Chapter 7: Alternating Current Assertion-Reason Questions | physics Class 12 CBSE

Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 1

Logic Check

Assertion (A): In a pure resistor connected to an ac source, the voltage and current are in phase. Reason (R): For a resistor, Ohm's law v = iR holds instantaneously, so both v and i have the same sinusoidal form with no phase shift.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 2

Logic Check

Assertion (A): The average power dissipated in a pure inductor over one complete ac cycle is zero. Reason (R): In a pure inductor, the current lags the voltage by π/2, so the power factor cos(π/2) = 0.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 3

Logic Check

Assertion (A): In a purely capacitive ac circuit, the current leads the voltage by π/2. Reason (R): For a capacitor, current i = dq/dt = C(dv/dt), which gives a cosine function when v is sinusoidal, meaning current is π/2 ahead of voltage.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 4

Logic Check

Assertion (A): Inductive reactance increases with frequency. Reason (R): XL = ωL = 2πνL, which is directly proportional to the frequency ν of the ac source.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 5

Logic Check

Assertion (A): Capacitive reactance decreases with increasing frequency. Reason (R): XC = 1/(ωC) = 1/(2πνC), which is inversely proportional to frequency ν.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 6

Logic Check

Assertion (A): At resonance in a series LCR circuit, the impedance is maximum. Reason (R): At resonance, XL = XC, so the net reactance is zero and Z = √(R² + 0) = R, which is the minimum possible value of impedance.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 7

Logic Check

Assertion (A): A step-up transformer violates the law of conservation of energy. Reason (R): A step-up transformer increases voltage, so energy output must exceed energy input.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 8

Logic Check

Assertion (A): The power factor of an ac circuit is always between 0 and 1. Reason (R): Power factor = cos φ, where φ is the phase angle between voltage and current, and cos φ ranges from 0 to 1 for all practical ac circuits.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 9

Logic Check

Assertion (A): Resonance can occur in a series RL circuit (without a capacitor). Reason (R): For resonance, the net reactance must be zero, which requires both inductive and capacitive reactance to cancel each other.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 10

Logic Check

Assertion (A): Phasors representing voltage and current in ac circuits are true vectors with spatial direction. Reason (R): In reality, voltage and current are scalar quantities; phasors are rotating mathematical constructs introduced only to simplify addition of sinusoidal quantities.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 11

Logic Check

Assertion (A): The rms value of alternating current is the equivalent dc current that produces the same average Joule heating. Reason (R): Joule heating depends on i², and the rms value I = im/√2 is defined such that I²R gives the correct average power.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 12

Logic Check

Assertion (A): In a series LCR circuit, the sum of instantaneous voltages across R, L, and C equals the source voltage at every instant. Reason (R): By Kirchhoff's loop rule, the algebraic sum of all voltages in a closed loop is zero at every instant.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 13

Logic Check

Assertion (A): Eddy current losses in a transformer are reduced by using a laminated iron core. Reason (R): Lamination divides the core into thin insulated sheets, increasing resistance to eddy current flow and thus reducing the magnitude of eddy currents.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 14

Logic Check

Assertion (A): In a series LCR circuit, power is dissipated only in the resistor R. Reason (R): The phase difference between voltage and current is exactly π/2 for both pure inductor and pure capacitor, so the average power for each is VI cos(π/2) = 0.

Reasoning Options

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Assertion-Reason

Hard difficulty • Check the statement and reason

Hard

Question 15

Logic Check

Assertion (A): The algebraic sum of rms voltages across R, L, and C in a series LCR circuit can be greater than the rms source voltage. Reason (R): VR, VL, and VC have different phases and must be added as phasors, not algebraically; their phasor sum equals the source voltage.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 16

Logic Check

Assertion (A): Electrical energy is transmitted over long distances at high voltage and low current. Reason (R): Power loss in transmission lines is P_loss = I²R; transmitting at high voltage reduces current (for the same power), thus greatly reducing I²R losses.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 17

Logic Check

Assertion (A): A capacitor fully blocks direct current in steady state but allows alternating current to flow. Reason (R): In dc, the capacitor charges to source voltage and current ceases; in ac, it alternately charges and discharges, allowing sustained current with reactance XC = 1/(ωC).

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 18

Logic Check

Assertion (A): At resonance in a series LCR circuit, the power factor is 1 and maximum power is transferred to the circuit. Reason (R): At resonance, XC = XL, phase angle φ = 0, cos φ = 1, and impedance Z = R is minimum, giving maximum current and power P = I²R.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 19

Logic Check

Assertion (A): The resonant frequency of a series LCR circuit depends on the value of resistance R. Reason (R): Resonant frequency ω₀ = 1/√(LC) depends only on L and C, and is completely independent of R.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 20

Logic Check

Assertion (A): When an iron rod is inserted into an inductor in series with a bulb and ac source, the bulb glows brighter. Reason (R): The iron rod increases inductance L, increasing inductive reactance XL and total impedance, thereby reducing current and bulb brightness.

Reasoning Options

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Chapter sections

Chapter 7: Alternating Current | Assertion-Reason

Assertion (A): In a pure resistor connected to an ac source, the voltage and current are in phase. Reason (R): For a resistor, Ohm's law v = iR holds instantaneously, so both v and i have the same sinusoidal form with no phase shift.

Assertion (A): The average power dissipated in a pure inductor over one complete ac cycle is zero. Reason (R): In a pure inductor, the current lags the voltage by π/2, so the power factor cos(π/2) = 0.

Assertion (A): In a purely capacitive ac circuit, the current leads the voltage by π/2. Reason (R): For a capacitor, current i = dq/dt = C(dv/dt), which gives a cosine function when v is sinusoidal, meaning current is π/2 ahead of voltage.

Assertion (A): Inductive reactance increases with frequency. Reason (R): XL = ωL = 2πνL, which is directly proportional to the frequency ν of the ac source.

Assertion (A): Capacitive reactance decreases with increasing frequency. Reason (R): XC = 1/(ωC) = 1/(2πνC), which is inversely proportional to frequency ν.

Assertion (A): At resonance in a series LCR circuit, the impedance is maximum. Reason (R): At resonance, XL = XC, so the net reactance is zero and Z = √(R² + 0) = R, which is the minimum possible value of impedance.

Assertion (A): A step-up transformer violates the law of conservation of energy. Reason (R): A step-up transformer increases voltage, so energy output must exceed energy input.

Assertion (A): The power factor of an ac circuit is always between 0 and 1. Reason (R): Power factor = cos φ, where φ is the phase angle between voltage and current, and cos φ ranges from 0 to 1 for all practical ac circuits.

Assertion (A): Resonance can occur in a series RL circuit (without a capacitor). Reason (R): For resonance, the net reactance must be zero, which requires both inductive and capacitive reactance to cancel each other.

Assertion (A): Phasors representing voltage and current in ac circuits are true vectors with spatial direction. Reason (R): In reality, voltage and current are scalar quantities; phasors are rotating mathematical constructs introduced only to simplify addition of sinusoidal quantities.

Assertion (A): The rms value of alternating current is the equivalent dc current that produces the same average Joule heating. Reason (R): Joule heating depends on i², and the rms value I = im/√2 is defined such that I²R gives the correct average power.

Assertion (A): In a series LCR circuit, the sum of instantaneous voltages across R, L, and C equals the source voltage at every instant. Reason (R): By Kirchhoff's loop rule, the algebraic sum of all voltages in a closed loop is zero at every instant.

Assertion (A): Eddy current losses in a transformer are reduced by using a laminated iron core. Reason (R): Lamination divides the core into thin insulated sheets, increasing resistance to eddy current flow and thus reducing the magnitude of eddy currents.

Assertion (A): In a series LCR circuit, power is dissipated only in the resistor R. Reason (R): The phase difference between voltage and current is exactly π/2 for both pure inductor and pure capacitor, so the average power for each is VI cos(π/2) = 0.

Assertion (A): The algebraic sum of rms voltages across R, L, and C in a series LCR circuit can be greater than the rms source voltage. Reason (R): VR, VL, and VC have different phases and must be added as phasors, not algebraically; their phasor sum equals the source voltage.

Assertion (A): Electrical energy is transmitted over long distances at high voltage and low current. Reason (R): Power loss in transmission lines is P_loss = I²R; transmitting at high voltage reduces current (for the same power), thus greatly reducing I²R losses.

Assertion (A): A capacitor fully blocks direct current in steady state but allows alternating current to flow. Reason (R): In dc, the capacitor charges to source voltage and current ceases; in ac, it alternately charges and discharges, allowing sustained current with reactance XC = 1/(ωC).

Assertion (A): At resonance in a series LCR circuit, the power factor is 1 and maximum power is transferred to the circuit. Reason (R): At resonance, XC = XL, phase angle φ = 0, cos φ = 1, and impedance Z = R is minimum, giving maximum current and power P = I²R.

Assertion (A): The resonant frequency of a series LCR circuit depends on the value of resistance R. Reason (R): Resonant frequency ω₀ = 1/√(LC) depends only on L and C, and is completely independent of R.

Assertion (A): When an iron rod is inserted into an inductor in series with a bulb and ac source, the bulb glows brighter. Reason (R): The iron rod increases inductance L, increasing inductive reactance XL and total impedance, thereby reducing current and bulb brightness.

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