re A.18a is a typical ac resis- tive circuit. The phase relations between voltage (VR) and current (IR) is illustrated in Figure A.18b. A simple inductive circuit is shown in Figure A.19a, and the current and voltage phase relationships of this circuit are shown in Figure A.19b. In this circuit, because it is predominantly inductive, the potential difference (voltage VL) across the inductor leads the current by 90°. Figure A.20a is a simple ac capacitive circuit. In this case the current (Ic) leads the voltage (Vc) by 90°. This is illustrated in Figure A.20b. The magnitudes of current and voltage at some moment in time are usually analyzed using vectors. Vector analysis is beyond the scope of this appendix. Effective Emf and Current Values. In a practical sense, an arbitrary standard has been adopted SO that only the value of current or voltage need be given to define it, its position in Figure A.18a An ac circuit that is predominantly resistive. Phase Figure A.18b Phase relationship between voltage and current in an ac circuit that is resistive. Here the voltage and current remain in phase. A.8 ALTERNATING CURRENTS Figure A.19a A simple ac inductive circuit. Here the current (IL) lags the potential difference (voltage VL) by 90°. G is the ac emf source. Phase Figure A.19b Phase relationship between current (IL) and potential difference (voltage VL) in an ac inductive circuit. Figure A.20a A simple ac capacitive circuit. time being understood by the convention adopted. One approach is to state the maximum value of voltage or current. However, this approach has some disadvantages. Another and often more useful approach is the average value over a complete half-cycle (i.e., over IT radians). For a sine wave this value equals 0.636 of the maximum value. One of the most applicable values is based on the heating effect of a given value of alternating current in a resistor that will be exactly the same as the heating value of direct current in the same resistor. This eliminates the disadvantage of thinking that the eff