1. 21-11 LR Circuit Any inductor will have some resistance. An LR circuit has some properties similar to an RC circuit. See formula and diagrams on page 641. See Example 21-12 p 641.

2. 21-12 AC Circuits and Impedance I=I 0 cos2  ft …ac current produced. t is time…I 0 is peak current… V rms =V 0 I rms =I 0 Resistors…when an ac source is connected to a resistor as in Fig.2—31a, the current increases and decreases with ohm’s law V=IR or I=V/R…current and voltage are in phase…(peaks aligned) P=IV=I 2 rms R=V 2 rms /R

3. 21-12 AC Circuits and Impedance Inductor …an inductor, of inductance L, represented by a curly-coil symbol…ignoring the small internal resistance…The voltage applied to the inductor will equal the back emf…(Kirchhoff’s rule of closed circuits) V-L(  I/  t)=0 V=L(  I/  t) …This equation tell you I is increasing most rapidly when V has its maximum value, V=V 0 I will be decreasing most rapidly when V=-V 0 These 2 instances correspond to points b and d p643 figure 21-32 Current and voltage are 90 0 out of phase.

4. 21-12 AC Circuits and Impedance …no energy, on average is transformed in an inductor because of this… and no energy is dissipated as thermal energy. As mentioned in section 21-9, the back emf of an inductor impedes the flow of an ac current. The magnitude of the current in an inductor is directly proportional to the applied ac voltage at a given frequency. …so V=IX L …where X is called the inductive reactance or impedance of the inductor….the values can be both rms or peak for both.

5. 21-12 AC Circuits and Impedance The constant of proportionality for impedance is 2 , so X L = 2  fL See Example 21-13 on pg 644. Capacitors…..when a capacitor is connected to a battery, the capacitor plates quickly acquire equal and opposite charges; but no steady current. See page 645 figure 21-33 Current leads the voltage by 90 0 This is opposite of an inductor.

6. 21-12 AC Circuits and Impedance In an ac circuit, only resistance will dissipate energy by heat. V=IX C …where X is called the capacitive reactance or impedance of the inductor. See pg 646 21-13.