
(a)
The average power supplied to the circuit.
(a)

Answer to Problem 70P
Pav=933.38W
Explanation of Solution
Given:
RMS voltage, Vrms=120V
Frequency, f=60Hz
RMS current, Irms=11A
Phase difference between the current and the voltage, ϕ=45o
Calculation:
The average power is
Pav=VrmsIrmscosϕPav=120V×11A×cos 45oPav=933.38W
Conclusion:
The average power supplied to the circuit is Pav=933.38W .
(b)
The resistance of the circuit.
(b)

Answer to Problem 70P
R=7.71Ω
Explanation of Solution
Given:
RMS voltage, Vrms=120V
Frequency, f=60Hz
RMS current, Irms=11A
Phase difference between the current and the voltage, ϕ=45o
Average power, Pav=933.38W
Calculation:
The resistance is,
Pav=Irms2RR=PavIrms2R=933.38W(11A)2R=7.71Ω
Conclusion:
R=7.71Ω
This is the resistance of the circuit.
(c)
The capacitance of the circuit.
(c)

Answer to Problem 70P
C=2.4×10−4F
Explanation of Solution
Given:
Rms voltage, Vrms=120V
Frequency, f=60Hz
Rms current, Irms=11A
Phase difference between the current and the voltage, ϕ=45o
Average power, Pav=933.38W
Inductance, L=50mH
Resistance, R=7.71Ω
Formula used:
Capacitive reactance, XC=12πfC
Impedance, Z2=R2+(XL−XC)2=ε2Irms2
Calculation:
The capacitive impedance is,
Z2=R2+(XL−XC)2ε2Irms2=R2+(2πfL−XC)2(120V11A)2=7.71Ω2+(2π×60Hz×50×10-3−XC)2119.01=59.44+(18.85−XC)218.85−XC=√119.01−59.44XC=18.85−7.71XC=11.14 Ω
The capacitance is derived as,
C=12πfXCC=12π×60Hz×11.14 ΩC=2.4×10−4F
Conclusion:
The capacitance is C=2.4×10−4F .
(d)
The change in the capacitance to make the power factor equal to 1.
(d)

Answer to Problem 70P
ΔC=1×10−4F
Explanation of Solution
Given:
Rms voltage, Vrms=120V
Frequency, f=60Hz
Rms current, Irms=11A
Phase difference between the current and the voltage, ϕ=45o
Average power, Pav=933.38W
Inductance, L=50mH
Capacitance, C=2.4×10−4F
Formula used:
Capacitive reactance, XC=12πfC
Frequency, 2πf=1√LC
Calculation:
The capacitance at power factor 1 is Cpf=1 .The frequency is independent on the power factor and the inductance is remaining constant as per the conditions. So, Cpf=1 is establish as,
CPf=1=1(2πf)2LCPf=1=1(2π×60Hz)2×0.05HCPf=1=1.4×10−4F
The change in the capacitance for which the power factor is 1 is,
ΔC=C−CPf=1ΔC=(2.4×10−4−1.4×10−4)FΔC=1×10−4F
Conclusion:
The capacitance must change ΔC=1×10−4F
(e)
The change in inductance to make the power factor 1.
(e)

Answer to Problem 70P
ΔL=0.02H
Explanation of Solution
Given:
RMS voltage, Vrms=120V
Frequency, f=60Hz
RMS current, Irms=11A
Phase difference between the current and the voltage, ϕ=45o
Average power, Pav=933.38W
Inductance, L=50mH=0.05H
Capacitance, C=2.4×10−4F
Formula used:
Capacitive reactance, XC=12πfC
Frequency, 2πf=1√LC
Calculation:
The capacitance at power factor 1 is Lpf=1 . The frequency is independent of the power factor and the capacitance is remains constant as per the conditions. So, Lpf=1 is established as:
LPf=1=1(2πf)2CLPf=1=1(2π×60Hz)2×2.4×10−4FLPf=1=0.03 H
The change in the inductance for which the power factor is 1 is
ΔL=L−LPf=1ΔL=(0.05−0.03) HΔL=0.02H
Conclusion:
The change in inductance to make power factor 1 is ΔL=0.02H .
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Chapter 29 Solutions
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