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702 ⏐⏐⏐ SERIES AND PARALLEL ac CIRCUITS<br />

+<br />

+<br />

E<br />

–<br />

E<br />

–<br />

I s = 2 A ∠ 0°<br />

Y T<br />

I s = 2 mA ∠ 20°<br />

I R<br />

R 2 �<br />

FIG. 15.136<br />

Problem 27.<br />

I R<br />

R 10 k�<br />

FIG. 15.137<br />

Problem 28.<br />

X C<br />

X L<br />

I C<br />

I L<br />

5 �<br />

20 k�<br />

+<br />

a c<br />

SECTION 15.8 Parallel ac Networks<br />

27. For the circuit of Fig. 15.136:<br />

a. Find the total admittance YT in polar form.<br />

b. Draw the admittance diagram.<br />

c. Find the voltage E and the currents IR and IL in phasor<br />

form.<br />

d. Draw the phasor diagram of the currents Is, IR, and IL, and the voltage E.<br />

e. Verify Kirchhoff’s current law at one node.<br />

f. Find the average power delivered to the circuit.<br />

g. Find the power factor of the circuit, and indicate<br />

whether it is leading or lagging.<br />

h. Find the sinusoidal expressions for the currents and<br />

voltage if the frequency is 60 Hz.<br />

i. Plot the waveforms for the currents and voltage on the<br />

same set of axes.<br />

28. Repeat Problem 27 for the circuit of Fig. 15.137, replacing<br />

I L with I C in parts (c) and (d).<br />

29. Repeat Problem 27 for the circuit of Fig. 15.138, replacing<br />

E with I s in part (c).<br />

+<br />

E = 60 V ∠ 0°<br />

–<br />

I s<br />

Y T<br />

I R<br />

R 12 �<br />

FIG. 15.138<br />

Problems 29 and 48.<br />

X L<br />

I L<br />

10 �<br />

30. For the circuit of Fig. 15.139:<br />

a. Find the total admittance Y T in polar form.<br />

b. Draw the admittance diagram.<br />

c. Find the value of C in microfarads and L in henries.<br />

d. Find the voltage E and currents I R, I L, and I C in phasor<br />

form.<br />

e. Draw the phasor diagram of the currents I s, I R, I L, and<br />

I C, and the voltage E.<br />

f. Verify Kirchhoff’s current law at one node.<br />

g. Find the average power delivered to the circuit.<br />

h. Find the power factor of the circuit, and indicate<br />

whether it is leading or lagging.<br />

i. Find the sinusoidal expressions for the currents and<br />

voltage.<br />

j. Plot the waveforms for the currents and voltage on the<br />

same set of axes.<br />

is = 3 sin(377t + 60°) e R 1.2 � XL 2 � XC 5 �<br />

–<br />

FIG. 15.139<br />

Problem 30.<br />

i R i L i C

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