CBSE Board Exam 2020: Important MCQs (with Answers) for Class 12 Physics - Chapter 7 - Alternating Current; Also useful for JEE Main, UPSEE, WBJEE& Other Engineering Entrance Exams

Students preparing for CBSE 12th Physics Board Exam often ask about important MCQs (Multiple Choice Questions) and in this article, we are going to provide important questions, based on Chapter 7 (Alternating Current) of Class 12 Physics NCERT textbook. Here you will also get important links to access some important articles for the preparation of CBSE 12th board exams 2020.

CBSE Board Exam 2020: Important MCQs (with Answers) for Class 12 Physics, Chapter 7 - Alternating Current

1. An inductor and a resistor are connected in series in an AC circuit with voltage source having root mean square voltage of 6 V. The reactance of inductor is 1 Ω and resistance of resistor is 2 Ω. What is the average power dissipated in the circuit?

(a) 2.7 W

(b) 4.2 W

(c) 8 W

(d) 14.4 W

Sol: (d)

Given, inductive reactance = X_L= 1 Ω, resistance = R = 2 Ω and root mean square voltage = V_rms = 6 V.

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2. An AC voltage source of variable angular frequency ω and fixed amplitude V₀ is connected in series with a capacitance C and an electric bulb of resistance R (inductance zero). When ω is increased then,

(a) The bulb glows dimmer

(b) The bulb glows brighter

(c) Total impedance of the circuit is unchanged

(d) Total impedance of the circuit increases

Sol: (b)

3. The correct variation of capacitive reactance of a capacitor with frequency is represented by

Sol: (d)

4. Given below is the graph for the variation of current with time. Find the magnitude of virtual current.

(a) 1 ampere

(b) 3.5 ampere

(c) 5 ampere

(d) None of these

Sol: (c)

From the graph, the value of virtual current or root mean square current is

5. The correct formula to determine the Q factor of series resonance circuit is

Sol: (d)

6. The dimensions of Inductance/Resistance is same as the dimension of

(a) time

(b) capacitance

(c) capacitive reactance

(d) inductive reactane

Sol: (a)

7. A bulb connected in series with a solenoid is lit by an AC source. If a soft iron core is introduced in the solenoid then,

(a) the bulb will glow brighter

(b) the bulb will glow dimmer

(c) there will be no effect on the light produced by the bulb

(d) bulb may glow more brighter or dimmer

Sol: (b)

The bulb will glow dimmer because when a soft iron core is introduced in the solenoid then, its inductive reactance (X_L = ωL) will increase, as a result of which the current through the bulb decreases.

8. The correct variation of inductive reactance (X_L) of an inductor with frequency (v) is represented by

Sol: (a)

9. A 60 microfarad capacitor is connected to a 110 V, 60 Hz AC supply. What is the value of root mean square value of current in the circuit?

(a) 1.49 ampere

(b) 2.49 ampere

(c) 3.62 ampere

(d) 4.12 ampere

Sol: (b)

Given, capacitance = C = 60 μF = 60 × 10^-6 F, root mean square voltage = E_v = 110 V, frequency = v = 60 Hz, root mean square current = I_v = ?

10. A transformer has an efficiency of 80 % and works at 100 volts and 4 kW. If the secondary voltage is 240 volts, then the values of current in primary and secondary coil are

(a) 40 ampere and 13.3 ampere respectively

(b) 13.3 ampere and 40 ampere respectively

(c) 8 ampere and 16 ampere respectively

(d) 16 ampere and 8 ampere respectively

Sol: (b)

Let, current in primary = I_p and current in secondary = I_s.