CBSE Class 12 Physics Alternating Current Worksheet Set A

CBSE Class 12 Physics Alternating Current Worksheet Set A

Question. An inductive circuit contains resistance of 10 ohms and an inductance of 2 henry. If an A.C. voltage of 120 Volts and frequency 60 Hz is applied to this circuit, the current would be nearly
(a) 0.32 Amp
(b) 0.16 Amp
(c) 0.48 Amp
(d) 0.80 Amp

Answer: B

Question. Which of the following will have the dimensions of time
(a) LC
(b) R/L
(c) L/R
(d) C/L

Answer: C

Question. 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) number of turns in the coil is reduced
(b) a capacitance of reactance X_C = X_L is included in the same circuit
(c) an iron rod is inserted in the coil
(d) frequency of the AC source is decreased

Answer: C

Question. Using an A.C. voltmeter the potential difference in the electrical line in a house is read to be 234 volt. If the line frequency is known to be 50 cycles/second, the equation for the line voltage is
(a) V = 165 sin (100 π t)
(b) V = 331 sin (100 π t)
(c) V = 220 sin (100 π t)
(d) V = 440 sin (100 π t)

Answer: B

Question. A step down transformer reduces 220 V to 110 V. The primary draws 5 ampere of current and secondary supplies 9 ampere.
The efficiency of transformer is
(a) 20%
(b) 44%
(c) 90%
(d) 100%

Answer: C

Question. In LCR circuit if resistance increases quality factor
(a) increases finitely
(b) decreases finitely
(c) remains constant
(d) None of these

Answer: B

Question. A resistance 'R' draws power 'P' when connected to an AC source. If an inductance is now placed in series with the resistance, such that the impedance of the circuit becomes 'Z', the power drawn will be
(a) P√R/Z
(b) P(R/Z)
(c) P
(d) P(R/Z)²

Answer: D

Question. A series R-C circuit is connected to an alternating voltage source. Consider two situations: 
(A) When capacitor is air filled.
(B) When capacitor is mica filled.
Current through resistor is i and voltage across capacitor is V then :
(a) V_a > V_b
(b) i_a > i_b
(c) V_a = V_b
(d) V_a < V_b

Answer: A

Question. In the circuit of Fig, the bulb will become suddenly bright if 

Answer: C

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

Answer: A

Question. An alternating voltage of 220 V, 50 Hz frequency is applied across a capacitor of capacitance 2 μF. The impedence of the circuit is
(a) π/5000
(b) 1000/π
(c) 500 π
(d) 5000/π

Answer: D

Question. The primary winding of a transformer has 100 turns and its secondary winding has 200 turns. The primary is connected to an A.C. supply of 120 V and the current flowing in it is 10 A. The voltage and the current in the secondary are
(a) 240 V, 5 A
(b) 240 V, 10 A
(c) 60 V, 20 A
(d) 120 V, 20 A

Answer: A

Question. In an a.c. circuit V and I are given by
V = 100 sin (100 t) volts
I = 100 sin (100 t + π/3) mA
the power dissipated in the circuit is
(a) 104 watt
(b) 10 watt
(c) 2.5 watt
(d) 5.0 watt

Answer: C

Question. A transformer is used to light a 140 W, 24 V bulb from a 240 V a.c. mains. The current in the main cable is 0.7 A. The efficiency of the transformer is
(a) 63.8 %
(b) 83.3 %
(c) 16.7 %
(d) 36.2 %

Answer: B

Question. In the given circuit, the current drawn from the source is 

(a) 20 A
(b) 10 A
(c) 5 A
(d) 5 √2 A

Answer: D

Question. In series combination of R, L and C with an A.C. source at resonance, if R = 20 ohm, then impedence Z of the combination is
(a) 20 ohm
(b) zero
(c) 10 ohm
(d) 400 ohm

Answer: A

Question. Fleming's left and right hand rules are used in
(a) DC motor and AC generator
(b) DC generator and AC motor
(c) DC motor and DC generator
(d) Both rules are same, any one can be used

Answer: C

Question. In the question 86, if the switch is opened after the capacitor has been charged, it will discharges with a time constant
(a) RC
(b) 2RC
(c) 1/2 RC
(d) RC ln 2

Answer: B

Question. A step down transformer is connected to 2400 volts line and 80 amperes of current is found to flow in output load. The ratio of the turns in primary and secondary coil is 20 : 1. If transformer efficiency is 100%, then the current flowing in the primary coil will be
(a) 1600 amp
(b) 20 amp
(c) 4 amp
(d) 1.5 amp

Answer: C

Question. For the circuit shown in the fig., the current through the inductor is 0.9 A while the current through the condenser is 0.4 A. Then
(a) current drawn from generator I = 1.13 A
(b) ω = 1/(1.5 L C)
(c) I = 0.5 A 
(d) I = 0.6 A 

Answer: C

Question. An inductor of self inductance 100 mH and a resistor of resistance 50Ω, are connected to a 2 V battery. The time required for the current to half its steady value is
(a) 2 milli second
(b) 2 ln (0.5) milli second
(c) 2 ln (3) milli second
(d) 2 ln (2) milli second

Answer: D

Question. An inductor 20 mH, a capacitor 50 μF and a resistor 40W are connected in series across a source of emf V = 10 sin 340 t.
The power loss in A.C. circuit is : 
(a) 0.51 Ω
(b) 0.67 Ω
(c) 0.76 Ω
(d) 0.89 Ω

Answer: A

Question. The time taken by the current to rise to 0.63 of its maximum value in a d.c. circuit containing inductance (L) and resistance (R) depends on
(a) L only
(b) R only
(c) L/R
(d) LR

Answer: C

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

Answer: C

Question. Figure shows a circuit that contains three identical resistors with resistance R = 9.0 Ω each, two identical inductors with inductance L = 2.0 mH each, and an ideal battery with emf ε = 18 V. The current 'i' through the battery just after the switch closed is
(a) 0.2 A
(b) 2 A
(c) 0
(d) 2 mA

Answer: B
 

              Very Short Answer

Q1)       What is the tesla coil?

Q2)       Define alternating voltage?

Q3)       What is phasors?

Q4)       What happen when a capacitor is connected to a voltage source in a dc circuit?

Q5)       How can we represent the AC current and voltage?

              Short Answer

Q6)       What are the two reasons of LC oscillations is not realistic?

Q7)       What do you mean by LCR series circuits?

Q8)       How AC voltage applied to a resistor?

Q9)       What do you mean by the term phasors-diagram?

Q10)     How AC voltage applied to an inductor?

              Long Answer

Q11)     Write short note on resonance?

Q12)     Explain LC oscillations?

Q13)     State the difference between alternating current and direct current?

Q14)     Explain the power factors in the power in AC circuit?

Q15)     Explain transformers and what are reasons for losing the energy in transformers?