Physics – Alternating Current
Class 12 Physics (Chapter 7) • 100 Multiple Choice Questions
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Question 1
What is the mean value of alternating current over a complete cycle?
Correct Answer: Zero
Question 2
The relation between root mean square (rms) value and peak value (I₀) of alternating current is:
Correct Answer: I_rms = I₀ / √2
Question 3
In a pure resistive AC circuit, current and voltage are:
Correct Answer: In same phase
Question 4
In a pure inductive AC circuit, current:
Correct Answer: Lags behind voltage by 90°
Question 5
In a pure capacitive AC circuit, current:
Correct Answer: Leads voltage by 90°
Question 6
Inductive reactance (X_L) is given by the formula:
Correct Answer: X_L = ω L = 2π f L
Question 7
Capacitive reactance (X_C) is given by the formula:
Correct Answer: X_C = 1 / (ω C) = 1 / (2π f C)
Question 8
Impedance (Z) of an LCR series circuit is expressed as:
Correct Answer: Z = √[R² + (X_L - X_C)²]
Question 9
Resonance condition in an LCR series circuit occurs when:
Correct Answer: X_L = X_C
Question 10
Resonant frequency (ω_r) of an LCR series circuit is given by:
Correct Answer: ω_r = 1 / √(L C)
Question 11
Power factor (cos φ) in an AC circuit is defined as the ratio of:
Correct Answer: True power to apparent power (R / Z)
Question 12
What is the power factor of a purely inductive or purely capacitive AC circuit?
Correct Answer: Zero
Question 13
What is the power factor of a purely resistive AC circuit?
Correct Answer: 1
Question 14
What is 'wattless current'?
Correct Answer: Current in a purely inductive or capacitive circuit that consumes no average power
Question 15
An ideal choke coil has:
Correct Answer: High inductance and zero resistance
Question 16
What is the primary function of a choke coil in an AC circuit?
Correct Answer: To control AC current without wasting much electrical energy
Question 17
An AC generator (dynamo) works on the principle of:
Correct Answer: Electromagnetic induction (Faraday's Law)
Question 18
What is the frequency of household AC supply in India?
Correct Answer: 50 Hz
Question 19
What is the frequency of household AC supply in USA?
Correct Answer: 60 Hz
Question 20
A transformer works on the principle of:
Correct Answer: Mutual induction
Question 21
A step-up transformer converts:
Correct Answer: Low voltage AC to high voltage AC
Question 22
In a step-up transformer, the number of turns in the secondary coil (N_s) compared to primary coil (N_p) is:
Correct Answer: N_s > N_p
Question 23
Can a transformer operate on direct current (DC)?
Correct Answer: No, because DC produces no changing magnetic flux
Question 24
The core of a transformer is laminated to minimize energy loss due to:
Correct Answer: Eddy currents
Question 25
In an LC oscillating circuit, energy oscillates between:
Correct Answer: Electric field of capacitor and magnetic field of inductor
Question 26
What is the frequency of electrical oscillation in an ideal LC circuit?
Correct Answer: f = 1 / (2Ï€ √(L C))
Question 27
Quality factor (Q-factor) of a series resonant circuit is defined as:
Correct Answer: (1 / R) · √(L / C)
Question 28
What instrument is used to measure alternating current?
Correct Answer: Hot wire ammeter
Question 29
Why do hot-wire instruments have a non-linear scale?
Correct Answer: Because heat produced is proportional to the square of current (H ∝ I²)
Question 30
In a step-down transformer, if input voltage is 220V and transformation ratio is 1:10, what is the output voltage?
Correct Answer: 22 V
Question 31
What is the phase difference between voltage and current in a purely resistive circuit?
Correct Answer: 0°
Question 32
What is the phase difference between voltage and current in a purely inductive circuit?
Correct Answer: 90° (Voltage leads current)
Question 33
What is the phase difference between voltage and current in a purely capacitive circuit?
Correct Answer: 90° (Current leads voltage)
Question 34
The unit of impedance (Z) is:
Correct Answer: Ohm (Ω)
Question 35
The unit of inductive reactance is:
Correct Answer: Ohm (Ω)
Question 36
The unit of capacitive reactance is:
Correct Answer: Ohm (Ω)
Question 37
What is the impedance of an LC circuit at resonance?
Correct Answer: Zero
Question 38
What is the impedance of an LCR series circuit at resonance?
Correct Answer: Minimum (equal to resistance R)
Question 39
At resonance, the current in an LCR series circuit is:
Correct Answer: Maximum
Question 40
What is the phase angle between voltage and current at resonance in an LCR series circuit?
Correct Answer: 0°
Question 41
If the frequency of an AC source connected to an inductor is doubled, its inductive reactance becomes:
Correct Answer: Doubled
Question 42
If the frequency of an AC source connected to a capacitor is doubled, its capacitive reactance becomes:
Correct Answer: Halved
Question 43
What is the inductive reactance for direct current (DC, f = 0)?
Correct Answer: Zero
Question 44
What is the capacitive reactance for direct current (DC, f = 0)?
Correct Answer: Infinite (Blocks DC completely)
Question 45
The average power consumed in a pure inductor over a complete cycle is:
Correct Answer: Zero
Question 46
The average power consumed in a pure capacitor over a complete cycle is:
Correct Answer: Zero
Question 47
The average power consumed in a pure resistor connected to an AC source is:
Correct Answer: P_avg = V_rms × I_rms = I_rms² R
Question 48
In a step-down transformer, power losses are minimized by using:
Correct Answer: Laminated soft iron core and thick copper wires
Question 49
Why is electrical power transmitted over long distances at very high voltages?
Correct Answer: To reduce transmission power loss (I²R loss)
Question 50
What is the peak value of AC whose rms value is 10 A?
Correct Answer: 14.14 A (10√2 A)
Question 51
The phase relationship between voltage and current in an AC circuit containing only a capacitor is:
Correct Answer: Current leads voltage by 90°
Question 52
The alternating emf is represented by e = e₀ sin(ωt). The angular frequency ω is related to frequency f as:
Correct Answer: ω = 2π f
Question 53
What is the value of peak factor (ratio of peak value to rms value) for a sinusoidal AC?
Correct Answer: √2 (≈ 1.414)
Question 54
What is the form factor (ratio of rms value to average value over half cycle) for sinusoidal AC?
Correct Answer: Ï€ / (2√2) (≈ 1.11)
Question 55
In an AC circuit containing resistance R and inductance L in series, the impedance Z is:
Correct Answer: Z = √(R² + X_L²)
Question 56
In an AC circuit containing resistance R and capacitance C in series, the impedance Z is:
Correct Answer: Z = √(R² + X_C²)
Question 57
What is the average value of alternating current over a half-cycle (positive half)?
Correct Answer: 2 I₀ / Ï€ (≈ 0.637 I₀)
Question 58
The quality factor (Q) of a resonance circuit can also be expressed as the ratio of:
Correct Answer: Voltage across L (or C) to voltage across R
Question 59
In an LCR circuit, if X_L > X_C, the circuit is:
Correct Answer: Inductive (voltage leads current)
Question 60
In an LCR circuit, if X_C > X_L, the circuit is:
Correct Answer: Capacitive (current leads voltage)
Question 61
What is the expression for bandwidth (Δω) of a series LCR resonant circuit?
Correct Answer: Δω = R / L
Question 62
Why are alternating currents preferred over direct currents for long-distance power transmission?
Correct Answer: AC can be easily stepped up and down using transformers with low loss
Question 63
In an AC circuit, what is the ratio of virtual voltage to virtual current known as?
Correct Answer: Impedance (Z)
Question 64
What is the power factor of a circuit containing resistance R and inductance L in series?
Correct Answer: R / √(R² + X_L²)
Question 65
If a 220V AC supply is given, what does 220V signify?
Correct Answer: RMS value (Root Mean Square voltage)
Question 66
What is the peak voltage of a 220V AC mains supply?
Correct Answer: 311 V (220 × √2)
Question 67
What is the dimensional formula of impedance (Z)?
Correct Answer: [M L² T⁻³ A⁻²]
Question 68
What is the dimensional formula of inductive reactance (X_L)?
Correct Answer: [M L² T⁻³ A⁻²]
Question 69
Which device transforms mechanical energy into electrical energy using alternating current principles?
Correct Answer: AC Generator (Alternator)
Question 70
In a step-down transformer, the output current compared to the input current is:
Correct Answer: Higher (Step up in current)
Question 71
What is the efficiency (η) of a real transformer defined as?
Correct Answer: (Output Power / Input Power) × 100%
Question 72
Flux linkage loss in a transformer can be minimized by:
Correct Answer: Winding primary and secondary coils closely over one another
Question 73
What is 'Copper loss' in a transformer?
Correct Answer: Heat loss due to the electrical resistance of the copper windings (I²R loss)
Question 74
What is 'Hysteresis loss' in a transformer?
Correct Answer: Energy loss due to repeated magnetization and demagnetization of the iron core
Question 75
In a series LCR circuit, if the voltage across R is 40V, across L is 80V, and across C is 50V, what is the applied source voltage?
Correct Answer: 50 V
Question 76
What is the impedance of a parallel resonant circuit (rejector circuit) at resonance?
Correct Answer: Maximum (Infinite practically)
Question 77
What is the current in a parallel resonant circuit at resonance?
Correct Answer: Minimum
Question 78
Which of the following circuits is known as an acceptor circuit?
Correct Answer: Series LCR circuit
Question 79
Which of the following circuits is known as a rejector circuit?
Correct Answer: Parallel resonant circuit
Question 80
What is the unit of angular frequency ω in AC circuits?
Correct Answer: Radian per second (rad/s)
Question 81
What is the sharpness of resonance (or Q-factor) proportional to?
Correct Answer: Inductance and inversely to resistance (ω_r L / R)
Question 82
If an alternating voltage e = 200 sin(100t) is applied across a resistor of 50 Ω, what is the rms current?
Correct Answer: 2.82 A (2√2 A)
Question 83
In a step-up transformer, the turns ratio (N_s / N_p) is:
Correct Answer: Greater than 1
Question 84
In a step-down transformer, the turns ratio (N_s / N_p) is:
Correct Answer: Less than 1
Question 85
What is the reactance of a capacitor of capacitance 10 µF at frequency 50 Hz?
Correct Answer: 318.3 Ω
Question 86
What is the inductive reactance of an inductor of inductance 0.1 H at frequency 50 Hz?
Correct Answer: 31.4 Ω
Question 87
Which device can be used to block AC while allowing DC to pass through?
Correct Answer: Inductor (Choke coil)
Question 88
Which device can be used to block DC while allowing AC to pass through?
Correct Answer: Capacitor
Question 89
The power in an AC circuit is given by P = V_rms · I_rms · cos φ. What does cos φ represent?
Correct Answer: Power factor
Question 90
What is the resonance frequency of an LC circuit if L = 2 H and C = 2 µF?
Correct Answer: 79.6 Hz
Question 91
Which component causes a voltage drop that is in phase with current in an AC circuit?
Correct Answer: Resistor (R)
Question 92
In an alternating voltage source, the maximum value of voltage is called:
Correct Answer: Peak voltage or amplitude (V₀)
Question 93
What is the phase relationship between the alternating voltage and current in an ideal resistor?
Correct Answer: Zero phase difference
Question 94
Which of the following describes apparent power in an AC circuit?
Correct Answer: V_rms × I_rms
Question 95
What is the true power in an AC circuit equal to?
Correct Answer: Apparent power × Power factor (V_rms · I_rms · cos φ)
Question 96
What is the function of a step-down transformer in power distribution?
Correct Answer: To reduce high transmission voltage to safe lower voltage for consumers
Question 97
In an LCR series circuit, at frequencies lower than the resonant frequency, the circuit is predominantly:
Correct Answer: Capacitive
Question 98
In an LCR series circuit, at frequencies higher than the resonant frequency, the circuit is predominantly:
Correct Answer: Inductive
Question 99
What is the angle of lag or lead (phase angle φ) in an LCR circuit given by?
Correct Answer: tan φ = (X_L - X_C) / R
Question 100
What is the primary reason why core of a transformer is laminated?
Correct Answer: To reduce eddy current energy losses
