## MCQ Alternating Current Circuit Class 12 Physics Chapter 7

**1. In an ac circuit, V and I are given by V = 100 sin (100t) Volt, mA. The power dissipated in the circuit is**

a) 104 watt

b) 10 watt

c)2.5 watt

d)5 watt

Ans: c)2.5 watt

P = Vrms Irms Cos θ

**2. 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**

a) e.m.f. is ahead of current by / 2

b) Current is ahead of e.m.f. by / 2

c) Current lags behind e.m.f. by

d) Current is ahead of e.m.f. by

Ans: b) Current is ahead of e.m.f. by / 2

**3. Voltage and current in an ac circuit are given by V = 5 sin (100πt – 300) and I = 4 sin (100πt + 300)**

a) Voltage leads the current by o 30

b) Current leads the voltage by o 30

c) Current leads the voltage by o 60

d) Voltage leads the current by o 60

Ans: c) Current leads the voltage by o 60

**4. The given figure shows the variation of V and I vs t for a circuit element connected to A.C mains.** **Name the circuit element**

a) Resistance

b) Capacitor

c) Inductor

d) Alternator

Ans: b) Capacitor

**5.In a series LCR Series circuit, the voltages across Inductor, capacitor and Resistances are 20V,20V,40V ****respectively. The phase difference between the supplied voltage and current in the circuit is**

a) 300

b) 600

c) 900

d) 00

Ans: d) 00

Since voltage across Inductor and capacitor are same, circuit is in resonance. Phase difference is 00

**6. If an 8 resistance and 6 reactance are present in an ac series circuit then the impedance of the ****circuit will be**

a) 20 ohm

b) 5 ohm

c) 10 ohm

d) 14 2 ohm

Ans: c) 10 ohm

**7. The graphs given below depict the dependence of two reactive impedances X1 and X2 on the frequency ****of the alternating e.m.f. applied individually to them. We can then say that**

a) X1 is an inductor and X2 is a capacitor

b) X1 is a resistor and X2 is a capacitor

c) X1 is a capacitor and X2 is an inductor

d) X1 is an inductor and X2 is a resistor

Ans: c) X1 is a capacitor and X2 is an inductor

For capacitor, Capacitative Reactance is inversely proportional to frequency and For Inductor, Inductive Reactance is directly proportional to frequency

8. In the circuit given below, what will be the reading of the voltmeter

a)200V

b) 100 V

c) 300V

d) 400V

Ans: a) 200V

V2 = VR

2 + (VL-Vc)2

**9. A capacitor has capacitance C and reactance X, If the capacitance and frequency become double, then ****reactance will be**

a) 4X

b) X/2

c) X/4

d) 2x

Ans: c) X/4

X α (1/fC)

**10. A capacitor acts as an infinite resistance for**

a) DC

b) AC

c) Both DC and as well as AC

d) Neither for AC nor DC

Ans: a)DC

11. High Voltage transmission line is preferred as

a) its electric appliances are less costly

b) Thin power cables are required

c) Idle current is low

d) Power losses is low

Ans: d) Power losses is low

**12. In a series LR circuit, XL = R and power factor of the circuit is P1 . When capacitor with capacitance C is ****such that is put in series, the power factor becomes P2 . The ratio of P1/P2 is**

a) 2:1

b) 1: √2

c) √2 : 1

d) 1: 2

Ans: b) 1: √2

### Read the assertion and reason carefully to mark the correct option out of the options given below:

a) If both assertion and reason are true, the reason is the correct explanation of the assertion

b) If both assertion and reason are true, the reason is the not correct explanation of the assertion

c) If assertion is true but reason is false

d) If the assertion and reason both are false

13. Assertion : An inductance and a resistance are connected in series with an ac circuit. In this

circuit the current and the potential difference across the resistance lag behind potential difference across

the inductance by an angle /2.

Reason : In LR circuit voltage leads the current by phase angle which depends on the value of

inductance and resistance both.

Ans: b

14. Assertion : A capacitor of suitable capacitance can be used in an ac circuit in place of the choke coil.

Reason : A capacitor blocks dc and allows ac only.

Ans: b

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

Reason : Capacitive reactance is inversely proportional to frequency.

Ans: a

16. Assertion : When capacitive reactance is smaller than the inductive reactance in LCR current, e.m.f.

leads the current .

Reason : The phase angle is the angle between the alternating e.m.f. and alternating current of the

circuit.

Ans: b

17. Assertion: Direct current is more dangerous than Alternating current of same value.

Reason: An electrocuted person sticks to direct current line. While alternating current repels the person

from the line.

Ans: d

18. Assertion: A transformer can’t work on DC supply

Reason — DC changes neither in magnitude nor in direction

Answer – A

**CASE STUDY EXPERIMENT:**

According to Faraday’s law of electromagnetic induction, there will be an EMF induced in the second winding. If the circuit of this secondary winding is closed, then a current will flow through it. This is the basic working principle of a transformer.

A transformer that increases voltage between the primary to secondary windings is defined as a step-up transformer. Conversely, a transformer that decreases voltage between the primary to secondary windings

is defined as a step-down transformer.

Ideal transformer is not very practical. This is because in the open air only a very tiny portion of the flux produced from the first coil will link with the second coil. So the current that flows through the closed circuit connected to the secondary winding will be extremely small (and difficult to measure).

The rate of change of flux linkage depends upon the amount of linked flux with the second winding. So ideally almost all of the flux of primary winding should link to the secondary winding. This is effectively and efficiently done by using a core type transformer. This provides a low reluctance path common to both of the windings.

**19. If the supply frequency of a transformer increases, the secondary output voltage of the transformer**

a) Increase

b) Decrease

c) Remains the same

d) any of the above

Ans.(c) Remain the same

**20. Which quantity is increased in step-down transformer ?**

a) resistance

b) power

c) current

D) charge

Ans: c

**21. The voltage in the secondary coil of a transformer does not depend upon**

a) Frequency of the source

b) Voltage in Primary

c) Ratio of no. of turns in the two coils

d) Both (b) and (c)

Ans: a

**22. The primary winding of transformer has 500 turns whereas its secondary has 5000 turns. The primary ****is connected to an ac supply of 20 V, 50 Hz. The secondary will have an output of**

a) 200 V, 50 Hz

b) 2 V, 50 Hz

c) 200 V, 500 Hz

d) 2 V, 5 Hz

Ans: (a)

**23. The magnitude of the emf induced across the secondary of a transformer does not depend on**

a) the magnitude of the emf applied across the primary

b) the number of turns in the primary

c) the number of turns in the secondary

d) the resistance of the primary and the secondary

Ans: d

**24. Why does stepping voltage reduce power loss?**

a) Since the resistance of the conductor decreases with increase of voltage

b) Since current decreases with increase of the voltage

c) Both of the above

d) None of the above

Ans.(b)

Hint: Power is directly proportional to the current.

**25. The physical quantity that remains unchanged in a transformer is**

a) Voltage

b) current

c) Frequency

d) None of these

2. Electrical power transmission involves the bulk movement of electrical energy from a generating site, such as a power station or power plant, where voltage is stepped up and distributed to electrical substation and from electrical substation where voltage is stepped down and distributed to consumers or other substations. The reason electrical power is stepped up to these voltage levels is to make it more efficient by reducing the I2R losses

that take place when power is transmitted.

When voltage is stepped up, the current reduces relative to the voltage so that power remains constant, thus reducing these I2R losses.

**26. Name the device which is used to transform voltage from 12kV to 400kV from power plant to substation.**

a) AC generator

b) Step up Transformer

c) Moving coil Galvanometer

d) Step down transformer

Ans: B

**27. Laminated cores are used in such devices to reduce energy loss due to:**

a) Eddy currents

b) Flux leakage

c) Resistance of the windings

d) Hysteresis loss

Ans: A

28. A small town with a power demand of 600 kW at 220 V is situated 20 km away from power plant generating power at 440 V. The resistance of the two wire line carrying power is 0.4 Ω per km. If the power is transmitted to the substation in the town at 12 kV which the substation steps down to 220 V before supplying to the town, Estimate the line power loss during transmission in the form of heat.

a) 400 kW

b) 40 kW

c) 600 kW

d) 60 kW

Ans: B

( I = P/V = 600000/12000 = 600/12 = 50 A,

Power loss = I2 R = 50 x 50 x R = 50 x 50 x 0.4 x 40 = 40000 W = 40 kW )

**29. High voltage transmission line is preferred as**

a) Its appliances are less costly

b) Thin power cables are required

c) Idle current is very low

d) Power loss is very less

Ans: d

**30. Which of the following assumptions are used for deriving the relation:**

a) The primary resistance and current are small;

b) The same flux links both the primary and the secondary as very little flux escapes from the core

c) The secondary current is small

d) All of the above

Ans: B

## MCQ’S IN ALTERNATING CURRENTS

**1. A capacitor acts as an infinite resistance for**

a) DC

b) AC

c) DC as well as AC

d) neither AC nor DC.

**2. In an AC circuit current is lagging from the applied voltage then circuit can have**

a) Resistance and inductor

b) Resistance and capacitor

c) Both (a) and (b)

d) None of the above

**3. The peak voltage in a 220 V, AC source is**

a) 220 V

b) about 160 V

c) about 310 V

d) 440 V.

**4. An AC source is rated 220 V, 50 Hz. The average voltage is calculated in a time interval of 001 s. It**

a) must be zero

b) may be zero

c) is never zero

d) is (220/2)V.

**5. The magnetic field energy in an inductor charges form maximum value to minimum value in 50 ms ****when connected to an AC source. The frequency of the source is**

a) 20 Hz

b) 50 Hz

c) 200 Hz

d) 500 Hz.

**8. Transformers are used**

a) in DC circuits only

b) in AC circuits only

c) in both DC and AC circuits

d) neither in DC nor in AC circuits

**9. An alternating current having peak value 14 A is used to heat a metal wire. To produce the same heating effect, a constant current i can be used where i is**

a) 14 A

b) about 20 A

c) 7 A

d) about 10 A

Q 10. A constant current of 28 A exists in a resistor. The rms current is

(a) 28 A (b) about 2 A (c) 14 A (d) undefined for a directed current.

CASE STUDY QUESTIONS

A series LCR circuit containing a resistance of 120 has an angular resonance frequency

4105 rad/ sec . At resonance, voltage across resistance and inductance are 60 V and 40 V.

11. Value of inductance is

(A) 100H (B) 200H

(C) 100H (D) 200H

12. The value of capacitance is

(A) 32F (B) 16F

(C) 1/ 32F (D) 1/16F

14. The primary coil of a transformer has 100 turns and is connected to a 120 V AC source. How many

turns are in the secondary coil if there’s a 2400 V across it?

(a) 5 (b) 50 (c) 200 (d) 2000

15. A transformer with 40 turns in its primary coil is connected to a 120 V AC source. If 20 W of power

is supplied to the primary coil, how much power is developed in the secondary coil?

(a) 10 W (b) 20W (c) 80W (d) 160W

17. A 12 V battery is used to supply 2.0 mA of current to the 300 turns in the primary coil of a given

transformer. What is the current in the secondary coil if N2 = 150 turns ?

(a) zero (b) 1.0 mA (c) 2.0 mA (d) 4.0 mA

SOURCE STUDY QUESTIONS

An alternating voltage (in volts) varies with time t (in seconds) as V = 200 sin (100 t) to a

series combination of resistance of 10 ohm and inductor of Inductance 5 mH . Then

18. The peak value of the voltage is

(A) 200 V (B) 282.8 V

(C) 141.45 V (D) 100 V

19. The rms value of the voltage is

(A) 200 V (B) 282.8 V

(C) 141.45 V (D) 100 V

20. Inductive reactance in the circuit is

(A) 500π mΩ (B) 500 mΩ

(C) 200π mΩ (D) 500π Ω

Statement answers type questions:

A. Both Assertion and Reason are correct and Reason is the correct

explanation for Assertion.

B. Both Assertion and Reason are correct and Reason is not the correct

explanation for Assertion.

C. Assertion is correct but Reason is incorrect.

D. Both Reason and Assertion are incorrect.

21. Assertion: If the frequency of alternating current in an ac circuit consisting of an

inductance coil is increased then current gets decreased

Reason: The current is inversely proportional to frequency of alternating current.

22. Assertion: An alternating current does not show any magnetic effect.

Reason: Alternating current does not vary with time.

23. Assertion: In a series LCR circuit, at resonance condition power

consumed by circuit is maximum.

Reason: At resonance condition effective resistance of circuit is

maximum.

24. Assertion: When resistance and inductor are connected in series with AC

source. Voltage lead the current in inductor.

Reason: In series R, L, C circuit when connected by A-C source voltage

can lead, or lag in phase with current depends on L and C only.