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Published on: 23/05/2021
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Questions + Answers key
Take MCQ Physics Test1.
Obtain the resonant frequency \(({ \omega }_{ r })\) of a series L-C-R circuit with L = 2.0 H, \(C=32\mu F \) and \(R=10\Omega \) . What is the Q-value of this circuit?
2.
A series L-C-R circuit with \(R=20\Omega \), L = 1.5H and \(C=5\mu F\) is connected to a variable frequency of 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, what is the average complete cycle?
3.
In series L-C-R circuit, the plot of Imax\(\omega \) versus is shown in the figure. Find the bandwidth and mark in the figure.

4.
A capacitor C, a variable resistance R and a bulb B are connected in series to the AC mains in circuit as shown in the figure.The bulb glows of the bulb change, if
(i) a dielectric slab is introduced between the plates of the capacitor, keeping resistance R to be same;
(ii) the resistance R is increased keeping will capacitance?

1.
Given, L = 2.0 H, C = 32 x 10-6 F,
\(R=10\Omega \)
\({ \omega }_{ r }\) = ? Q = ?
\({ \omega }_{ r }=\frac { 1 }{ \sqrt { LC } } =\frac { 1 }{ \sqrt { 2.0\times 32\times { 10 }^{ -6 } } } \)
\(\frac { { 10 }^{ 3 } }{ 8 } =125rad/s\)
\( Q=\frac { 1 }{ R } \sqrt { \frac { L }{ C } } =\frac { 1 }{ 10 } \sqrt { \frac { 2 }{ 32\times { 10 }^{ -6 } } } \)
\(\frac { 1 }{ 10\times 4\times { 10 }^{ -3 } } =25\)
2.
When the frequency of the supply equals the natural frequency of the circuit, resonance occurs.
\({ Z }_{ r }=R=20\Omega \)
\({ I }_{ rms }=\frac { { V }_{ rms } }{ { Z }_{ r } } =\frac { 200 }{ 20\quad } =10A\)
Average power transferred in one cycle,
\({ P }_{ av }={ V }_{ rms }{ I }_{ rms }\cos { \phi } \)
= 200 x 10 x cos 00 [\(\phi ={ 0 }^{ 0 }\)]
= 2000W = 2kW
3.
Consider the diagram.
Bandwidth = \(\omega _{ 2 }-{ \omega }_{ 1 }\),

Where , \(\omega_1\) and \(\omega_2\) correspond to frequencies at which magnitude of current is \(\frac { 1 }{ \sqrt { 2 } } \) times of maximum value.
\({ I }_{ rms }=\frac { { I }_{ max } }{ \sqrt { 2 } } =\frac { 1 }{ \sqrt { 2 } } \approx 0.7A\)
Clearly, from the diagram, the corresponding frequencies are 0.8 rad/s and rad/s.
\(\triangle \omega \) = B and width = 1.2 - 0.8 = 0.4 rad/s
4.
(i) As the electric slab is introduced between the plates of the capacitor, its capacitance. Hence, the potential drop across the capacitor will i.e \(V=\frac { Q }{ C } \)
As a result, the potential drop across the bulb will increase as they are connected in series. Thus its brightness will increase.
(ii) As a resistance R is increased, the potential drop across the resistor will increase. As a result, the potential drop across the bulb will decrease as they are connected in series. Thus, its brightness will decrease.
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