03 March 2010

AC Circuits

                                                                          When only a resistance connected in series with an alternating source as shown in fig 11.1(a),both current and voltage are zero at same instant.Similarly they both reach maximum at the same instant.for this reason they are said to be"in phase".

                                                                  (a)Circuit Diagram

                                                    (b)Current and voltage waveform

                                                Fig.11.1 AC Through resistive circuit


      If the voltage rises, the current rises and if the voltage falls,the current falls and so on.It means that both the voltage and current pass their maximum and minimum values at the same instant and their instantaneous values are said to be in phase.This behavior is shown Fig.11.1(b).

Power In A Resistive Circuit:
                                                  In a circuit containing resistance,only the current is in phase with the voltage and power at any instant is equal to {(i power 2)Multiply by R} (v Multiply i) Where v and i are instantaneous values.The voltage and current waveform are shown in fig.11.2.

Fig.11.2 Power waveform


                  During  the first half cycle the instantaneous values of voltage and current are both positive, so that the power given by v Multiply i is positive.On the sound half cycle both v and i are negative,so that the product of -v and -i is also positive. The power waveform is shown shaded,it is sinusoidal and has a frequency of twice that of the supply.Thus power is absorbed form the supply on both positive and negative half cycles.

AC Through Inductance:
                                            When only a inductor connected is series with an alternating source as shown in fig.11.3(a),then the current lags the applied voltage by 90` or voltage leads the current by 90`.Because the voltage and current are not zero at the same instant or maximum at the same instant, they are said to be out of phase.The phase.The phase difference corresponds to maximum value 90` after the voltage,the current reaches to maximum value 90`after the voltage,the current is said to lag the voltage by 90`.The behavior is shown in Fig.11.3(b).

                                             Fig.11.3 AC through inductive circuit 

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