# Case Study Chapter 7 Alternating Current Class 12 Physics

Please refer to below Case Study Chapter 7 Alternating Current Class 12 Physics. These Case Study Questions Class 12 Physics will be coming in your examinations. Students should go through the Chapter 7 Alternating Current Case Study based questions in their Class 12 Physics CBSE, NCERT, KVS book as this will help them to secure more marks in upcoming exams.

## Case Study Based Questions Physics Class 12 – Chapter 7 Alternating Current

LCR Circuit. When a pure resistance R, pure inductor L and an ideal capacitor of capacitance C is connected in series to a source of alternating emf then current at any instant through the three elements has the same amplitude and is represented as I = I 0 sinϖt. However, voltage across each element has a different phase relationship with the current as shown in graph. The effective resistance of RLC circuit is called impedance (Z) of the circuit and the voltage leads the current by a phase angle φ.

A resistor of 12 Ω, a capacitor of reactance 14 Ω and a pure inductor of inductance 0.1 H are joined in series and placed across 200 V, 50 Hz a.c. supply.

Question. The value of inductive reactance is
(a) 15 Ω
(b) 31.4 Ω
(c) 20 Ω
(d) 30 Ω

B

Question. The value of impedance is
(a) 20 Ω
(b) 15 Ω
(c) 30 Ω
(d) 21.13 Ω

D

Question. What is the value of current in the circuit?
(a) 5 A
(b) 15 A
(c) 10 A
(d) 9.46 A

D

Question. What is the value of the phase angle between current and voltage?
(a) 53º9’
(b) 63º9’
(c) 55º4’
(d) 50º

C

Question. From graph, which one is true from following?
(a) VL ≥ VC
(b) VL < VC
(c) VL > VC
(d) VL = VC

C

AC Voltage Applied to a Capacitor. Let a source of alternating emf E = E0sinwt be connected to a capacitor of capacitance C. If ‘I’is the instantaneous value of current in the circuit at instant t, then I =

The capacitive reactance limits the amplitude of current in a purely in a capacitive circuit and it is given by XC = 1/ϖC

Question. What is the unit of capacitive reactance?
(b) ampere
(c) ohm
(d) ohm-1

C

Question. The capacitive reactance of a 5 μF capacitor for a frequency of 106 Hz is
(a) 0.032 Ω
(b) 2.52 Ω
(c) 1.25 Ω
(d) 4.51 Ω

A

Question. In a capacitive circuit, resistance to the flow of current is offered by
(a) resistor
(b) capacitor
(c) inductor
(d) frequency

B

Question. In a capacitive circuit, by what value of phase angle does alternating current leads the emf?
(a) 45º
(b) 90º
(c) 75º
(d) 60º

B

Question. One microfarad capacitor is joined to a 200 V, 50 Hz alternator. The rms current through capacitor is
(a) 6.28 × 10-2 A
(b) 7.5 × 10-4 A
(c) 10.52 × 10-2 A
(d) 15.25 × 10-2 A

A

Average power associated with an inductor and capacitor. The power averaged over one full cycle of a.c. is known as average power. IT is also known as true power. Pav = VrmsIrms cos φ = V0I0/2 cos φ. Root mean square or simply rms watts refer to continuous power. A circuit containing a 80 mH inductor and a 60 μF capacitor in series is connected to a 230 V, 50 Hz supply. The resistance of the circuit is negligible.

Question. The value of current amplitude is
(a) 15 A
(b) 11.63 A
(c) 17.65 A
(d) 6.33 A

B

Question. Find rms value.
(a) 6 A
(b) 5.25 A
(c) 8.23 A
(d) 7.52 A

C

Question. The average power transferred to inductor is
(a) zero
(b) 7 W
(c) 2.5 W
(d) 5 W

A

Question. The average power transferred to the capacitor is
(a) 5 W
(b) 0
(c) 11 W
(d) 15 W

B

Question. What is the total average power absorbed by the circuit?
(a) zero
(b) 10 W
(c) 2.5 W
(d) 15 W

A

Transformer. A transformer is essentially an a.c. device. It cannot work on d.c. It changes alternating voltages or currents. It does not affect the frequency of a.c. It is based on the phenomenon of mutual induction. A transformer essentially consists of two coils of insulated copper wire having different number of turns and wound on the same soft iron core. The number of turns in the primary and secondary coils of an ideal transformer are 2000 and 50 respectively. The primary coil is connected to a main supply of 120 V and secondary coil is connected to a bulb of resistance 0.6 Ω.

Question. The value of voltage across the secondary coil is
(a) 5 V
(b) 2 V
(c) 3 V
(d) 10 V

C

Question. The value of current in the bulb is
(a) 7 A
(b) 15 A
(c) 3 A
(d) 5 A

D

Question. The value of current in primary coil is
(a) 0.125 A
(b) 2.25 A
(c) 1.51 A
(d) 3.52 A

A

Question. Power in primary coil is
(a) 20 W
(b) 5 W
(c) 10 W
(d) 15 W

D

Question. Power is secondary coil is
(a) 15 W
(b) 20 W
(c) 7 W
(d) 8 W

A

Resonant Series LCR Circuit. When the frequency of ac supply is such that the inductive reactance and capacitive reactance become equal, the impedance of the series LCR circuit is equal to the ohmic resistance in the circuit. Such a series LCR circuit is known as resonant series LCR circuit and the frequency of the ac supply is known as resonant frequency. Resonance phenomenon is exhibited by a circuit only if both L and C are present in the circuit. We cannot have resonance in a RL or RC circuit. A series LCR circuit with L = 0.12 H, C = 480 nF, R = 23 Ω is Connect to a 230 V variable frequency supply.

Question. Find the value of source for which current amplitude is maximum.
(a) 222.32 Hz
(b) 550.52 Hz
(c) 663.48 Hz
(d) 770 Hz

C

Question. The value of maximum current is
(a) 14.14 A
(b) 22.52 A
(c) 50.25 A
(d) 47.41 A

A

Question. The value of maximum power is
(a) 2200 W
(b) 2299.3 W
(c) 5500 W
(d) 4700 W

B

Question. What is the Q-factor of the given circuit?
(a) 25 A
(b) 42.21 A
(c) 35.42 A
(d) 21.74 A