# NCERT solutions for Physics Class 12 chapter 7 - Alternating Current [Latest edition]

## Solutions for Chapter 7: Alternating Current

Below listed, you can find solutions for Chapter 7 of CBSE NCERT for Physics Class 12.

Exercise
Exercise [Pages 266 - 268]

### NCERT solutions for Physics Class 12 Chapter 7 Alternating Current Exercise [Pages 266 - 268]

Exercise | Q 7.1 | Page 266

A 100 Ω resistor is connected to a 220 V, 50 Hz ac supply.

1. What is the rms value of current in the circuit?
2. What is the net power consumed over a full cycle?
Exercise | Q 7.2 (a) | Page 266

The peak voltage of an ac supply is 300 V. What is the rms voltage?

Exercise | Q 7.2 (b) | Page 266

The rms value of current in an ac circuit is 10 A. What is the peak current?

Exercise | Q 7.3 | Page 266

A 44 mH inductor is connected to 220 V, 50 Hz ac supply. Determine the rms value of the current in the circuit.

Exercise | Q 7.4 | Page 266

A 60 µF capacitor is connected to a 110 V, 60 Hz ac supply. Determine the rms value of the current in the circuit.

Exercise | Q 7.5 | Page 266

What is the net power absorbed by each circuit over a complete cycle. Explain your answer.

Exercise | Q 7.6 | Page 266

Obtain the resonant frequency ωr of a series LCR circuit with L = 2.0 H, C = 32 µF and R = 10 Ω. What is the Q-value of this circuit?

Exercise | Q 7.7 | Page 266

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

Exercise | Q 7.8 | Page 266

Suppose the initial charge on the capacitor is 6 mC. What is the total energy stored in the circuit initially? What is the total energy at later time?

Exercise | Q 7.9 | Page 266

A series LCR circuit with R = 20 Ω, L = 1.5 H and C = 35 µF is connected to a variable-frequency 200 V ac supply. When the frequency of the supply equals the natural frequency of the circuit, what is the average power transferred to the circuit in one complete cycle?

Exercise | Q 7.10 | Page 266

A radio can tune over the frequency range of a portion of MW broadcast band: (800 kHz to 1200 kHz). If its LC circuit has an effective inductance of 200 µH, what must be the range of its variable capacitor?

[Hint: For tuning, the natural frequency i.e., the frequency of free oscillations of the LC circuit should be equal to the frequency of the radio wave.]

Exercise | Q 7.11 | Page 266

Figure shows a series LCR circuit connected to a variable frequency 230 V source. L = 5.0 H, C = 80 µF, R = 40 Ω.

(a) Determine the source frequency which drives the circuit in resonance.

(b) Obtain the impedance of the circuit and the amplitude of current at the resonating frequency.

(c) Determine the rms potential drops across the three elements of the circuit. Show that the potential drop across the LC combination is zero at the resonating frequency.

Exercise | Q 7.12 | Page 267

An LC circuit contains a 20 mH inductor and a 50 µF capacitor with an initial charge of 10 mC. The resistance of the circuit is negligible. Let the instant the circuit is closed be t = 0.

(a) What is the total energy stored initially? Is it conserved during LC oscillations?

(b) What is the natural frequency of the circuit?

(c) At what time is the energy stored
(i) completely electrical (i.e., stored in the capacitor)?
(ii) completely magnetic (i.e., stored in the inductor)?

(d) At what times is the total energy shared equally between the inductor and the capacitor?

(e) If a resistor is inserted in the circuit, how much energy is eventually dissipated as heat?

Exercise | Q 7.13 | Page 267

A coil of inductance 0.50 H and resistance 100 Ω is connected to a 240 V, 50 Hz ac supply.

(a) What is the maximum current in the coil?

(b) What is the time lag between the voltage maximum and the current maximum?

Exercise | Q 7.14 | Page 267

Obtain if the circuit is connected to a high-frequency supply (240 V, 10 kHz). Hence, explain the statement that at very high frequency, an inductor in a circuit nearly amounts to an open circuit. How does an inductor behave in a dc circuit after the steady state?

Exercise | Q 7.15 | Page 267

A 100 µF capacitor in series with a 40 Ω resistance is connected to a 110 V, 60 Hz supply.

(a) What is the maximum current in the circuit?

(b) What is the time lag between the current maximum and the voltage maximum?

Exercise | Q 7.16 | Page 267

Obtain if the circuit is connected to a 110 V, 12 kHz supply? Hence, explain the statement that a capacitor is a conductor at very high frequencies. Compare this behaviour with that of a capacitor in a dc circuit after the steady state.

Exercise | Q 7.17 | Page 267

Keeping the source frequency equal to the resonating frequency of the series LCR circuit, if the three elements, L, C and R are arranged in parallel, show that the total current in the parallel LCR circuit is minimum at this frequency. Obtain the current rms value in each branch of the circuit for the elements and source specified for this frequency.

Exercise | Q 7.18 | Page 267

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.

(a) Obtain the current amplitude and rms values.

(b) Obtain the rms values of potential drops across each element.

(c) What is the average power transferred to the inductor?

(d) What is the average power transferred to the capacitor?

(e) What is the total average power absorbed by the circuit?
[‘Average’ implies ‘averaged over one cycle’.]

Exercise | Q 7.19 | Page 267

Suppose the circuit has a resistance of 15 Ω. Obtain the average power transferred to each element of the circuit, and the total power absorbed.

Exercise | Q 7.20 | Page 268

A series LCR circuit with L = 0.12 H, C = 480 nF, R = 23 Ω is connected to a 230 V variable frequency supply.

(a) What is the source frequency for which current amplitude is maximum. Obtain this maximum value.

(b) What is the source frequency for which average power absorbed by the circuit is maximum. Obtain the value of this maximum power.

(c) For which frequencies of the source is the power transferred to the circuit half the power at resonant frequency? What is the current amplitude at these frequencies?

(d) What is the Q-factor of the given circuit?

Exercise | Q 7.21 | Page 268

Obtain the resonant frequency and Q-factor of a series LCR circuit with L = 3.0 H, C = 27 µF, and R = 7.4 Ω. It is desired to improve the sharpness of the resonance of the circuit by reducing its ‘full width at half maximum’ by a factor of 2. Suggest a suitable way.

Exercise | Q 7.22 (a) | Page 268

In any ac circuit, is the applied instantaneous voltage equal to the algebraic sum of the instantaneous voltages across the series elements of the circuit? Is the same true for rms voltage?

Exercise | Q 7.22 (b) | Page 268

A capacitor is used in the primary circuit of an induction coil.

Exercise | Q 7.22 (c) | Page 268

An applied voltage signal consists of a superposition of a dc voltage and an ac voltage of high frequency. The circuit consists of an inductor and a capacitor in series. Show that the dc signal will appear across C and the ac signal across L.

Exercise | Q 7.22 (d) | Page 268

A choke coil in series with a lamp is connected to a dc line. The lamp is seen to shine brightly. Insertion of an iron core in the choke causes no change in the lamp’s brightness. Predict the corresponding observations if the connection is to an ac line.

Exercise | Q 7.22 (e) | Page 268

Why is choke coil needed in the use of fluorescent tubes with ac mains? Why can we not use an ordinary resistor instead of the choke coil?

Exercise | Q 7.23 | Page 268

A power transmission line feeds input power at 2300 V to a stepdown transformer with its primary windings having 4000 turns. What should be the number of turns in the secondary in order to get output power at 230 V?

Exercise | Q 7.24 | Page 268

At a hydroelectric power plant, the water pressure head is at a height of 300 m and the water flow available is 100 m3 s–1. If the turbine generator efficiency is 60%, estimate the electric power available from the plant (g = 9.8 ms–2).

Exercise | Q 7.25 | Page 268

A small town with a demand of 800 kW of electric power at 220 V is situated 15 km away from an electric plant generating power at 440 V. The resistance of the two wire line carrying power is 0.5 Ω per km. The town gets power from the line through a 4000-220 V step-down transformer at a sub-station in the town.

(a) Estimate the line power loss in the form of heat.

(b) How much power must the plant supply, assuming there is negligible power loss due to leakage?

(c) Characterise the step up transformer at the plant.

Exercise | Q 7.26 | Page 268

Do the same with the replacement of the earlier transformer by a 40,000-220 V step-down transformer (Neglect, as before, leakage losses though this may not be a good assumption any longer because of the very high voltage transmission involved). Hence, explain why high voltage transmission is preferred?

Exercise

## NCERT solutions for Physics Class 12 chapter 7 - Alternating Current

Shaalaa.com has the CBSE Mathematics Physics Class 12 CBSE solutions in a manner that help students grasp basic concepts better and faster. The detailed, step-by-step solutions will help you understand the concepts better and clarify any confusion. NCERT solutions for Mathematics Physics Class 12 CBSE 7 (Alternating Current) include all questions with answers and detailed explanations. This will clear students' doubts about questions and improve their application skills while preparing for board exams.

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Concepts covered in Physics Class 12 chapter 7 Alternating Current are LC Oscillations, Reactance and Impedance, Peak and Rms Value of Alternating Current Or Voltage, Alternating Currents, Different Types of AC Circuits: AC Voltage Applied to an Inductor, Different Types of AC Circuits: AC Voltage Applied to a Series LCR Circuit, Different Types of AC Circuits: AC Voltage Applied to a Capacitor, Representation of AC Current and Voltage by Rotating Vectors - Phasors, Different Types of AC Circuits: AC Voltage Applied to a Resistor, Alternating Currents and Direct Currents, Forced Oscillations and Resonance, Transformers, Power in AC Circuit: the Power Factor.

Using NCERT Physics Class 12 solutions Alternating Current exercise by students is an easy way to prepare for the exams, as they involve solutions arranged chapter-wise and also page-wise. The questions involved in NCERT Solutions are essential questions that can be asked in the final exam. Maximum CBSE Physics Class 12 students prefer NCERT Textbook Solutions to score more in exams.

Get the free view of Chapter 7, Alternating Current Physics Class 12 additional questions for Mathematics Physics Class 12 CBSE, and you can use Shaalaa.com to keep it handy for your exam preparation.

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