Physics MCQs for Class 12 with Answers Chapter 7 Alternating Current

CBSE Class 12 Physics MCQs Solution All Chapters

Free PDF of CBSE Physics Multiple Choice Questions for Class 12 with Answers Chapter 7 Alternating Current. Physics MCQs for Class 12 Chapter Wise with Answers PDF was Prepared Based on Latest Exam Pattern. Students can solve NCERT Class 12 Physics Alternating Current MCQs Pdf with Answers to know their preparation level.

Alternating Current MCQ Chapter 7

Below are some of the very important NCERT Alternating Current MCQ Class 12 Physics Chapter 7 with answers. These Alternating Current MCQ have been prepared by expert teachers and subject experts based on the latest syllabus and pattern of CBSE Term 1 examination.

We have given these Alternating Current MCQ Class 12 Physics questions with answers to help students understand the concept.

MCQ Questions for Class 12 Physics are very important for the latest CBSE Term 1 and Term 2 pattern. These MCQs are very important for students who want to score high in CBSE Board, NEET and JEE exam.

MCQ (1-50)

1. Alternating current cannot be measured by DC ammeter because

  1. AC is virtual
  2. AC changes its direction
  3. AC cannot pass through DC ammeter
  4. average value of complete cycle is zero
Answer/Explanation

4

2. An alternating current of equivalent value of Io/√2 is

  1. RMS current
  2. DC current
  3. current
  4. all of these
Answer/Explanation

1

3. In an AC circuit I = 100sin200πt. The time required for current to achieve its peak value will be

  1. 1/200 second
  2. 1/400 second
  3. 1/100 second
  4. 1/300 second
Answer/Explanation

2

4. The ratio of mean value over half cycle to RMS value of AC is

  1. √2:1
  2. 2:π
  3. 2√2:π
  4. √2:π
Answer/Explanation

3

5. The peak value of an alternating EMF E given by E = Eocosωt is 10V and its frequency is 50 Hz. At that time t=1/600 s, the instantaneous EMF is

  1. 5√3 V
  2. 5 V
  3. 10 V
  4. 1 V
Answer/Explanation

1

6. The frequency of an alternating voltage is 50 cps and its amplitude is 120 V. Then the RMS value of voltage is

  1. 56.5 V
  2. 70.7 V
  3. 101.3 V
  4. 84.8 V
Answer/Explanation

4

7. An electric heater of 40 ohm is connected to a 200V, 50 Hz main supply. The peak value of electric current flowing in the circuit is approximately

  1. 10 A
  2. 5 A
  3. 7 A
  4. 2.5 A
Answer/Explanation

3

8. In the case of an inductor

  1. voltage leads the current by π/4
  2. voltage leads the current by π/3
  3. voltage leads the current by π/2
  4. voltage lacks the current by π/2
Answer/Explanation

3

9. A resistance of 20 ohm is connected to a source of an alternating potential V = 220sin(100πt). The time taken by the current to change from its peak value to RMS value is

  1. 2.5 x 10-3 s
  2. 25 x 10-3 s
  3. 0.25 s
  4. 0.2 s
Answer/Explanation

1

10. The RMS value of an AC of 50 Hz is 10A. The time taken by the alternating current in reaching from zero to maximum value and the peak value of current will be

  1. 1 x 10-2 s and 7.07 A
  2. 2 x 10-2 s and 14.14 A
  3. 5 x 10-2 s and 14.14 A
  4. 5 x 10-3 s and 7.07 A
Answer/Explanation

3

11. Determine The RMS value of the EMF given by

E (in V) = 8 sin(ωt) + 6 sin(2ωt)

  1. 10√2 V
  2. 10 V
  3. 5√2 V
  4. 7√2 V
Answer/Explanation

3

12. An alternating current of frequency f is flowing in a circuit containing a resistor of resistance R and a choke of inductance L in series. The impedance of the circuit is

  1. R + 2πfπL
  2. √(R2 + L2)
  3. √(R2 + 2fπL)
  4. √(R2 + 4π2f2L2)
Answer/Explanation

4

13. A generator produces a voltage that is given by V = 240 sin 120t V, where t is in seconds. The frequency and RMS voltage and nearly

  1. 19 Hz and 120 V
  2. 19 Hz and 170 V
  3. 60 Hz and 240 V
  4. 754 Hz and 170 V
Answer/Explanation

2

14. The instantaneous voltage through a device of impedance 20 ohm is e = 80sin100πt. The effective value of the current is

  1. 1.732 A
  2. 2.828 A
  3. 3 A
  4. 4 A
Answer/Explanation

2

15. A 15μF capacitor is connected to 220V, 50Hz source. Find the capacitive reactance and the RMS current

  1. 212.1 Ω; 1.037 A
  2. 212.1 Ω; 2.037 A
  3. 412.1 Ω; 1.037 A
  4. 412.1 Ω; 2.037 A
Answer/Explanation

1

16. In an AC circuit an alternating voltage V = 200√2 sin100t is connected to a capacitor of capacity 1μF. The RMS value of the current in the circuit is

  1. 10mA
  2. 20mA
  3. 100mA
  4. 200mA
Answer/Explanation

2

17. In an LR circuit, the value of L is (0.4/π) and the value of R is 30Ω. If in the circuit, an alternating EMF of 200V at 50 cps is connected, the impedance of the circuit and current will be

  1. 50 Ω, 4 A
  2. 40.4 Ω, 5 A
  3. 30.7 Ω, 6.5 A
  4. 11.4 Ω, 17.5 A
Answer/Explanation

1

18. In an AC circuit the voltage applied is E = Eosinωt. The resulting current in the circuit is I = Iosin(ωt – π/2). The power consumption in the circuit is given by

  1. P = EoIo / 2
  2. P = EoIo / √2
  3. P = √EoIo
  4. P = 0
Answer/Explanation

4

19. In an LCR circuit AC circuit, the voltage across each of the components L, C and R is 50V. The voltage across the LC combination will be

  1. 0 V
  2. 50 V
  3. 50√2 V
  4. 100 V
Answer/Explanation

1

20. Find the capacitive reactance of a 10μF capacitor, when it is part of a circuit whose frequency is 100 Hertz.

  1. 159.2 Ω
  2. 412.1 Ω
  3. 612.1 Ω
  4. 812.1 Ω
Answer/Explanation

1

21. The resonant frequency of a circuit is f. If the capacitance is made 4 times the initial values, than the resonant frequency will become

  1. f/2
  2. f
  3. 2f
  4. f/4
Answer/Explanation

1

22. A coil of 10Ω and 10mH is connected in parallel to a capacitor of 0.1μF. The impedance of the circuit at resonance is

  1. 103Ω
  2. 106Ω
  3. 102Ω
  4. 104Ω
Answer/Explanation

4

23. Which of the following curves correctly represent the variation of capacitive reactance (Xc) with frequency (f)?

  1. (a)
  2. (b)
  3. (c)
  4. (d)
Answer/Explanation

2

24. How does the current in an RC circuit vary when the charge on the capacitor builds up?

  1. it decreases linearly
  2. it increases linearly
  3. it decreases exponentially
  4. it increases exponentially
Answer/Explanation

3

25. The impedance in a circuit containing a resistance of 1Ω and an inductance of 0.1 H in series for AC of 50 Hz is

  1. √10 Ω
  2. 10√10 Ω
  3. 100 Ω
  4. 100√10 Ω
Answer/Explanation

2

26. An AC circuit contains a resistance R, capacitance C and inductance L in series with the source of EMF e=eosin(ωt+f). The current through the circuit is maximum when

  1. ω2 = LC
  2. ωL = 1/ωC
  3. R = L = C
  4. ω = LCR
Answer/Explanation

2

27. A charged 30μF capacitor is connected to a 27 mH inductor. The angular frequency of free oscillations of the circuit is

  1. 1.1 x 103 rad s-1
  2. 2.1 x 103 rad s-1
  3. 3.1 x 103 rad s-1
  4. 4.1 x 103 rad s-1
Answer/Explanation

1

28. The frequency of the output signal becomes ________ times by doubling the value of the capacitance in the LC oscillator circuit.

  1. ½
  2. 2
  3. √2
  4. 1/√2
Answer/Explanation

4

29. In an LCR circuit, the sharpness of resonance depends on

  1. resistance
  2. capacitance
  3. inductance
  4. all of these
Answer/Explanation

4

30. The average power dissipation in a pure capacitor in AC circuit is

  1. CV2
  2. 2CV2
  3. CV2/2
  4. zero
Answer/Explanation

4

31. In a series resonant circuit, having L, C and R as its elements, the resonant current is ‘i’. The power dissipated in circuit at resonance is

  1. Zero
  2. i2R
  3. i2ωL
  4. i2R/(ωL-(1/ωC))
Answer/Explanation

2

32. An AC supply gives 30 VRMS which passes through 10Ω. The power dissipated in it is

  1. 45√2 W
  2. 90√2 W
  3. 45 W
  4. 90 W
Answer/Explanation

4

33. In a series LCR circuit alternating EMF(e) and current(i) are given by equation v=vosin(ωt), i=iosin(wt+π/3). The average power dissipated in the circuit over a cycle of AC is

  1. Zero
  2. voio/2
  3. voio/4
  4. (√3/2)voio
Answer/Explanation

3

34. In an AC circuit, the current flowing in inductance is I = 5sin(100t-π/2)A and the potential difference V = 200sin(100t)V. The power consumption is equal to

  1. Zero
  2. 20 W
  3. 40 W
  4. 1000 W
Answer/Explanation

1

35. The power factor in an AC series LR circuit is

  1. L/R
  2. √(R2 + L2ω2)
  3. R√(R2 + L2ω2)
  4. R/(√R2 + L2ω2)
Answer/Explanation

4

36. A transformer is employed in

  1. Convert DC into AC
  2. Convert AC into DC
  3. Obtain a suitable DC voltage
  4. Obtain a suitable AC voltage
Answer/Explanation

4

37. The loss of energy in the form of heat in the iron core of a transformer is

  1. Copper loss
  2. Iron loss
  3. Mechanical loss
  4. None of these
Answer/Explanation

2

38. The core of any transformer is laminated so as to

  1. Make it light weight
  2. Make it robust and strong
  3. Increase the secondary voltage
  4. Reduce the energy loss due to eddy currents
Answer/Explanation

4

39. A step up transformer has a transformation ratio 5:3. What is the voltage in secondary if voltage in primary is 60 V?

  1. 60 V
  2. 180 V
  3. 20 V
  4. 100 V
Answer/Explanation

4

40. A transformer has 50 turns in the primary and 100 in the secondary. If the primary is connected to a 220 V DC supply, what will be the voltage across the secondary?

  1. 19 V
  2. 30 V
  3. 62 V
  4. 0 V
Answer/Explanation

4

41. The primary of a transformer has 400 turns while the secondary has 2000 turns. If the power output from the secondary at 1000 V is 12kW, what is the primary voltage?

  1. 200 V
  2. 400 V
  3. 300 V
  4. 500 V
Answer/Explanation

1

42. A step down transformer is used on a 1000V line to deliver 20A at 120V at the secondary coil. If the efficiency of the transformer is 80% the current drawn from the line is

  1. 0.3 A
  2. 3 A
  3. 30 A
  4. 24 A
Answer/Explanation

2

43. If the RMS current in a 50 Hz AC circuit is 5 A, the value of the current 1/300 s after its value becomes zero is

  1. 5√2 A
  2. 5√(3/2) A
  3. ⅚ A
  4. 5√2 A
Answer/Explanation

2

44. An alternating current generator has an internal resistance Rg and an internal reactance Xg. It is used to supply power to a passive load consisting of a resistance Rg and a reactance XL. For maximum power to be delivered from the generator to the laid, the value of XL is equal to

  1. Zero
  2. Xg
  3. -Xg
  4. Rg
Answer/Explanation

3

45. When a voltage measuring device is connected to AC mains, the meter shows the steady input voltage of 220 V. This means

  1. Input voltage cannot be AC voltage, but a DC voltage
  2. Maximum input voltage is 220 V
  3. The meter reads not v but <v2> and is calibrated to read √<v2>
  4. The pointer of the meter is stuck by some mechanical defect
Answer/Explanation

3

46. To reduce the resonant frequency in an LCR series circuit with a generator

  1. The generator frequency should be reduced
  2. Another capacitor should be added in parallel to the first
  3. The iron core of the inductor should be removed
  4. Dielectric in the capacitor should be removed
Answer/Explanation

2

47. Which of the following combinations should be selected for better tuning of an LCR circuit used for communication?

  1. R = 20Ω, L = 1.5 H, C = 35 μF
  2. R = 25Ω, L = 2.5 H, C = 45 μF
  3. R = 15Ω, L = 3.5 H, C = 30 μF
  4. R = 25Ω, L = 1.5 H, C = 45 μF
Answer/Explanation

3

48. An inductor of reactance 1Ω and a resistor of 2Ω are connected in series to the terminals of a 6V(rms) AC source. The power dissipated in the circuit is

  1. 8 W
  2. 12 W
  3. 14.4 W
  4. 18 W
Answer/Explanation

3

49. The selectivity of a series LCR AC circuit is large when

  1. L is large, R is large
  2. L is small, T is small
  3. L is large , R is small
  4. L = R
Answer/Explanation

3

50. The phase difference between the current and the voltage in series LCR circuit at resonance is

  1. π
  2. π/2
  3. π/3
  4. zero
Answer/Explanation

4


Assertion-Reasoning Based MCQ

Code

  1. Both assertion and reason are true and reason is the correct explanation of assertion.
  2. Both assertion and reason are true but reason is not the correct explanation of assertion.
  3. Assertion is true but reason is false.
  4. Assertion is false but reason is true.

1. Assertion AC is more dangerous in use than DC

Reason It is because the peak value of AC is greater than indicated value

Answer/Explanation

1. (1)

AC is more dangerous in use than DC. It is because the peak value of AC is greater than the indicated value.

2. Assertion Average value of AC over a complete cycle is always zero

Reason average value of AC is always defined over half cycle

Answer/Explanation

2. (2)

The mean or average value of alternating current or EMF during half cycle is given by

Im = 0.636 Io
Em = 0.6363 Eo

During the next half cycle, the mean value of AC will be equal in magnitude but opposite in direction. For this reason the average value of AC over a complete cycle is always zero. So the average value is always defined over a half cycle of AC.

3. Assertion The alternating current lags behind the EMF by a phase angle of when AC flows through and inductor

Reason The inductive reactance increases as the frequency of AC source decreases

Answer/Explanation

3. (3)

When AC flows through an inductor current lags behind the EMF, by phase of π/2 inductive reactance

XL = ωL = 2πfL

So, when frequency increases correspondingly inductive reactance also increases.

4. Assertion Capacitor serves as a block for DC and offers an easy path to AC

Reason Capacitive reactance is inversely proportional to frequency

Answer/Explanation

4. (1)

The capacitive reactance of capacitor is given by

XC = 1/ ωC = 1/2πfC

So this is infinite for DC and has a very small value for AC. Hence, a capacitor blocks DC.

5. Assertion In series LCR resonant circuit the impedance is equal to the ohmic resistance

Reason At resonance the inductive reactance exceeds the capacitive reactance

Answer/Explanation

5. (3)

In series resonance circuit inductive reactance is equal to capacitive reactance.

ωL = 1/ωC

alternating current mcq
Z = R

6. Assertion An alternating current shows magnetic effect

Reason Alternating current varies with time

Answer/Explanation

6. (2)

Like direct current, an AC also produces magnetic field. But the magnitude and direction of the field goes on changing continuously with time.

7. Assertion In series LCR circuit resonance can take place

Reason Resonance takes place in inductance and capacitive reactance are equal and opposite

Answer/Explanation

7. (1)

At resonant frequency,

XL = XC, Z = R (minimum)

8. Assertion Power factor correction is must in heavy machinery

Reason A low power factor implies larger power loss in transmission

Answer/Explanation

8. (2)

A heavy machinery requires a large power.

The average power is given by,

Pav = ErmsIrmscosΦ

The required power can be supplied to the heavy machinery either by supplying larger current or by improving power factor. The first method is costly. Hence, the second one is used.

9. Assertion Choke coil is preferred over a registered to adjust current in an AC circuit

Reason Power factor for inductance is zero

Answer/Explanation

9. (1)

We can use a capacitor of suitable capacitance as a choke coil, because average power consumed per cycle in an ideal capacitor is zero. Therefore, like a choke coil a condenser can reduce AC without power dissipation.

10. Assertion When AC circuit containing resistor only its power is minimum

Reason Power of a circuit is independent of phase angle

Answer/Explanation

10. (4)

The power of an AC circuit is given by,

P = EIcosΦ

Where cosΦ is a power factor and is Φ phase angle. In case of circuit containing resistance only, phase angle is zero and power factor is equal to 1. Therefore power is maximum in case of circuit containing resistor only.

11. Assertion A transformer cannot work on DC supply

Reason DC change is neither in magnitude nor in direction

Answer/Explanation

11. (1)

Transformer works on AC only AC changes in magnitude as well as in direction and induced EMF.

12. Assertion A laminated core is used in transformer to increase eddy currents

Reason The efficiency of a transformer increases with increase in eddy currents

Answer/Explanation

12. (4)

Large eddy currents are produced in non laminated iron core of the transformer by induced EMF, as the resistance of bulk iron core is very small. By using thin iron sheets are score the resistance is increased. Laminating the core substantially reduces the eddy currents. Eddy currents heat up the core of the transformer. More the eddy current greater the loss of energy and efficiency goes down.

13. Assertion Soft iron is used as a core of transformer

Reason Area of hysteresis loop for soft iron is small

Answer/Explanation

13. (1)

Hysteresis loss in the core of transformer is directly proportional to the hysteresis loop area of the core material. Since soft iron has narrow hysteresis loop area, that is why soft iron core is used in transformer.

14. Assertion An AC generator is based on the phenomenon of electromagnetic induction

Reason In single coil we consider self induction only

Answer/Explanation

14. (2)

According to electromagnetic induction, whenever the magnetic flux changes and EMF will be induced in the coil.

 


Case-Study Based MCQ

1. The figure shows a series LCR circuit.

For such a citcuit, the impedance Z is given by Z = √R2 + (XL – XC)2 where XL and XC are inductive and capacitive resistances respectively. As the frequency of AC is increased, at a particular frequency, XL becomes equal to XC. For that frequency, maximum current occurs. This is because the impedcane becomes equal to the least value which is R.

Current through the circuit is I = V/R. This circuit behaves like a pure resistive circuit and current and voltage will be in phase. This is called resonance. Frequency of AC at which resonance occurs is called resonant frequency. If frequency is less than the resonant frequency, then the capacitive reactance will be more. The circuit will be capacitive in nature.

If frequency is more than the resonant frequency, inductive reactance will be more. Circuit is inductive in nature and the current lags behind the voltage by a phase of π/2.

An LCR circuit which has a resistance 50 ohm has a resonant angular frequency 2 x 103 rad/s. At resonance, the voltage across the resistance and inductance are 25 V and 20 V respectively.

(i) The value of inductance is

(a) 20mH
(b) 10mH
(c) 40mH
(d) 25mH

Answer/Explanation

1. (i) (a) XL = VL/I

I = VR/R = 25/50 = 1/2

XL = 20/(1/2) = 40 ohm

XL = ωL ; L = 40 / (2 x 103) = 20 x 10-3 = 20mH

(ii) The value of capacitive reactance is

(a) 25μF
(b) 1μF
(c) 2μF
(d) 12.5 μF

Answer/Explanation

(ii) (d) ω2 = 1/LC ; C = 1/( ω2L) = 1 / ((2 x 103)2 x 20 x 10-3) = 12.5 μF

(iii) The impedance at resonance is

(a) 50 ohm
(b) 16 ohm
(c) 64 ohm
(d) 25 ohm

Answer/Explanation

(iii) (a) At resonance, the impedance equal just resistance.

(iv) Which of the following angular frequency of AC will see the circuit as inductive in nature?

(a) 1.5 x 103 rad/s
(b) 103 rad/s
(c) 2 x 103 rad/s
(d) 5 x 103 rad/s

Answer/Explanation

(iv) (d) For inductive nature, ω > ωr

(v) At angular frequency 103 rad/s, the nature of circuit is

(a) inductive
(b) capacitive
(c) resisitive
(d) none of these

Answer/Explanation

(v) (b) If ω < ωr, the circuit will be capacitive in nature.

2. A series LCR circuit consist of series combination of a resistance, an inductor and a capacitance. A similar series LCR circuit is shown in figure. The given series LCR circuit is connected across a 200 V 60 Hz line consisting of capacitive reactance 30 ohm a non-inductive resistor of 44 ohm and a coil of inductive reactance 90 ohm and resistance 36 ohm.

(i) Calculate the total impedance of the circuit.

(a) 1000 ohm
(b) 100 ohm
(c) 3600 ohm
(d) 4900 ohm

Answer/Explanation

2. (i) (b) Z = √((R1 + R2)2 + (XL – XC)2) = √((44 + 36)2 + (90 – 30)2) = 100 ohm

(ii) Calculate the current flowing in the circuit.

(a) 1 A
(b) 5 A
(c) 2 A
(d) 10 A

Answer/Explanation

(ii) (c) Current, I = V/Z = 200/100 = 2A

(iii) What is the impedance of the coil?

(a) 97 ohm
(b) 87 ohm
(c) 100 ohm
(d) 110 ohm

Answer/Explanation

(iii) (a) Impedance of the coil, ZL = √(R22 + XL2) = √((36)2 + (90)2) = 97 ohm

(iv) what is the potential difference across the coil?

(a) 194 V
(b) 186 V
(c) 180 V
(d) 190 V

Answer/Explanation

(iv) (a) Potential difference across the coil, VL = IZL = 2 x 97 = 194 V

(v) Calculate the power dissipated in the coil.

(a) 100 W
(b) 122 W
(c) 130 W
(d) 144 W

Answer/Explanation

(v) (d) Power dissipated in the inductive coil, P = I2R2 = (2)2 x 36 = 144 W

Alternating Current Class 12 Physics MCQs

1. The phase difference between the alternating current and emf is π/2. Which of the following cannot be the constituent of the circuit?
(a) C alone
(b) L alone
(c) L and C
(d) R and L

Answer/Explanation

Answer: c
Explaination:
(c) When both L and C components are there then phase difference could be zero, less than or greater than \(\frac{\pi}{2}\).


2. In an LCR-series ac circuit, the voltage across each of the component L, C and R is 50 V. The voltage across the LC-combination will be
(a) 50 V
(b) 50√2 V
(c) 100 V
(d) zero

Answer/Explanation

Answer: d
Explaination:
(d) The voltage across L and C are out of phase. Hence the voltage across the LC combination is zero.


Physics MCQs with Answers for Class 12 Question 3. An ac circuit has a resistance of 12 ohm and an impedance of 15 ohm. The power factor of the circuit will be
(a) 0.8
(b) 0.4
(c) 0.125
(d) 1.25

Answer/Explanation

Answer: a
Explaination:
(a) Power factor, cos Φ = \(\frac{R}{Z}\) = \(\frac{12}{15}\) = 0.8


4. In an ac circuit the voltage applied is ε = ε 0 sin wt. The resulting current in the circuit is I = I0 sin (wt – π/2). The power consumption in the circuit is given by
Physics MCQ Questions for Class 12 Pdf Download

Answer/Explanation

Answer: c
Explaination:
(c) Here, Φ = \(\frac{\pi}{2}\),
SO pav = εrms Irms COS Φ = 0


5. In an LCR circuit, capacitance is charged from C to 2C. For resonant frequency to remain unchanged, the inductance should be changed from L to
(a) 4 L
(b) 2 L
(c) L/2
(d) L/4

Answer/Explanation

Answer: c
Explaination:
(c) Since, Resonance frequency,
\(v_{r}=\frac{1}{2 \pi \sqrt{L C}}\)
when C is changed 2C, L should be change to L/2.


6. The core of any transformer is laminated so as to
(a) reduce the energy loss due to eddy currents.
(b) make it light weight.
(c) make it robust and strong.
(d) increase the secondary voltage.

Answer

Answer: a


7. In an a.c. generator, a coil with N turns, all of the same area A and total resistance R, rotates with frequency ω in a magnetic field B the maximum value of emf generated in the coil is
(a) NABR
(b) NABω
(c) NABRω
(d) NAB

Answer/Explanation

Answer: b
Explaination:
(b) emf induced, ε = NABGω sin ωt,
ε0 = NBAω


8. If coil is open, then L and R becomes
(a) infinity, zero
(b) zero, infinity
(c) infinity, infinity.
(d) zero, zero

Answer/Explanation

Answer: b
Explaination:
(b) No current flows through an open circuit so magnetic flux is zero and hence, L = 0.
Also as I = 0, so R = ∞.


Question 9. In a series, LCR-circuit, resonant frequency depends on
Physics MCQs for Class 12 with Answers Chapter 7 Alternating Current 9

Answer/Explanation

Answer: c
Explaination: (c) Resonance frequency, vr = \(\frac{1}{\sqrt{LC}}\)


10. Which quantity is increased in a step-down transformer?
(a) Current
(b) Voltage
(c) Power
(d) Frequency

Answer/Explanation

Answer: a
Explaination: (a) In a step-down transformer current increases and voltage decreases.


11. If an AC voltage is applied to an ICi? circuit, which of the following is true?
(a) I and V are out of phase with each other inR.
(b) I and V are in phase in L while in C, they are out of phase.
(c) I and V are out of phase in both C and L.
(d) I and V are out of phase in L and in phase in C.

Answer

Answer: c


12. The peak value of ac voltage on a 220 V mains is
(a) 200√2 V
(b) 230√2 V
(c) 220√2 V
(d) 240√2 V

Answer/Explanation

Answer: c
Explaination:
(c) Peak value of ac voltage V0 = √2 Vrms.
= 220√2 V.


13. Series ac circuit has inductance L, resistance R and angular frequency to, the quality factor Q is
Class 12 Physics MCQ Pdf

Answer/Explanation

Answer: b
Explaination:
(b) Quality factor is measure of sharpness of resonance, is equal to \(\frac{\omega L}{R}\).


14. A transformer is used to light a 100 W and 110 V lamp from a 220 V mains. If the main current is 0.5 A, the efficiency of the transformer is approximately
(a) 30%
(b) 50%
(c) 90%
(d) 10%

Answer/Explanation

Answer: c
Explaination:
(c) Since efficiency of a transformer,
Physics MCQs for Class 12 Chapter Wise with Answers Pdf Download


15. Choose the correct statement.
(a) A capacitor can conduct a dc circuit but not an inductor.
(b) In a dc circuit the inductor can conduct but not a capacitor.
(c) In dc circuit both the inductor and capacitor cannot conduct.
(d) The inductor has infinite resistance in a dc circuit.

Answer/Explanation

Answer: b
Explaination:
(b) In dc circuit, an inductor offers zero resistance to dc, while a capacitor offers infinite resistance.


16. What is the value of inductance L for which the current is maximum in a series LCR- circuit with C = 10 μF and ω = 1000 s-1?
(a) 100 mH
(b) 1 mH
(c) 10 mH
(d) cannot be calculated unless R is known

Answer/Explanation

Answer: a
Explaination:
(a) Current is maximum at resonance, when
Alternating Current Class 12 MCQ


17. A coil of self-inductance L is connected in series with a bulb B and an ac source. Brightness of the bulb decreases when

(a) frequency of the ac source is decreased.
(b) number of turns in the coil is reduced.
(c) a capacitance of reactance Xc = XL in included.
(d) an iron rod is inserted in the coil.

Answer/Explanation

Answer: d
Explaination:
(d) Brightness of the bulb decreases when inductive reactance increases, i.e. when iron rod is inserted in the coil, its inductance L increases, hence current decreases.


18. The reactance of a capacitor C is X. If both the frequency and capacitance be doubled, then new reactance will be
(a) X
(b) 2X
(c) 4X
(d) \(\frac{X}{4}\)

Answer/Explanation

Answer:
Explaination:
Physics MCQs for Class 12 with Answers Chapter 7 Alternating Current 15
when f and C both are doubled.


19. A transformer works on the principle of
(a) converter.
(b) inverter.
(c) mutual inductance.
(d) self-inductance.

Answer/Explanation

Answer: c
Explaination: (c) Transformer works on the principle of mutual inductance.


20. The line that draws power supply to your house from street has
(a) zero average current.
(b) 220 V average voltage.
(c) voltage and current out of phase by 90°.
(d) voltage and current are in phase.

Answer/Explanation

Answer: a
Explaination: (a) zero average current.


21. Alternating current cannot be measured by dc ammeter, because
(a) ac cannot pass through ac ammeter.
(b) ac charges direction.
(c) average value of current of complete cycle is zero.
(d) ac ammeter will get damaged.

Answer/Explanation

Answer: a
Explaination: (a) ac cannot pass through ac ammeter.


22. Average power generated in an inductor connected to an a.c. source is
(a) \(\frac{1}{2}\)LI²
(b) LI²
(c) zero
(d) none of these

Answer/Explanation

Answer: c
Explaination:
(c) Average power dissipated in an inductor connected to an ac source is zero,
Pav = V0I0 cos = 0(Phase angle is \(\frac{\pi}{2}\))


23. The power factor varies between
(a) 2 and 2.5
(b) 3.5 to 5
(c) 0 to 1
(d) 1 to 2

Answer/Explanation

Answer: c
Explaination: (c) Power factor cos Φ in an ac circuit varies between 0 and 1.


24. Reciprocal of impedance is
(a) susceptance
(b) conductance
(c) admittance
(d) transconductance

Answer/Explanation

Answer: c
Explaination: (c) admittanc


25. Alternating voltage (V) is represented by the equation
(a) V(t) = Vm eωt
(b) V(t) = Vm sin ωt
(c) V(t) = Vm cot ωt
(d) V(t) = Vm tan ωt

Answer

Answer: b


26. The rms value of potential difference V shown in the figure is
Physics MCQs for Class 12 with Answers Chapter 7 Alternating Current 1

Answer

Answer: c


27. The phase relationship between current and voltage in a pure resistive circuit is best represented by
Physics MCQs for Class 12 with Answers Chapter 7 Alternating Current 2
Physics MCQs for Class 12 with Answers Chapter 7 Alternating Current 3

Answer

Answer: b


28. In the case of an inductor
(a) voltage lags the current by \(\frac{π}{2}\)
(b) voltage leads the current by \(\frac{π}{2}\)
(c) voltage leads the current by \(\frac{π}{3}\)
(d) voltage leads the current by \(\frac{π}{4}\)

Answer

Answer: b


29. Which of the following graphs represents the correct variation of inductive reactance XL with frequency u?
Physics MCQs for Class 12 with Answers Chapter 7 Alternating Current 4

Answer

Answer: b


30. In a pure capacitive circuit if the frequency of ac source is doubled, then its capacitive reactance will be
(a) remains same
(b) doubled
(c) halved
(d) zero

Answer

Answer: c


31. Which of the following graphs represents the correct variation of capacitive reactance Xc with frequency v u?
Physics MCQs for Class 12 with Answers Chapter 7 Alternating Current 5

Answer

Answer: c


32. In an alternating current circuit consisting of elements in series, the current increases on increasing the frequency of supply. Which of the following elements are likely to constitute the circuit?
(a) Only resistor
(b) Resistor and inductor
(c) Resistor and capacitor
(d) Only inductor

Answer

Answer: c


33. In which of the following circuits the maximum power dissipation is observed?
(a) Pure capacitive circuit
(b) Pure inductive circuit
(c) Pure resistive circuit
(d) None of these

Answer

Answer: c


34. In series LCR circuit, the phase angle between supply voltage and current is
Physics MCQs for Class 12 with Answers Chapter 7 Alternating Current 6

Answer

Answer: a


35. In a series LCR circuit the voltage across an inductor, capacitor and resistor are 20 V, 20 V and 40 V respectively. The phase difference between the applied voltage and the current in the circuit is
(a) 30°
(b) 45°
(c) 60°
(d) 0°

Answer

Answer: d


36. At resonance frequency the impedance in series LCR circuit is
(a) maximum
(b) minimum
(c) zero
(d) infinity

Answer

Answer: b


37. At resonant frequency the current amplitude in series LCR circuit is
(a) maximum
(b) minimum
(c) zero
(d) infinity

Answer

Answer: a


38. Quality factor and power factor both have the dimensions of
(a) time
(b) frequency
(c) work
(d) angle

Answer

Answer: d


39. The natural frequency (ca0) of oscillations in LC circuit is given by
Physics MCQs for Class 12 with Answers Chapter 7 Alternating Current 7

Answer

Answer: c


40. A transformer works on the principle of
(a) self induction
(b) electrical inertia
(c) mutual induction
(d) magnetic effect of the electrical current

Answer

Answer: c


41. For an ideal-step-down transformer, the quantity which is constant for both the coils is
(a) current in the coils
(b) voltage across the coils
(c) resistance of coils
(d) power in the coils

Answer

Answer: d


42. If the rms current in a 50 Hz ac circuit is 5 A, the value of the current 1/300 seconds after its value becomes zero is [NCERT Exemplar]
(a) 5√2A
(b) \(5 \sqrt{3 / 2}\)A
(c) 5/6 A
(d) 5√2A

Answer

Answer: b


43. An alternating current generator has an internal resistance Rg and an internal reactance Xg. It is used to supply power to a passive load consisting of a resistance Rg and a reactance XL For maximum power to be delivered from the generator to the load, the value of XL is equal to [NCERT Exemplar]
(a) zero.
(b) Xg.
(c) -Xg.
(d) Rg.

Answer

Answer: c


44. When a voltage measuring device is connected to AC mains, the meter shows the steady input voltage of 220 V. This means [NCERT Exemplar]
(a) input voltage cannot be AC voltage, but a DC voltage.
(b) maximum input voltage is 220V.
(c) the meter reads not v but < v² > and is . calibrated to read √< v² > .
(d) the pointer of the meter is stuck by some mechanical defect.

Answer

Answer: c


45. To reduce the resonant frequency in an LCR series circuit with a generator [NCERT Exemplar]
(a) the generator frequency should be reduced.
(b) another capacitor should be added in parallel to the first.
(c) the iron core of the inductor should be removed.
(d) dielectric in the capacitor should be removed.

Answer

Answer: b


46. Which of the following combinations should be selected for better tuning of an LCR circuit used for communication? [NCERT Exemplar]
(a) R = 20, L= 1.5 H, C = 35 μF.
(b) R = 25 , L = 2.5 H, C = 45 μF.
(c) R = 15 , L = 3.5 H, C = 30 μF.
(d) R = 25 , L = 1.5 H, C = 45 μF.

Answer

Answer: c


47. An inductor of reactance 1 Ω and a resistor of 2 Ω are connected in series to the terminals of a 6 V (rms) a.c. source. The power dissipated in the circuit is [NCERT Exemplar]
(a) 8 W.
(b) 12 W.
(c) 14.4 W.
(d) 18 W.

Answer

Answer: c


48. The output of a step-down transformer is measured to be 24 V when connected to a 12 watt light bulb. The value of the peak current is [NCERT Exemplar]
(a) 1/√2 A.
(b) √2A.
(c) 2 A.
(d) 2√2 A.

Answer

Answer: a


49. As the frequency of an ac circuit increases, the current first increases and then decreases. What combination of circuit elements is most likely to comprise the circuit? [NCERT Exemplar]
(a) Inductor and capacitor.
(b) Resistor and inductor.
(c) Resistor and capacitor.
(d) Inductor only.

Answer

Answer: a


50. In an alternating current circuit consisting of elements in series, the current increases on increasing the frequency of supply. Which of the following elements are likely to constitute the circuit?
(a) Only resistor.
(b) Resistor and an inductor.
(c) Resistor and a capacitor.
(d) Only an inductor.

Answer

Answer: c


51. Electrical energy is transmitted over large distances at high alternating voltages. Which of the following statements is not correct?
(a) For a given power level, there is a lower current.
(b) Lower current implies less power loss.
(c) Transmission lines can be made thinner.
(d) It is easy to reduce the voltage at the receiving end using step-down transformers.

Answer

Answer: c


52. For an LCR circuit, the power transferred from the driving source to the driven oscillator is P = I²Z cos Φ. Which of the following is incorrect?
(a) Here, the power factor cos Φ ≥ 0, P ≥ 0.
(b) The driving force can give no energy to the oscillator (P = 0) in some cases.
(c) The driving force cannot syphon out (P < 0) the energy out of oscillator.
(d) The driving force can take away energy out of the oscillator.

Answer

Answer: d


53. When an AC voltage of 220 V is applied to the capacitor C
(a) the maximum voltage between plates is 220 V.
(b) the current is in phase with the applied voltage.
(c) the charge on the plates is not in phase with the applied voltage.
(d) power delivered to the capacitor is zero.

Answer

Answer: d


54. Average value of ac over a complete cycle _________ .

Answer/Explanation

Answer:
Explaination: zero


55. Inductive reactance increases with _________ in frequency of ac.

Answer/Explanation

Answer:
Explaination: increase


56. In ac generator _________ is converted into electrical energy of alternating form.

Answer/Explanation

Answer:
Explaination: mechanical energy


57. Electric current that varies in magnitude dontinuously and reverses its direction periodically is known as _________ .

Answer/Explanation

Answer:
Explaination: alternating current.


58. Current which flows with a constant magnitude in same fixed direction is known as _________ .

Answer/Explanation

Answer:
Explaination: direct current.


59. Average or mean value of ac over a half cycle is _________ .

Answer/Explanation

Answer:
Explaination: 0.637 I0


60. Rms or virtual value of alternating current is _________ .

Answer/Explanation

Answer:
Explaination: 0.707 I0


61. Current in the inductive a.c. circuit _________ the voltage.

Answer/Explanation

Answer:
Explaination: lag behind


62. Capacitive reactance increases with _________ in frequency of ac.

Answer/Explanation

Answer:
Explaination: increase


63. Capacitive reactance Xc increases with _________ in frequency of ac and current in the capacitive ac circuit _________ the ac voltage.

Answer/Explanation

Answer:
Explaination: decrease, leading ahead


64. Can ever the rms value be equal to the peak value of an ac?

Answer/Explanation

Answer:
Explaination: Yes, when the ac is a square wave.


65. Which is more dangerous 220 ac or 220 dc and why?

Answer/Explanation

Answer:
Explaination:
Physics MCQs for Class 12 with Answers Chapter 7 Alternating Current 16
∴ ac is more dangerous than dc because its peak value is very high and also ac is in nature.


66. The peak value of emf in ac is E0. Write its
(i) rms, and (ii) average value over a complete cycle. [Foreign 2011]

Answer/Explanation

Answer:
Explaination: (i) Erms = \(\frac{E_{0}}{\sqrt{2}}\) (ii) Zero.


67. The current flowing through a pure inductor of inductance 4mH is i= 12 cos 300 t ampere. What is (i) rms, and (ii) average value of the current for a complete cycle? [Foreign 2011]

Answer/Explanation

Answer:
Explaination: (i) Irms = \(\frac{12}{\sqrt{2}}\) (ii) Iav = zero


68. Calculate the rms value of the alternating current shown in the figure. [HOTS]
Physics MCQs for Class 12 with Answers Chapter 7 Alternating Current 11

Answer/Explanation

Answer:
Explaination:
Physics MCQs for Class 12 with Answers Chapter 7 Alternating Current 17
The rms value of the alternating current shown in the figure is 2 A.


69. When an alternating current is passed through a moving coil galvanometer, it shows no deflection. Why?

Answer/Explanation

Answer:
Explaination: A moving coil galvanometer measures an average value of current, which is zero for ac. Hence, no deflection is shown by galvanometer


70. Which of the following curves may represent the reactance of a series LC combination? [DoE]
Physics MCQs for Class 12 with Answers Chapter 7 Alternating Current 12

Answer/Explanation

Answer:
Explaination:
(b) AS XC-XL = \(\frac{1}{2 \pi v C}\) – 2πvL


71. In a series LCR circuit, the voltages across an inductor, a capacitor and a resistor are 30 V, 30V and 60V respectively. What is the phase difference between the applied voltage and the current in the circuit?

Answer/Explanation

Answer:
Explaination: Zero. As VL = VC, circuit is resistive in nature.


72. Can a capacitor of suitable capacitance replace a inductor coil in an AC circuit?

Answer/Explanation

Answer:
Explaination: Yes, because average power consumed in both is least while controlling an AC.


73. At an airport, a person is made to walk through the doorway of a metal detector, for security reasons. Is she/he is carrying anything made of metal, the metal detector emits a sound. On what principle does this detector work? [NCERT Exempler]

Answer/Explanation

Answer:
Explaination: The metal detector works on the principle of resonances in ac circuits.


74. A capacitor blocks dc and allows ac. Why?

Answer/Explanation

Answer:
Explaination:
The capacitive reactance is XC =\(\frac{1}{2 \pi v C}\).
For dc, v = 0 ⇒ XC = ∞
i.e. a capacitor offers infinite resistance to dc and hence blocks it.
For ac, v ≠ 0, ⇒ XC ≠ ∞ , but has some finite value.
Therefore, an ac can pass through the capacitor.


75. A bulb and a capacitor are connected in series to an ac source of variable frequency. How will the brightness of the bulb change on increasing the frequency of the ac source?

Answer/Explanation

Answer:
Explaination:
The brightness will increase (∵ XC ∝ \(\frac{1}{v}\)), and heat produced H ∝ I², where
Physics MCQs for Class 12 with Answers Chapter 7 Alternating Current 18


76. Is there are any device by which direct current can be controlled without any loss of energy?

Answer/Explanation

Answer:
Explaination:
By the definition, value of the alternating voltage is equal to 110V.


77. A 110 V dc heater is used on an ac source, such that the heat produced is the same. What would be the value of the alternating voltage?

Answer/Explanation

Answer:
Explaination: cos Φ = 1.


78. The power factor of an ac circuit is 0.5. What is the phase difference between voltage and current in this circuit? [Foreign 2015]

Answer/Explanation

Answer:
Explaination:
∵ Power factor, cos Φ=0.5
∴ Φ = 60° or π/3
So, the phase difference between voltage and current is \(\frac{\pi}{3}\).


79. Why does a low power factor imply large power loss in transmission for circuits used for transporting electric power? [NCERT Example]

Answer/Explanation

Answer:
Explaination:
We know that P = TV cos Φ, where cos Φ is the power factor. To supply a given power, at a given voltage, if cos Φ is small, we have to increase the current accordingly. But this will lead to large power loss (I²R) in transmission.


80. The instantaneous current and voltage of an ac circuitaregivenby I = 10sin314tA and V=50sin (314 t + \(\frac{\pi}{2}\)) V. What is the power of dissipation m the circuit?

Answer/Explanation

Answer:
Explaination:
Phase difference between the current and voltage is π/2. So, the power dissipation Pav = Prms cos Φ is zero.


81. Why is the use of ac voltage preferred over dc voltage? [AI2014]

Answer/Explanation

Answer:
Explaination:
An ac voltage can be stepped up or down using a transformer, but not the dc voltage.


82. Does a step down transformer violate the principle of conservation of energy?

Answer/Explanation

Answer:
Explaination:
No. In a transformer, if a voltage is increased, the current is decreased in the same ratio and the product VI (power) remains the same.


83. Can we measure 110 V, 50 Hz a.c. using moving coil galvanometers? How can we measure it.

Answer/Explanation

Answer:
Explaination:
No.
Reason: As average value of a.c. voltage over one complete cycle is zero.
We can measure the rms value of a.c. voltage using hot wire meters.


84. What is an acceptor circuit and where it is used?

Answer/Explanation

Answer:
Explaination: Series LCR circuit is called acceptor circuit radio or TV sets.


85. Why do we prefer carbon brushes than copper in an ac generator? [DoE]

Answer/Explanation

Answer:
Explaination: Corrosion free and small expansion on heating maintains proper contact.


86. In a series LCR circuit, XL, Xc and R are the inductive reactance, capacitive reactance and resistance respectively at a certain frequency f If the frequency of ac is doubled, what will be the values of reactances and resistance of the circuit? [DoE]

Answer/Explanation

Answer:
Explaination:
Resistance R remains unchanged; XL will be doubled and XC will be halved.


87. In a series L-R circuit, XL = R and power factor of the circuit is P1 When a capacitor with capacitance C such that XL = Xc is put in series, the power factor becomes P2 Find P1/P2

Answer/Explanation

Answer:
Explaination: 1 : √2


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