NCERT Exemplar Solutions for Class 12 Physics Chapter 7 – Alternating Current are available here. In this article, you will get solutions from question number 7.14 to question number 7.20. These are basically very short answer type questions (or VSA). Solutions of MCQ I and MCQ II are already available and you can access them from the links given below

**NCERT Exemplar Solutions for Class 12 Physics - Chapter 7: Alternating Current (MCQ I)**

**NCERT Exemplar Solutions for Class 12 Physics - Chapter 7: Alternating Current (MCQ II)**

These questions can be asked in competitive exams like NEET, WBJEE, JEE Main, UPSEE etc & CBSE Class 12 Physics board exams.

**NCERT Exemplar Solutions for CBSE Class 12 Physics, Chapter 7 (from question number 7.14 to 7.20) are given below:**

**Question 7.14:**

If a L-C circuit is considered analogous to a harmonically oscillating spring block system, which energy of the L-C circuit would be analogous to potential energy and which one analogous to kinetic energy?

**Solution 7.14:**

An L-C circuit is considered analogous to a harmonically oscillating spring block system in which Magnetic energy analogous to kinetic energy and electrical energy analogous to potential energy.

**Question 7.15:**

Draw the effective equivalent circuit of the circuit shown in Fig 7.1, at very high frequencies and find the effective impedance.

**Solution 7.15:**

At high frequencies, capacitor ≈ short circuit (low reactance) and inductor ≈ open circuit (high reactance). Therefore, the equivalent circuit Z ≈ R_{1} + R_{3} as shown in the figure given below

**Question 7.16:**

Study the circuits (a) and (b) shown in Fig 7.2 and answer the following questions.

(*a*) Under which conditions would the rms currents in the two circuits be the same?

(*b*) Can the rms current in circuit (b) be larger than that in (a)?

**Solution 7.16:**

(*a*) Yes, if rms voltage in the two circuits are same then at resonance, the rms current in LCR will be same as that in R circuit.

(*b*) No, because *R* ≤ Z, so *I*_{a} ≥ I_{b}.

**Question 7.17:**

Can the instantaneous power output of an ac source ever be negative? Can the average power output be negative?

**Solution 7.17:**

Yes, No.

**Question 7.18:**

In series LCR circuit, the plot of *I*_{max} vs ω is shown in Fig 7.3. Find the bandwidth and mark in the figure.

**Solution 7.18:**

Bandwidth corresponds to frequencies at which I_{m} = (*I*_{max})/√2 ≈ 0.7 *I*_{max}

It is shown in the Figure given below

It is shown in the Fig.

Δω = 1.2 – 0.8 = 0.4 rad/s

**NCERT Exemplar Textbook: CBSE Class 12 Physics – All Chapters**

**Question 7.19:**

The alternating current in a circuit is described by the graph shown in Fig 7.4 . Show rms current in this graph.

**Solution 7.19:**

*I*_{rms }= 1.6A (shown in Fig. by dotted line).

**Question 7.20:** How does the sign of the phase angle φ , by which the supply voltage leads the current in an LCR series circuit, change as the supply frequency is gradually increased from very low to very high values.

**Solution 7.20:**

From negative to zero to positive; zero at resonant frequency.