Alternating Current
Q. A capacitor in an AC circuit has a capacitive reactance of 50 ohms. If the frequency of the AC source is increased, what happens to the capacitive reactance?
A.
Increases
B.
Decreases
C.
Remains the same
D.
Becomes infinite
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Solution
Capacitive reactance (X_C) is given by X_C = 1/(2πfC). If the frequency (f) increases, X_C decreases.
Correct Answer: B — Decreases
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Q. A capacitor in an AC circuit has a capacitive reactance of 50 ohms. What is the frequency if the capacitance is 10 microfarads?
A.
1 kHz
B.
10 kHz
C.
100 Hz
D.
1000 Hz
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Solution
Capacitive reactance (X_C) is given by X_C = 1 / (2πfC). Rearranging gives f = 1 / (2πX_CC). Substituting X_C = 50 ohms and C = 10 x 10^-6 F gives f = 318.31 Hz, approximately 1 kHz.
Correct Answer: A — 1 kHz
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Q. If the frequency of an AC source is 60 Hz, what is the time period of the AC signal?
A.
0.0167 s
B.
0.0333 s
C.
0.05 s
D.
0.1 s
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Solution
The time period (T) is the reciprocal of frequency (f). T = 1/f = 1/60 Hz = 0.0167 s.
Correct Answer: B — 0.0333 s
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Q. If the frequency of an AC source is increased, what happens to the capacitive reactance?
A.
It increases
B.
It decreases
C.
It remains the same
D.
It becomes infinite
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Solution
Capacitive reactance X_C = 1/(ωC) decreases as frequency increases.
Correct Answer: B — It decreases
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Q. If the inductance of a coil is 2 H and the frequency is 50 Hz, what is the inductive reactance?
A.
100 π
B.
100
C.
314
D.
628
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Solution
Inductive reactance X_L = 2πfL = 2π(50)(2) = 628 Ω.
Correct Answer: C — 314
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Q. If the peak current in an AC circuit is 5 A, what is the average current over one complete cycle?
A.
5 A
B.
2.5 A
C.
0 A
D.
7.07 A
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Solution
The average current over one complete cycle is I_avg = I_peak/√2 = 5/√2 = 2.5 A.
Correct Answer: B — 2.5 A
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Q. If the peak voltage of an AC source is 200 V, what is the RMS voltage?
A.
100 V
B.
141.42 V
C.
200 V
D.
282.84 V
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Solution
The RMS voltage (V_rms) is given by V_rms = V_peak / √2. Therefore, V_rms = 200 V / √2 = 141.42 V.
Correct Answer: B — 141.42 V
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Q. If the peak voltage of an AC source is 220 V, what is the RMS voltage?
A.
110 V
B.
154 V
C.
220 V
D.
311 V
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Solution
The RMS voltage (V_rms) is given by V_rms = V_peak / √2. Therefore, V_rms = 220 V / √2 = 110 V.
Correct Answer: A — 110 V
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Q. In a parallel RLC circuit, what happens to the total current if the frequency is increased?
A.
Increases
B.
Decreases
C.
Remains the same
D.
Depends on R
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Solution
In a parallel RLC circuit, increasing frequency generally increases the total current due to lower reactance.
Correct Answer: A — Increases
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Q. In a series RLC circuit, at resonance, what is the relationship between inductive reactance and capacitive reactance?
A.
X_L > X_C
B.
X_L < X_C
C.
X_L = X_C
D.
X_L + X_C = 0
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Solution
At resonance in a series RLC circuit, the inductive reactance (X_L) equals the capacitive reactance (X_C), hence X_L = X_C.
Correct Answer: C — X_L = X_C
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Q. In a series RLC circuit, if the resistance is increased, what happens to the bandwidth of the resonance?
A.
Increases
B.
Decreases
C.
Remains the same
D.
Becomes zero
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Solution
Increasing the resistance in a series RLC circuit decreases the bandwidth of the resonance because the quality factor (Q) is inversely proportional to resistance.
Correct Answer: B — Decreases
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Q. In a series RLC circuit, if the resistance is increased, what happens to the bandwidth?
A.
Increases
B.
Decreases
C.
Remains the same
D.
Becomes zero
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Solution
In a series RLC circuit, increasing the resistance decreases the bandwidth because the quality factor (Q) is inversely proportional to resistance.
Correct Answer: B — Decreases
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Q. In a series RLC circuit, if the resistance is increased, what happens to the bandwidth of the resonance peak?
A.
Increases
B.
Decreases
C.
Remains the same
D.
Becomes zero
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Solution
Increasing the resistance in a series RLC circuit decreases the bandwidth of the resonance peak.
Correct Answer: B — Decreases
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Q. In a series RLC circuit, what happens to the current at resonance?
A.
Maximum
B.
Minimum
C.
Zero
D.
Constant
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Solution
At resonance, the current is maximum in a series RLC circuit.
Correct Answer: A — Maximum
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Q. In a series RLC circuit, what happens to the current when the frequency is increased beyond the resonant frequency?
A.
Increases
B.
Decreases
C.
Remains constant
D.
Becomes zero
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Solution
Beyond the resonant frequency, the circuit becomes more inductive, causing the current to decrease.
Correct Answer: B — Decreases
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Q. In a series RLC circuit, what is the condition for resonance?
A.
R = 0
B.
L = C
C.
ωL = 1/ωC
D.
V = I
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Solution
The condition for resonance in a series RLC circuit is ωL = 1/ωC.
Correct Answer: C — ωL = 1/ωC
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Q. In an AC circuit, if the current lags the voltage by 30 degrees, what is the type of load?
A.
Resistive
B.
Inductive
C.
Capacitive
D.
None of the above
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Solution
If the current lags the voltage, it indicates an inductive load, as inductors cause the current to lag behind the voltage.
Correct Answer: B — Inductive
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Q. In an AC circuit, if the current lags the voltage by 30 degrees, what type of circuit is it?
A.
Resistive
B.
Inductive
C.
Capacitive
D.
None of the above
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Solution
If the current lags the voltage, it indicates an inductive circuit, where the current phase is behind the voltage phase.
Correct Answer: B — Inductive
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Q. In an AC circuit, if the current lags the voltage by 30 degrees, what type of load is present?
A.
Resistive
B.
Inductive
C.
Capacitive
D.
None of the above
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Solution
If the current lags the voltage, it indicates the presence of an inductive load, as inductors cause the current to lag behind the voltage.
Correct Answer: B — Inductive
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Q. In an AC circuit, if the current lags the voltage by 45 degrees, what is the type of load?
A.
Resistive
B.
Inductive
C.
Capacitive
D.
None of the above
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Solution
If the current lags the voltage, it indicates an inductive load, as inductors cause the current to lag behind the voltage.
Correct Answer: B — Inductive
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Q. In an AC circuit, if the frequency is doubled, what happens to the inductive reactance?
A.
It doubles
B.
It halves
C.
It remains the same
D.
It quadruples
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Solution
Inductive reactance (X_L) is given by X_L = 2πfL. If the frequency (f) is doubled, X_L also doubles.
Correct Answer: B — It halves
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Q. In an AC circuit, if the frequency is doubled, what happens to the reactance of an inductor?
A.
Doubles
B.
Halves
C.
Remains the same
D.
Quadruples
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Solution
The reactance of an inductor is given by X_L = 2πfL. If frequency is doubled, reactance halves.
Correct Answer: B — Halves
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Q. In an AC circuit, if the power factor is 0.5, what is the angle between voltage and current?
A.
30 degrees
B.
60 degrees
C.
90 degrees
D.
45 degrees
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Solution
The power factor is cos(θ). If PF = 0.5, then θ = cos^(-1)(0.5) = 60 degrees.
Correct Answer: B — 60 degrees
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Q. In an AC circuit, if the power factor is 1, what type of load is present?
A.
Inductive
B.
Capacitive
C.
Resistive
D.
Reactive
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Solution
A power factor of 1 indicates a purely resistive load.
Correct Answer: C — Resistive
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Q. In an AC circuit, if the voltage is given by V(t) = V_0 sin(ωt), what is the expression for the current through a resistor R?
A.
I(t) = (V_0/R) sin(ωt)
B.
I(t) = (V_0/R) cos(ωt)
C.
I(t) = (R/V_0) sin(ωt)
D.
I(t) = (R/V_0) cos(ωt)
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Solution
For a resistor, the current is in phase with the voltage. Therefore, I(t) = V(t)/R = (V_0/R) sin(ωt).
Correct Answer: A — I(t) = (V_0/R) sin(ωt)
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Q. In an RLC circuit, what is the condition for resonance?
A.
XL = XC
B.
R = 0
C.
L = C
D.
f = 0
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Solution
Resonance occurs when the inductive reactance (XL) equals the capacitive reactance (XC).
Correct Answer: A — XL = XC
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Q. In an RLC series circuit, if the resistance is increased while keeping the inductance and capacitance constant, what happens to the quality factor (Q)?
A.
Increases
B.
Decreases
C.
Remains the same
D.
Becomes zero
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Solution
The quality factor (Q) is given by Q = (1/R)√(L/C). Increasing R decreases Q.
Correct Answer: B — Decreases
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Q. In an RLC series circuit, what is the condition for resonance?
A.
XL = XC
B.
R = 0
C.
XL > XC
D.
R > XL
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Solution
Resonance in an RLC series circuit occurs when the inductive reactance (X_L) equals the capacitive reactance (X_C), i.e., X_L = X_C.
Correct Answer: A — XL = XC
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Q. What is the average power consumed in an AC circuit with a power factor of 0.8 and a maximum power of 100 W?
A.
80 W
B.
100 W
C.
120 W
D.
160 W
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Solution
Average power (P_avg) is given by P_avg = P_max × power factor. Therefore, P_avg = 100 W × 0.8 = 80 W.
Correct Answer: A — 80 W
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Q. What is the average power consumed in an AC circuit with a power factor of 0.8 and an RMS voltage of 100 V with a current of 5 A?
A.
200 W
B.
400 W
C.
300 W
D.
500 W
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Solution
Average power (P) is given by P = V_rms * I_rms * PF. Therefore, P = 100 V * 5 A * 0.8 = 400 W.
Correct Answer: C — 300 W
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