Grade 12 : Physics - Alternating Current (Set 1)

Question 1

An alternating current supply provides a peak current $I_0 = 10 ext{ A}$.

What is the root-mean-square (RMS) value of this alternating current?

Accepted Answer

A) $5\sqrt{2}	ext{ A}$

Answer

B) $10\sqrt{2}	ext{ A}$

Answer

C) 5 A

Answer

D) 10 A

Question 2

An AC voltage is applied across a purely inductive circuit element.

What is the phase angle by which alternating voltage leads the alternating current in a purely inductive circuit?

Accepted Answer

B) $90^\circ$

Answer

A) $0^\circ$

Answer

C) $180^\circ$

Answer

D) $45^\circ$

Question 3

What is the inductive reactance $X_L$ of an inductor with inductance $L = \frac{1}{\pi} ext{ H}$ operating at a frequency $f = 50 ext{ Hz}$?

Accepted Answer

B) $100\ \Omega$

Answer

A) $50\ \Omega$

Answer

C) $200\ \Omega$

Answer

D) $25\ \Omega$

Question 4

A capacitor of capacitance $C$ is connected across a steady Direct Current (DC) power source.

What is the capacitive reactance offered by the capacitor to DC?

Accepted Answer

B) Infinite

Answer

A) Zero

Answer

C) $100\ \Omega$

Answer

D) $1\ \Omega$

Question 5

What is the average power dissipated per complete cycle in an ideal purely capacitive AC circuit?

Accepted Answer

B) Zero

Answer

A) Maximum power

Answer

C) Half of maximum power

Answer

D) Double maximum power

Question 6

Calculate the resonant angular frequency $\omega_r$ of a series LCR circuit containing $L = 0.25 ext{ H}$ and $C = 4\ \mu ext{F}$.

Accepted Answer

A) 1000 rad/s

Answer

B) 500 rad/s

Answer

C) 2000 rad/s

Answer

D) 250 rad/s

Question 7

What is the power factor of a purely resistive AC circuit?

Accepted Answer

B) 1

Answer

A) 0

Answer

C) 0.5

Answer

D) $\sqrt{2}$

Question 8

At electrical resonance in a series LCR circuit, what is the value of the total impedance $Z$?

Accepted Answer

B) $Z = R$

Answer

A) $Z = \sqrt{R^2 + X_L^2}$

Answer

C) $Z = X_L - X_C$

Answer

D) $Z = 0$

Question 9

An AC power socket has a peak voltage rating of $220\sqrt{2} ext{ V}$.

What is the effective RMS voltage measured by an AC voltmeter?

Accepted Answer

A) 220 V

Answer

B) 110 V

Answer

C) 440 V

Answer

D) 311 V

Question 10

In a series LCR circuit, RMS voltage drops across individual elements are measured as: $V_R = 30 ext{ V}$, $V_L = 40 ext{ V}$, and $V_C = 40 ext{ V}$.

What is the total RMS voltage applied across the combination?

Accepted Answer

C) 30 V

Answer

A) 110 V

Answer

B) 50 V

Answer

D) 70 V

Question 11

A step-up transformer increases primary voltage from 100 V to 400 V. If the primary coil has 200 turns, how many turns are in the secondary coil?

Accepted Answer

B) 800

Answer

A) 400

Answer

C) 100

Answer

D) 1600

Question 12

What standard frequency of alternating current (AC) is commonly supplied for domestic utilities in India?

Accepted Answer

A) 50 Hz

Answer

B) 60 Hz

Answer

C) 100 Hz

Answer

D) 0 Hz

Question 13

Which formula correctly represents the Quality factor ($Q$-factor) of a series LCR resonant circuit?

Accepted Answer

A) $Q = \frac{1}{R}\sqrt{\frac{L}{C}}$

Answer

B) $Q = R\sqrt{\frac{L}{C}}$

Answer

C) $Q = \frac{1}{R}\sqrt{\frac{C}{L}}$

Answer

D) $Q = \frac{\omega_r R}{L}$

Question 14

Under what phase condition between alternating voltage and alternating current does current become 'wattless'?

Accepted Answer

C) Phase difference of $90^\circ$

Answer

A) Phase difference of $0^\circ$

Answer

B) Phase difference of $45^\circ$

Answer

D) Phase difference of $180^\circ$

Question 15

An instantaneous AC voltage equation is given by $V = 100 \sin(100\pi t)$ volts.

What is the frequency $f$ of this voltage supply?

Accepted Answer

A) 50 Hz

Answer

B) 100 Hz

Answer

C) 25 Hz

Answer

D) 60 Hz

Question 16

Why is the iron core of a transformer laminated into thin insulated sheets?

Accepted Answer

C) To minimize energy loss due to eddy currents

Answer

A) To eliminate hysteresis loss

Answer

B) To reduce copper loss in windings

Answer

D) To prevent flux leakage

Question 17

An AC circuit comprises a resistor $R = 6\ \Omega$ and a capacitor reactance $X_C = 8\ \Omega$ in series. What is the total impedance $Z$ of the circuit?

Accepted Answer

B) $10\ \Omega$

Answer

A) $14\ \Omega$

Answer

C) $2\ \Omega$

Answer

D) $48\ \Omega$

Question 18

What is the mean (average) value of a sinusoidal alternating current over one complete time period?

Accepted Answer

C) Zero

Answer

A) Equal to peak value $I_0$

Answer

B) Equal to RMS value $I_{	ext{rms}}$

Answer

D) $\frac{2I_0}{\pi}$

Question 19

An ideal step-down transformer has $N_p = 500$ primary turns and $N_s = 100$ secondary turns. The primary current measured is 2 A.

What is the current output from the secondary coil?

Accepted Answer

A) 10 A

Answer

B) 0.4 A

Answer

C) 4 A

Answer

D) 2.5 A

Question 20

What is the phase difference between current and voltage in a purely resistive AC circuit?

Accepted Answer

A) 0 rad

Answer

B) $\frac{\pi}{2}	ext{ rad}$

Answer

C) $\pi	ext{ rad}$

Answer

D) $\frac{\pi}{4}	ext{ rad}$