An AC source operating at 60. Hz with a maximum voltage of 170 V is connected in series with a resistor (R = 1.2 kΩ) and a capacitor (C = 2.5 μF). (a) What is the maximum value of the current in the circuit? (b) What are the maximum values of the potential difference across the resistor and the capacitor? (c) When the current is zero, what are the magnitudes of the potential difference across the resistor, the capacitor, and the AC source? How much charge is on the capacitor at this instant? (d) When the current is at a maximum, what are the magnitudes of the potential differences across the resistor, the capacitor, and the AC source? How much charge is on the capacitor at this instant?
(a)

Answer to Problem 30P
Explanation of Solution
Given Info: The maximum voltage of the AC source is 170 V and has a frequency of 60 Hz. A resistor of resistance 1.2 kΩ and a capacitor of capacitance 2.5 μF are connected in series in the circuit.
Formula to calculate the capacitive reactance is,
XC=12πfC
- C is the capacitance of the capacitor
- f is the frequency of the ac source
Substitute 60 Hz for f, 2.5 μF for C to determine the capacitive reactance,
XC=12π(60 Hz)(2.5 μF)=1.1×103 Ω
The formula to calculate the impedance of the circuit where the components are in series is given by,
Z=√R2+(XL−XC)2
- R is the resistance of the resistor
- XL is the inductive reactance
- XC is the capacitive reactance
Substitute 1.2 kΩ for R, 0 for XL, 1.1×103 Ω for XC to determine the impedance of the circuit,
Z=√(1.2 kΩ)2+(0−1.1×103 Ω)2=1.6×103 Ω
Conclusion:
The maximum current in the circuit is given by,
Im=VmZ
- Vm is the maximum voltage supplied by the source
- Z is the impedance of the circuit
Substitute 1.6×103 Ω for Z and 170 V for Vm to determine the maximum current in the circuit,
Im=170 V1.6×103 Ω=0.11 A
The maximum current in the circuit is 0.11 A.
b)

Answer to Problem 30P
Explanation of Solution
Given Info: The maximum voltage of the AC source is 170 V and has a frequency of 60 Hz. A resistor of resistance 1.2 kΩ and a capacitor of capacitance 2.5 μF are connected in series in the circuit.
The maximum potential across the capacitor is
Vm,C=ImXC
- Im is the maximum current in the circuit
- XC is capacitive reactance
Substitute 0.11 A for Im and 1.1×103 Ω for XC to determine the maximum potential difference across the resistor,
Vm,C=(0.11 A)(1.1×103 Ω)=1.2×102 V
Conclusion: The maximum voltage across the capacitor is 1.2×102 V.
c)

Answer to Problem 30P
Explanation of Solution
Given Info: The maximum voltage of the AC source is 170 V and has a frequency of 60 Hz. A resistor of resistance 1.2 kΩ and a capacitor of capacitance 2.5 μF are connected in series in the circuit.
The current and the voltage across a capacitor are 90° out of phase with each other. Hence when the current reaches its maximum, the voltage across the inductor will reach its minimum. When the current is minimum the voltage will be maximum.
The maximum potential across the capacitor is
Vm,C=ImXC
- Im is the maximum current in the circuit
- XC is capacitive reactance
Substitute 0.11 A for Im and 1.1×103 Ω for XC to determine the maximum potential difference across the resistor,
Vm,C=(0.11 A)(1.1×103 Ω)=1.2×102 V
When Kirchoff’s law is applied to the closed circuit,
ΔVs−ΔVR−ΔVC=0
- ΔVs is the voltage across the AC source
- ΔVR is the voltage across the resistor
- ΔVC is the voltage across the capacitor
The voltage across the AC source will be,
ΔVs=ΔVR+ΔVC
Substitute 0 for ΔVR, 1.2×102 V for ΔVC to determine the voltage across the AC source,
ΔVs=0+1.2×102 V=1.2×102 V
The charge accumulated in the capacitor is given by,
q=CVC
- VC is the potential across the capacitor at the instant the current is zero
- C is the capacitance
From above section the potential across the capacitor is when current is zero is 1.2×102 V.
Substitute 1.2×102 V for VC and 2.5 μF for C to determine the charge across the capacitor,
q=(2.5 μF)(1.2×102 V)=(2.5×10−6 F)(1.2×102 V)=3.0×10−4 C=300 μC
Conclusion:
The magnitudes of potential difference across the resistor, the capacitor, and the AC source when the current is zero and the charge on the capacitor at that instant is 0, 1.2×102 V, 1.2×102 V and 300 μC respectively.
d)

Answer to Problem 30P
Explanation of Solution
Given Info: The maximum voltage of the AC source is 170 V and has a frequency of 60 Hz. A resistor of resistance 1.2 kΩ and a capacitor of capacitance 2.5 μF are connected in series in the circuit.
The current and the voltage across a resistor are in phase with each other. So when the current is maximum, the voltage across the resistor will be maximum.
The maximum potential across the resistor is
Vm,R=ImR
- Im is the maximum current in the circuit
- R is resistance
Substitute 0.11 A for Im and 1.2 kΩ for R to determine the maximum potential difference across the resistor,
Vm,R=(0.11 A)(1.2 kΩ)=1.3×102 V
The maximum voltage of the AC source is 170 V and has a frequency of 60 Hz. A resistor of resistance 1.2 kΩ and a capacitor of capacitance 2.5 μF are connected in series in the circuit.
The current and the voltage across a capacitor are 90° out of phase with each other. Hence when the current reaches its maximum, the voltage across the capacitor will reach its minimum.
Hence when the current reaches maximum, the voltage across the capacitor will be zero.
When Kirchoff’s law is applied to the closed circuit,
ΔVs−ΔVR−ΔVC=0
- ΔVs is the voltage across the AC source
- ΔVR is the voltage across the resistor
- ΔVC is the voltage across the capacitor
The voltage across the AC source will be,
ΔVs=ΔVR+ΔVC
Substitute 1.3×102 V for ΔVR, 0 for ΔVC to determine the voltage across the AC source,
ΔVs=1.3×102 V+0=1.3×102 V
The charge accumulated in the capacitor is given by,
q=CVC
- VC is the potential across the capacitor at the instant the current is maximum
- C is the capacitance
From above section the potential across the capacitor is when current is maximum is 0.
Substitute 0 for VC and 2.5 μF for C to determine the charge across the capacitor,
q=(2.5 μF)(0)=0
Conclusion:
The magnitudes of potential difference across the resistor, the capacitor, and the AC source when the current is maximum and the charge on the capacitor at that instant is 1.3×102 V, 0 , 1.3×102 V, and 0 respectively.
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