# NCERT Physics Class 12 Exemplar Ch 7 Alternating Current Part 6

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**30** For an *LCR* circuit driven at frequency, the equation reads

(i) Multiply the equation by *i* and simplify where possible.

(ii) Interpret each term physically.

(iii) Cast the equation in the form of a conservation of energy statement.

(iv) Integrate the equation over one cycle to find that the phase difference between *v* and i must be acute.

Ans:

rate of change of energy stored in an inductor. Joule heating loss

rate of change of energy stored in the capacitor.

Rate at which driving force pours in energy. It goes into (i) ohmic loss and (ii) increase of stored energy.

**if phase difference, a constant is acute.**

**31** In the *LCR* circuit shown in Figure, the ac driving voltage is

(i) Write down the equation of motion for

(ii) At, the voltage source stops and *R* is short circuited. Now write down how much energy is stored in each of *L* and *C*.

Describe subsequent motion of charges.

Ans:

(ii)

(iii) Left to itself, it is an *LC* oscillator. The capacitor will go on discharging and all energy will go to *L* and back and forth.