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NEET • PHYSICS • ALTERNATING CURRENT

Alternating current practice · Mixed difficulty

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Showing 1-30 of 154 questions

1 Medium

1 of 154

Analyzing +4 | -1

Assertion (A) : In an

\mathrm{AC}

circuit, potential difference across the capacitor may be greater than the applied voltage.

Reason (R) :

V_C=I X_C

, whereas

V=I Z

and

X_C

can be greater than

Z

also.

View answer and explanation

Correct option: A

If both Assertion and Reason are true and the Reason is correct explanation of the Assertion.

Option A is correct because the potential difference across a capacitor in an AC circuit can be greater than the applied voltage due to resonance, and the reason correctly explains this with the relationship between capacitive reactance, impedance, and voltage.

2 Medium

2 of 154

Understanding +4 | -1

When power is drawn from the secondary coil of the transformer, the dynamic resistance

View answer and explanation

Correct option: A

Increases

Option A (Increases) is correct because when power is drawn, the current in the secondary coil increases, which reflects as an increased load on the primary side, effectively increasing the dynamic resistance.

3 Easy

3 of 154

Understanding +4 | -1

An alternating e.m.f. is applied to purely capacitive circuit. The phase relation between e.m.f. and current flowing in the circuit is or
In a circuit containing capacitance only

View answer and explanation

Correct option: B

Current is ahead of e.m.f. by

\pi /2

Option B (Current is ahead of e.m.f. by

\pi /2

) is correct because in a purely capacitive circuit, the current leads the voltage by 90 degrees.

4 Medium

4 of 154

Understanding +4 | -1

Average power in the L-C-R circuit depends upon

View answer and explanation

Correct option: D

Current, emf and phase difference

Option D is correct because the average power in an L-C-R circuit depends on the current, emf, and the phase difference between them, as described by the formula P = Vrms * Irms * cos(phi).

5 Easy

5 of 154

Application +4 | -1

A generator produces a voltage that is given by

V = 240\sin 120\;t

, where t is in seconds. The frequency and r.m.s. voltage are

View answer and explanation

Correct option: C

19 Hz and 170 V

Option C (19 Hz and 170 V) is correct because the frequency is

f = \frac{\omega}{2\pi} = \frac{120}{2\pi} \approx 19

Hz and the RMS voltage is

V_{rms} = \frac{V_0}{\sqrt{2}} = \frac{240}{\sqrt{2}} \approx 170

6 Medium

6 of 154

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 (

{X_L} \gg R

) is correct because a choke coil is designed to have a high inductive reactance and low resistance to minimize power loss.

7 Medium

7 of 154

Analyzing +4 | -1

Assertion: With increase in frequency of an A.C supply, the impedance of an L-C-R series circuit, decreases at first, becomes minimum and then increases.

Reason: In L-C-R series circuit, impedance (z)

z=\sqrt{R^2+\left(X_L-X_C\right)^2}

View answer and explanation

Correct option: A

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

Option A is correct because both the assertion and the reason are true, and the reason correctly explains the assertion. The impedance of an L-C-R series circuit decreases, reaches a minimum at resonance, and then increases as frequency increases. The impedance formula explains this behavior.

8 Medium

8 of 154

Remembering +4 | -1

Which type of losses donot occur in the transformer?

View answer and explanation

Correct option: C

Mechanical losses

Option C (Mechanical losses) is correct because transformers are static devices and do not have moving parts, thus they do not experience mechanical losses.

9 Easy

9 of 154

Understanding +4 | -1

In an ideal choke, ratio of its inductance L to its DC resistance R is

View answer and explanation

Correct option: A

Infinity

Option A (Infinity) is correct because an ideal choke has zero DC resistance, so the ratio of inductance to DC resistance is infinite.

10 Medium

10 of 154

Understanding +4 | -1

There is a current of

20 \mathrm{~mA}

passing through an ideal inductor when connected to a

100 \mathrm{~V}, 20 \mathrm{~Hz}

supply. The power consumed is

View answer and explanation

Correct option: D

Zero

Option D (Zero) is correct because the power consumed by an ideal inductor in an AC circuit is zero due to the 90-degree phase difference between voltage and current.

11 Hard

11 of 154

Application +4 | -1

An LC circuit contains a 20 mH inductor and a 50 μF capacitor with an initial charge of 10 mC. The resistance of the circuit is negligible. Let the instant the circuit is closed be t = 0. At what time is the energy stored completely magnetic?

View answer and explanation

Correct option: B

t = 1.57 ms

Option A (88 – 198 pF) is correct because it represents the range of capacitance values needed to tune to the given frequency range.

12 Medium

12 of 154

Analyzing +4 | -1

Assertion: Average of sinusoidal A. C. can never be zero for half cycle.

Reason: Impedance given by inductance does not depends on frequency.

View answer and explanation

Correct option: C

If Assertion is True but the Reason is False

Option C is correct because the average of a sinusoidal AC over a half cycle is non-zero, while the impedance due to inductance depends on frequency.

13 Medium

13 of 154

Application +4 | -1

The potential difference V and the current i flowing through an instrument in an ac circuit of frequency

f

are given by V=5cos⁡ωt volts and I=2sin⁡ωt amperes (where ω=2πf). The power dissipated in the instrument is

View answer and explanation

Correct option: A

Zero

Option A (Zero) is correct because the voltage and current are 90 degrees out of phase, resulting in zero average power dissipation.

14 Medium

14 of 154

Understanding +4 | -1

A 10 ohm resistance, 5 mH coil and 10 μF capacitor are joined in series. When a suitable frequency alternating current source is joined to this combination, the circuit resonates. If the resistance is halved, the resonance frequency

View answer and explanation

Correct option: C

Remains unchanged

Option C is correct because the resonance frequency depends only on the inductance (L) and capacitance (C) values, not on the resistance (R). Therefore, halving the resistance does not change the resonance frequency.

15 Medium

15 of 154

Understanding +4 | -1

For a series L-C-R circuit at resonance, the statement which is not true is

View answer and explanation

Correct option: D

Power factor is zero

Option D (Power factor is zero) is correct because at resonance in a series LCR circuit, the power factor is 1, not 0.

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16 Easy

16 of 154

Understanding +4 | -1

The core of a transformer is laminated to reduce energy losses due to

View answer and explanation

Correct option: A

Eddy currents

Option A (Eddy currents) is correct because laminating the core increases the resistance to eddy current flow, thus reducing energy losses.

17 Medium

17 of 154

Analyzing +4 | -1

Given below are two statements:
Statement-I: The reactance of an ac circuit is zero. It is possible that the circuit contains a capacitor and an inductor.
Statement-II: In ac circuit, the average power delivered by the source never becomes zero.
In the light of the above statements, choose the correct answer from the options given below.

View answer and explanation

Correct option: C

Statement I is true but Statement II is false

Option C (Statement I is true but Statement II is false) is correct because reactance can be zero with both inductor and capacitor, and average power can be zero in an AC circuit.

18 Easy

18 of 154

Understanding +4 | -1

Power delivered by the source of the circuit becomes maximum, when

View answer and explanation

Correct option: B

wL = \frac{1}{{wc}}

Option B (

wL = \frac{1}{{wc}}

) is correct because maximum power is delivered at resonance, where the inductive reactance equals the capacitive reactance.

19 Medium

19 of 154

Analyzing +4 | -1

Assertion: In higher power transmission,

240 \mathrm{~V}

is preferred over

120 \mathrm{~V}

.
Reason: Power loss is more in case of 120 Vas compared to 240 V.

View answer and explanation

Correct option: D

If both Assertion & Reason are false

Option D (If both Assertion & Reason are false) is correct because higher voltage is preferred for power transmission to reduce power loss, and the reason provided is the opposite of the truth.

20 Easy

20 of 154

Remembering +4 | -1

In L – C – R series circuit the resonance condition in terms of capacitive reactance

\left( {{X_C}} \right)

and inductive reactance

\left( {{X_L}} \right)

View answer and explanation

Correct option: D

{X_C} - {X_L} = 0

Option D is correct because the resonance condition in an L-C-R series circuit is when the capacitive reactance equals the inductive reactance, i.e.,

{X_C} - {X_L} = 0

21 Easy

21 of 154

Understanding +4 | -1

A LCR circuit is at resonance for a capacitor C, inductance

\mathrm{L}

and resistance

\mathrm{R}

. Now the value of resistance is halved keeping all other parameters same. The current amplitude at resonance will be now:

View answer and explanation

Correct option: B

double

Option B is correct because at resonance, the current amplitude is inversely proportional to the resistance. Halving the resistance doubles the current amplitude.

22 Medium

22 of 154

Application +4 | -1

In a LCR circuit having

L = 8.0\;henry,\;C = 0.5\;\mu F

and

R = 100\;ohm

in series. The resonance frequency in per second is

View answer and explanation

Correct option: C

500 radian

Option C (500 radian) is correct because the resonance frequency is calculated using the formula

\omega = \frac{1}{\sqrt{LC}}

, resulting in 500 rad/s.

23 Medium

23 of 154

Application +4 | -1

A coil of wire of certain radius has 100 turns and a self inductance of 15 mH. The self inductance of a second similar coil of 500 turns will be

View answer and explanation

Correct option: B

375 mH

Option B (t = 1.57 ms) is correct because the energy is completely magnetic when the capacitor is fully discharged, which occurs at T/4.

24 Medium

24 of 154

Application +4 | -1

The intensity of EMW at a distance of 1 km from a source of power 12.56 KW is

View answer and explanation

Correct option: A

{10^{ - 3}}\,w{m^2}

Option A (

{10^{ - 3}}\,w{m^2}

) is correct because intensity is power per unit area, and the area is the surface area of a sphere with radius 1 km.

25 Easy

25 of 154

Analyzing +4 | -1

In series

L C R

circuit voltage drop across resistance is

8 {~V}

, across inductor is

6 {~V}

and across capacitor is

12 {~V}

. Then

View answer and explanation

Correct option: D

None of these

Option D (None of these) is correct because the source voltage, power factor, and voltage drop relationships are not as described in the other options for the given LCR circuit.

26 Easy

26 of 154

Application +4 | -1

The instantaneous value of current in an A.C. circuit is

I = 2\sin \left( {100\;\pi \;t + \pi /3} \right)A

The current will be maximum for the first time at

View answer and explanation

Correct option: D

t = \frac{1}{{600}}s

Option D (

t = \frac{1}{{600}}s

) is correct because the current is maximum when the argument of the sine function is

\pi/2

, so

100\pi t + \pi/3 = \pi/2

, which gives

t = \frac{1}{{600}}s

27 Easy

27 of 154

Application +4 | -1

An AC voltage source has an output of

\mathrm{\Delta }V = \left( {200\mathrm{V}} \right)\sin 2\pi ft.

, This source is connected to a 100 Ω resistor. RMS current in the resistance is

View answer and explanation

Correct option: A

1.41A

Option A (1.41A) is correct because the RMS voltage is

V_{rms} = \frac{200}{\sqrt{2}}

and the RMS current is

I_{rms} = \frac{V_{rms}}{R} = \frac{200}{\sqrt{2} \cdot 100} = \sqrt{2} \approx 1.41

28 Medium

28 of 154

Understanding +4 | -1

The ratio of secondary to primary turns is

\mathrm{9 : 4}

. If power input is

P

, what will be the ratio of power output (neglect all losses) to power input

View answer and explanation

Correct option: D

\mathrm{1 : 1}

Option D (1 : 1) is correct because in an ideal transformer, the power input equals the power output, regardless of the turns ratio.

29 Hard

29 of 154

Understanding +4 | -1

The sum of the instantaneous current values over one complete cycle is zero, and the average current is zero. This means average power consumed is

View answer and explanation

Correct option: D

\frac{1}{2} \mathrm{i}_{\mathrm{m}}^2 \mathrm{R}

Option D (

\frac{1}{2} \mathrm{i}_{\mathrm{m}}^2 \mathrm{R}

) is correct because the average power consumed in an AC circuit is related to the RMS current, which is

\frac{i_m}{\sqrt{2}}

30 Medium

30 of 154

Understanding +4 | -1

The instantaneous value of current and emf in an A.C. circuit are

I = \frac{1}{{\sqrt 2 }}\sin 314t

amp and

E = \sqrt 2 \sin \left( {314t - \frac{\pi }{6}} \right)V,

with respect to I) will be

View answer and explanation

Correct option: A

- \frac{\pi }{6}\mathrm{ rad}

Option A is correct. The phase difference is calculated by subtracting the phase of the current from the phase of the voltage, resulting in

-\frac{\pi}{6}

rad.

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Key Takeaways

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