, ac resistance varies quite widely from the equivalent dc resistance, given the same circuit. For example, the resistance of a coil wound on an iron core where the magnetizing effect demonstrates hysteresis ("the holding back") and resulting eddy currents add to the dc resistance. We find that these effects are a function of frequency, the higher the frequency (f), the greater are these effects. Certain comparable losses may occur in the dielectric materials of capacitors, which may have the effect of increasing the apparent resistance of the circuit. Still more important is the phenomenon called skin effect. As we increase frequency of an ac current being transported by wire means, a magnetic field is set up around the wire, penetrating somewhat into the wire itself. Counter currents are set up in the wire as a result of the magnetic field, and as frequency increases field penetration decreases, and the magnitude of the counter currents increases. The net effect is to force the current in the wire to flow nearer the surface of the wire instead of being evenly distributed across the cross section of the wire. Because the actual current flow is now through a smaller area, the apparent ac resistance is considerably greater than its effective dc resistance. When working in the radio frequency (RF) domain, this resistance may be very much greater than dc resistance. However, when working with power line frequencies (i.e., 60 Hz in North America and 50 Hz in many other parts of the world), skin effect is nearly insignificant. A.10 RESONANCE [Refs. 1 and 4] As we are aware, the value of inductive reactance 