Question

: In LC Oscillator , at time ,energy of inductor and energy of capacitor are equal ?
A. T/8
B. T/4
C. T/2
D. T

Answer 1

Hint:-
For solving the above problem we will use :
Energy of capacitor as: $\dfrac{1}{2}\dfrac{{{Q_0}^2}}{C}$
Energy of inductor as:$\dfrac{1}{2}L{I^2}$
Frequency of Oscillator :$\omega = \dfrac{1}{{\sqrt {LC} }}$

Complete step-by-step solution :Voltage in inductor given as : $L\dfrac{{di}}{{dt}}$ ..............1
Voltage in capacitor is given as: $\dfrac{q}{C}$ ..............2
Therefore we can write $L\dfrac{{di}}{{dt}}$ $=$ $\dfrac{q}{C}$...............3
Also we have $i$ = -$\dfrac{{dq}}{{dt}}$
In equation 3 we substitute the value of i
$\Rightarrow L\dfrac{{ddq}}{{dtdt}} = - \dfrac{q}{C} \\\ \Rightarrow \dfrac{{{d^2}q}}{{d{t^2}}} = - \dfrac{q}{{LC}} \\\$ ( equation becomes the differential equation)
On solving this differential equation we will get :
Q=Qcos, q is maximum when cos$\omega t$ =1, $\omega = \dfrac{1}{{\sqrt {LC} }}$
When energy of capacitor and inductor are equal :
$\dfrac{1}{2}\dfrac{{{Q_0}^2}}{C} =$ $\dfrac{1}{2}L{I^2}$
We will substitute the value of we have calculated above but before that calculate .
i = -$\dfrac{{dq}}{{dt}}$
$\Rightarrow i = - \dfrac{{dQ\cos \omega t}}{{dt}} \\\ \Rightarrow i = Q\omega \sin \omega t \\\$ (we have differentiated q equation with respect to t)
Now we will substitute the equation of q we have calculated above and the value of i we got after differentiation in the equalized equation of energy.
$\dfrac{1}{2}\dfrac{{{Q_0}^2}}{C} =$ $\dfrac{1}{2}L{I^2}$
$\Rightarrow \dfrac{1}{2}\dfrac{{{{(Q\cos \omega t)}^2}}}{C} = \dfrac{1}{2}L{(Q\omega \sin wt)^2} \\\ \Rightarrow \dfrac{{{Q^2}{{\cos }^2}\omega t}}{C} = L{Q^2}{\sin ^2}wt \\\$ (cancelling ½ term from LHS and RHS)
$\Rightarrow \dfrac{1}{{LC}} = \dfrac{{\omega {Q^2}{{\sin }^2}\omega t}}{{{Q^2}{{\cos }^2}\omega t}} \\\ \Rightarrow \dfrac{1}{{LC}} = \omega {\tan ^2}\omega t \\\$ (cancel Q2 and keep LC on LHS) (sin(wt) / cos(wt) = tan(wt))
$\Rightarrow \dfrac{1}{{\sqrt {LC} }} = \omega \tan \omega t$ (taking square roots on both LHS and RHS )
$\omega = \dfrac{1}{{\sqrt {LC} }}$,$\omega = \dfrac{{2\pi }}{T}$
So we can write :
$\Rightarrow \omega = \omega \tan \omega t \\\ \Rightarrow 1 = \tan \omega t \\\ \Rightarrow {\tan ^{ - 1}}(1) = \omega t \\\ \Rightarrow {45^0} = \dfrac{{2\pi t}}{T} \\\$ (Tan inverse 1 is equal to $45^0$ and substituting value of w from above )
We can write $45^0$ as $\dfrac{\pi}{4}$.
$\Rightarrow \dfrac{\pi }{4} = \dfrac{{2\pi t}}{T} \\\ \Rightarrow \dfrac{T}{4} = 2t \\\$ (cancelling $\pi$ from both sides )
$\Rightarrow t = \dfrac{T}{8}$
Option 1 is correct.

Note:- Oscillators are generally used to convert current flow form a direct current source to an alternating waveform which is of the frequency which is desired by us. Oscillators produce a continuous repeated, and alternating waveform without any input given to it.