Now consider an aç circuit consisting of a resistor of resistance R and an inductor of inductance L in series with an ac source generator.
Suppose in phasor diagram, current is taken along positive x-directin. Ten VR is also along positive x- direction and VL a1ong positive y-direction.
As we know potential difference across a resistance in ac is in phase with current, and it leads current in phase by 900 across the inductor, so we Can write
Here, is called impedance of the circuit. Impedance plays the same role in ac circuits as the ohmic resistance does in dc circuits. The modulus of impedance is,
The potential difference leads the current by an angle,
Illustration 2: An alternating voltage of 220 volt r.m.s. at a frequency of 40 cycles/second is supplied to a circuit containing a pure inductance of 0.01 H and a pure resistance of 6 ohms in series. Calculate (a) the current, (b) potential difference across the resistance,
(c) Potential difference across the inductance, (d) the time lag.
Solution: The impedance of L-R series circuit is given by
(a) RM.S. value of current
(b) The potential difference across the inductance is given by
volts inductance
(c) Potential difference across the resistance is given by
volts
(d) Phase angle