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JEE • PHYSICS • ALTERNATING CURRENT

Alternating current practice · Medium

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Practice format and chapter quiz

These sets use single-correct-answer MCQs. They do not cover every JEE Main or JEE Advanced question format and are not full-length mock papers.

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Showing 1-30 of 246 questions

1 Medium

1 of 246

Application +4 | -1

In a series RC circuit with an AC source,

R=300 \, \Omega

C=25 \, \mu \mathrm{F}, \varepsilon_0=50 \mathrm{~V}

and

v=50 / \pi \, \mathrm{Hz}

. Find the peak current and the average power dissipated in the circuit.

View answer and explanation

Correct option: A

0.10 \mathrm{~A}, 1 .5 \mathrm{~W}

Option A (0.10 A, 1.5 W) is correct because it is calculated by finding the peak current using impedance and then finding the average power dissipated using the rms values.

2 Medium

2 of 246

Application +4 | -1

In a series circuit

R=300 \Omega, L=0.9 \mathrm{~H}, C=2.0

\mu F

and

\omega=1000 \mathrm{~rad} / \mathrm{sec}

. The impedance of the circuit is:

View answer and explanation

Correct option: C

500 \Omega

Option C (

500 \Omega

) is correct because it is the result of calculating the impedance using the formula

Z = \sqrt{R^2 + (X_L - X_C)^2}

for a series RLC circuit.

3 Medium

3 of 246

Application +4 | -1

An L-C-R series circuit with a resistance of

100 \Omega

is connected to

200 \mathrm{~V}

(AC source) and angular frequency

300 \, \mathrm{rad} / \mathrm{s}

. When only the capacitor is removed, then the current lags behind the voltage by

60^{\circ}

. When only the inductor is removed the current leads the voltage by

60^{\circ}

. The average power dissipated in original L-C-R circuit is

View answer and explanation

Correct option: D

400 W

Option D (400 W) is correct because at resonance, the impedance equals the resistance, and power is calculated as V²/R.

4 Medium

4 of 246

Analyzing +4 | -1

Assertion (A) : In series

L-C \cdot R, \mathrm{AC}

circuit, current and voltage are in same phase at resonance.

Reason (R) : In series

L-C-R, \mathrm{AC}

circuit, resonant frequency does not depend on the value of resistance. Hence, current at resonance does not depend on resistance.

View answer and explanation

Correct option: C

If Assertion is true, but the Reason is false.

Option C is correct because at resonance in a series L-C-R AC circuit, current and voltage are in phase, but the resonant frequency does depend on the resistance.

5 Medium

5 of 246

Analyzing +4 | -1

A series

L-C-R

circuit is connected to an

\mathrm{AC}

voltage source. When

L

is removed from the circuit, the phase difference between current and voltage is

\frac{\pi}{4}

. If instead

C

is removed from the circuit, the phase difference is again

\frac{\pi}{4}

between current and voltage. The power factor of the circuit is

View answer and explanation

Correct option: B

Option B (1) is correct because when

X_L = X_C

, the circuit is at resonance, and the power factor is 1.

6 Medium

6 of 246

Understanding +4 | -1

A capacitor in an ideal LC circuit is fully charged by a DC source. Then it is disconnected from DC source, the current in the circuit

View answer and explanation

Correct option: A

Becomes zero instantaneously

Option A (Becomes zero instantaneously) is correct because if the capacitor is fully charged and then disconnected, the initial current in the circuit is zero.

7 Medium

7 of 246

Understanding +4 | -1

In a choke coil, the resistance

{X_L}

and resistance

R

are such that

View answer and explanation

Correct option: B

{X_L} \gg R

Option B is correct because in a choke coil, the inductive reactance is much greater than the resistance to minimize power dissipation.

8 Medium

8 of 246

Remembering +4 | -1

The power factor varies between

View answer and explanation

Correct option: C

0 to 1

Option C (0 to 1) is correct because the power factor, defined as the cosine of the phase angle between voltage and current, ranges from 0 (for purely reactive circuits) to 1 (for purely resistive circuits).

9 Medium

9 of 246

Analyzing +4 | -1

The voltage of an AC source varies with time according to the relation

E = 200\sin \left( {100\pi t} \right)\cos \left( {100\pi t} \right)

Select correct statement.

View answer and explanation

Correct option: D

The frequency of source voltage is 100 Hz

The frequency of the source voltage is 100 Hz because the given equation can be simplified to

E = 100\sin(200\pi t)

, and frequency is related to angular frequency by

f = \frac{\omega}{2\pi}

10 Medium

10 of 246

Application +4 | -1

A capacitor is charged and then made to discharge through a resistance. The time constant is

\tau

. In what time will the potential difference across the capacitor decrease by

10 \%

View answer and explanation

Correct option: C

\tau \,ln\, \frac{10}{9}

Option C (

\tau \,ln\, \frac{10}{9}

) is correct because it is derived from the exponential decay equation for capacitor discharge.

11 Medium

11 of 246

Analyzing +4 | -1

Assertion: The division are equally marked on the scale of A.C. ammeter.

Reason: Heat produced is directly proportional to the current.

View answer and explanation

Correct option: C

If Assertion is True but the Reason is False

Option C is correct because the divisions are not equally marked on the scale of an AC ammeter due to the heat produced being proportional to the square of the current.

12 Medium

12 of 246

Remembering +4 | -1

The principle of working of transformer is

View answer and explanation

Correct option: B

Mutual induction

Option B (Mutual induction) is correct because transformers operate based on the principle of mutual induction, where a changing magnetic field in one coil induces a voltage in another coil.

13 Medium

13 of 246

Remembering +4 | -1

Core of transformer is made up of

View answer and explanation

Correct option: A

soft iron

Option A (soft iron) is correct because soft iron has high permeability and low hysteresis loss, making it ideal for transformer cores.

14 Medium

14 of 246

Analyzing +4 | -1

An LCR circuit contains resistance of

100 \Omega

and a supply of

200 \mathrm{~V}

300 \mathrm{~rad}

angular frequency. If only capacitance is taken out from the circuit and the rest of the circuit is joined, current lags behind the voltage by

60^{\circ}

. If, on the other hand, only inductor is taken out, the current leads by

60^{\circ}

with the applied voltage. The current flowing in the circuit is

View answer and explanation

Correct option: C

2 \mathrm{~A}

Option C is correct because at resonance, the impedance is equal to the resistance, and the current is simply the voltage divided by the resistance, which is 2A.

15 Medium

15 of 246

Application +4 | -1

Calculate the wavelength of radiowaves radiated out by a circuit consisting of

0.02 \mu \mathrm{F}

capacitor and

8 \mu \mathrm{H}

inductor in series.

View answer and explanation

Correct option: A

7.54 \times 10^2 \mathrm{~m}

Option A (

7.54 \times 10^2

m) is correct because it is calculated using the resonant frequency of the LC circuit and the speed of light.

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16 Medium

16 of 246

Application +4 | -1

The average value of current

i=I_{{m}} \sin \omega t

from

t=\frac{\pi}{2 \omega}

t=\frac{3 \pi}{2 \omega}

is how many times of

I_{{m}}

View answer and explanation

Correct option: A

Option A (0) is correct because the average value of the sine function over the given interval is zero due to symmetry.

17 Medium

17 of 246

Application +4 | -1

A transformer has

\mathrm{100}

turns in the primary coil and carries

8\,A

current. If input power is one kilowatt, the number of turns required in the secondary coil to have

500\,\,V

output will be

View answer and explanation

Correct option: C

400

Option C (400) is correct because the number of turns in the secondary coil is directly proportional to the secondary voltage in a transformer.

18 Medium

18 of 246

Application +4 | -1

An ac is given by equation

I=I_1 \cos \omega t+I_2 \sin \omega t

. The rms value of current is given by

View answer and explanation

Correct option: C

\sqrt{\frac{I_1^2+I_2^2}{2}}

Option C (

\sqrt{\frac{I_1^2+I_2^2}{2}}

) is correct because it represents the RMS value of the given AC current, calculated by averaging the square of the current over a cycle.

19 Medium

19 of 246

Application +4 | -1

An inductor of inductance of 1 mH a condenser of

10\,\mu F

and a resistance of

50\,\,\Omega

are connected in series. The reactances of inductor and condensers are fame. The reactance of either of them will be

View answer and explanation

Correct option: D

10\,\,\Omega

Option D (

10\\,\Omega

) is correct because the reactance is calculated using the resonant frequency of the LC circuit.

20 Medium

20 of 246

Understanding +4 | -1

The capacity of a pure capacitor is 1 farad. In dc circuits, its effective resistance will be

View answer and explanation

Correct option: B

Infinite

Option B (Infinite) is correct because a capacitor blocks DC current, resulting in infinite resistance.

21 Medium

21 of 246

Understanding +4 | -1

Assertion: Power loss in ideal choke coil is zero.

Reason: Ideal choke coil has zero resistance.

View answer and explanation

Correct option: A

If both Assertion & Reason are True & the Reason is a correct explanation of the Assertion

Option A (If both Assertion & Reason are True & the Reason is a correct explanation of the Assertion) is correct because an ideal choke coil has zero resistance, leading to zero power loss.

22 Medium

22 of 246

Analyzing +4 | -1

Which of the following statements is/are incorrect?

View answer and explanation

Correct option: D

Below resonance, voltage leads the current while above it, current leads the voltage.

Option D (Below resonance, voltage leads the current while above it, current leads the voltage) is correct because it is the incorrect statement. Below resonance, current leads the voltage, and above resonance, voltage leads the current.

23 Medium

23 of 246

Application +4 | -1

A power transformer is used to step up an alternating e.m.f. of

220\,\,V

11\,\,kV

to transmit

4.4\,\,kW

of power. If the primary coil has

\mathrm{1000}

turns, what is the current rating of the secondary? Assume

\mathrm{100}\,\,\mathrm{\% }

efficiency for the transformer

View answer and explanation

Correct option: B

0.4\,A

Option B (0.4 A) is correct because the current in the secondary coil is calculated using the power and voltage in the secondary coil, assuming 100% efficiency.

24 Medium

24 of 246

Understanding +4 | -1

Statement (A): In a transformer a large alternating current at low voltage can be transformed into a small alternating current at high voltage.

Statement (B): Energy in current carrying coil is stored in the form of magnetic field.

View answer and explanation

Correct option: A

Both (A) and (B) are true

Option A is correct because transformers can step up voltage while decreasing current, and energy is stored in a coil's magnetic field.

25 Medium

25 of 246

Application +4 | -1

An inductor-coil of resistance

10 \, \Omega

and inductance

120 \mathrm{~mH}

is connected across a battery of emf

6 \mathrm{~V}

and internal resistance

2 \, \Omega

. Find the charge which flows through the inductor in (a)

10 \mathrm{~ms}

, (b)

20 \mathrm{~ms}

and (c)

100 \mathrm{~ms}

after the connections are made.

View answer and explanation

Correct option: A

(a)

1.8 \mathrm{~mC};

(b)

5.6 \mathrm{~mC};

(c)

45 \mathrm{~mC}

Option A is correct. The charge flowing through the inductor is calculated by integrating the current over time, considering the inductor's time constant and initial current.

26 Medium

26 of 246

Understanding +4 | -1

Statement (A): A lamp is connected in seres with a capacitor and ac source connected across their terminals consequently current flow in the circuit and the lamp will shine.

Statement (B): capacitor block dc current and allow ac current.

View answer and explanation

Correct option: A

Both (A) and (B) are true

Option A (Both (A) and (B) are true) is correct because a capacitor allows AC current to flow while blocking DC current, enabling the lamp to shine in an AC circuit.

27 Medium

27 of 246

Understanding +4 | -1

The

Q

-factor of a series L-C-R resonant circuit depends on

View answer and explanation

Correct option: D

L, C and R

Option D (L, C and R) is correct because the Q-factor depends on the inductance, capacitance, and resistance of the circuit.

28 Medium

28 of 246

Application +4 | -1

An AC supply gives 30 V r.m.s which passes through a

10\,\,\Omega

resistance. The power dissipated in it is [wats]

View answer and explanation

Correct option: B

Option B (90) is correct because the power dissipated is calculated using the formula

P = \frac{{V_{rms}^2}}{R}

29 Medium

29 of 246

Understanding +4 | -1

Assertion: Capacitor serves as a block for dc and offers an easy path to ac.

Reason: Capacitive reactance is inversely proportional to frequency.

View answer and explanation

Correct option: A

If both assertion and reason are true and the reason is the correct explanation of the assertion.

The correct answer is If both assertion and reason are true and the reason is the correct explanation of the assertion because capacitive reactance is inversely proportional to frequency, blocking DC and allowing AC.

30 Medium

30 of 246

Application +4 | -1

The armature of a DC motor has a resistance of 20 Ω. It draws a current of 1.5 A when run by 220 V DC. The value of peak emf induced in it will be

View answer and explanation

Correct option: C

190V

Option C (190V) is correct because it's calculated by subtracting the power loss due to resistance from the input power and then dividing by the current.

Revision notes and key takeaways

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Key Takeaways

✓ This set contains 246 single-correct-answer MCQs on ALTERNATING CURRENT.
✓ Try each question before opening its answer. Read the explanation where one is available.
✓ Sign in to save submitted answers. Revisit incorrect answers alongside your textbook or class notes.

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