CBSE Questions for Class 12 Medical Physics Alternating Current Quiz 11 - MCQExams.com

In an electric circuit applied ac emf is  $$e = 20 \sin 300 \ t$$  volt and the current is  $$i = 4 \sin 300 \ t$$  ampere.The reactance of the circuit is
  • $$5 \Omega$$
  • $$80 \Omega$$
  • $$1.8 \mathrm { k } \Omega$$
  • $$0$$
In an alternating current circuit, the phase difference between current l and voltage is $$\phi$$, then the Wattless component of current will be -
  • $$I\cos \phi$$
  • $$I \tan\phi$$
  • $$I \sin\phi$$
  • $$I \cos^2\phi$$
if value of R is changed then 
1499371_a9b18e8652754c32a4e6fd0c346076f4.png
  • voltage across L remains same
  • voltage across C remains sam
  • voltage across L-C combination remains same w
  • voltage across L-C combination changes
A current of 2A flow in an inductance of 10H. for generating 100V EMF in it the rate of change current will be.
  • 1 A/sec
  • 5 A/sec
  • 10 A/sec
  • 100 A/sec
In a RLC series circuit at resonance, the value of the power factor is :
  • infinity
  • zero
  • $$\dfrac{1}{\sqrt{2}}$$
  • $$1$$
A current $$I=3+8\sin 100t$$ is passing through a resistor of resistance $$10\Omega$$. The effective value of current is?
  • $$\sqrt{41} A$$
  • $$6.4 A$$
  • $$4\sqrt{2} A$$
  • $$3/\sqrt{2} A$$
An $$8\mu F$$ capacitor is connected to the terminals of an A.C. source whose $$V_{rms}$$ is $$150$$ volt and the frequency is $$60$$ Hz, the capacitive reactance is?
  • $$0.332\times 10^3\Omega$$
  • $$2.08\times 10^3\Omega$$
  • $$4.16\times 10^3\Omega$$
  • $$12.5\times 10^3\Omega$$
The self inductance of a choke coil is mH. when it is connected with a 10 VDC source then the loss of power is 20 watt. When it connected with 10 volt AC source loss of power is 10 watt. The frequency of AC source will bw-
  • 50 Hz
  • 60 Hz
  • 80 Hz
  • 100 Hz
Impedance of LR circuit is-
  • $$R^2+\omega ^2L^2$$
  • $$\sqrt {R+\omega L}$$
  • $$R+\omega L$$
  • $$\sqrt {R^2+\omega ^2 L^2}$$
An ideal inductor takes a current of 10 A when connected to a $$  125 {V}, 50 {Hz}  $$ ac supply.A pure resistor across the same source takes 12.5 A.If the two are connected in series across a $$  100 {V}, 40 {Hz}  $$ -supply,the current through the circuit will be
  • 7 A
  • 12.5 A
  • 20 A
  • 25 A
Impedance of a coil having inductance $$0.4H$$ at frequency of $$50Hz$$ will be
  • $$20\pi \Omega$$
  • $$40\pi \Omega$$
  • $$2\pi \Omega$$
  • $$4\pi \Omega$$
A capacitor C and inductor L are connected in parallel with a battery of emf e and internal resistance r. At time $$t=0$$, current through the cell be $$i_0$$ and at t$$ \rightarrow \infty $$,let the current be i, Then $$\frac{i_0}{i}$$ is equal to
1572142_44451203af0d4759a6fc31cf041a2adb.png
  • $$1$$
  • $$0$$
  • infinity
  • cannot be determine.
If an ac source is connected across  ideal capacitor and current passing through it is dentoed by curve then instantaneous power is denoted by curve
1572193_d014b991d3ed406e8f30da6f96695cec.PNG
  • $$c$$
  • $$b$$
  • $$e$$
  • $$a$$

The resonance frequency changes if .is a change in L-C-R series A.C. circuit. 
  • only R
  • only L
  • only C
  • L and C
Two inductors each of inductance L are connected in parallel . what is their equivalent inductance?
  • 2 L
  • L
  • L/ 2
  • L/ 4
Phase relationship between current (I) and applied voltage (E) for a series LCR circuit is shown here. $$ \omega_0$$ = Reasonat angular frequency of the circuit and $$ \omega $$ Applied angular frequency. 
1569999_f8077614f7f04caa919aa7ef6ee6f3d4.png
  • $$ \omega = \omega_o $$
  • $$ \omega > \omega_o, $$ exact relation can't be computed
  • $$ \omega < \omega_o, $$ exact relation can't be computed
  • $$ 2 \omega = \sqrt {3\omega_o} $$
The inductance of a coil is directly proportional to 
  • its length
  • the number of turns
  • the resistance of the coil
  • the square of the number of turns

In the phasors method for the only capacitive A.C. circuit, voltage is in +X direction then current is in direction. 
  • +X
  • -X
  • +Y
  • -Y
In LC oscillation resistance is $$100\Omega$$ and inductance and capacitance is $$1$$H and $$10\mu F$$. Find the half power of frequency.
  • $$266.2$$
  • $$366.2$$
  • $$166.2$$
  • $$233.2$$
A series LCR circuit containing a resistance of $$ 120 \Omega $$ has angular resonance frequency $$ 4 \times 10^2 rad^{-1} $$. At resonance the voltage across resistance and inductance are 60 V and respectively.
(a) The value of L and C are $$ 0.2 mH. \frac {1}{32} \mu F $$ 
 (b) $$ 8 \times 10^5 rad/s $$. The current lags the voltage by $$ 45^o $$
 (c) $$ 6 \times 10^5 rad/s, $$ the current lags the voltage by $$ 45^o $$ 
  • (a), (c) are correct
  • (b) and (c) are correct
  • (a), (b) are correct
  • (a), (b) (c) are wrong
The switch shown in figure is closed at $$t = 0$$ find the charge through battery till time $$t = \dfrac {L}{R}$$.
1613149_4add290dad784561948be69cf5f2444b.png
  • $$\dfrac {L_{\epsilon}}{eR^{2}}$$
  • $$\dfrac {L_{\epsilon}}{2eR^{2}}$$
  • $$\dfrac {2L_{\epsilon}}{eR^{2}}$$
  • None
If in an A.C., L-C series circuit $$X_c > X_L$$. Hence potential _________.
  • Lags behind the current by $$\pi/2$$
  • Leads the current by $$\pi$$ in phase
  • Leads the current by $$\pi/2$$ in phase
  • Lags behind the current by $$\pi$$ in phase
Find the time after which current in the circuit becomes $$80 \%$$ of its maximum value.
1612420_be75cb7148d44c2eba821c4f04cf3841.png
  • $$\dfrac{\ell n 2}{100}$$
  • $$\dfrac{\ell n 3}{100}$$
  • $$\dfrac{\ell n 5}{100}$$
  • $$\dfrac{\ell n 6}{100}$$
An oscillator circuit contains an inductor $$0.05H$$ and a capacitor of  capacity $$80\mu F$$.When the maximum voltage across the capacitor is $$200V$$, the maximum current (in amperes) in the circuit is 
  • $$2$$
  • $$4$$
  • $$8$$
  • $$10$$
  • $$16$$
In a series LCR circuit $$R=300\Omega, L=0.9H, C=2\mu F, \omega =1000\ rad/s$$. The impedance of the circuit is?
  • $$500\Omega$$
  • $$1300\Omega$$
  • $$400\Omega$$
  • $$900\Omega$$
$$L,C,R$$ represent the physical quantities inductances, capacitance and resistance respectively. Which of the following combinations have dimensions of freqeuency?
  • $$\cfrac { 1 }{ RC } $$
  • $$\cfrac { R }{ L } $$
  • $$\cfrac { 1 }{ \sqrt { LC } } $$
  • $$C/L$$
The natural frequency of the circuit shown in figure is 
1749297_a4c93e8238ae43ed97214a8f1ce56727.png
  • $$\dfrac{1}{\sqrt{LC}}$$
  • $$\dfrac{1}{\sqrt{2LC}}$$
  • $$\dfrac{2}{\sqrt{LC}}$$
  • none of these
The frequency of oscillation of current in the inductor  is 
1749114_56f8f15223324044986e3d21f032d3c8.png
  • $$\dfrac{1}{3\sqrt{LC}}$$
  • $$\dfrac{1}{6 \pi\sqrt{LC}}$$
  • $$\dfrac{1}{\sqrt{LC}}$$
  • $$\dfrac{1}{2 \pi\sqrt{LC}}$$
If $$ E_0 $$ represents the peak value of the voltage in an ac circuit, the r.m.s. value of the voltage will be[
  • $$ \dfrac{E_0}{\pi} $$
  • $$ \dfrac{E_0}{2} $$
  • $$ \dfrac{E_0}{\sqrt{\pi}} $$
  • $$ \dfrac{E_0}{\sqrt{2}} $$
The resistance of a coil for dc is in ohms. In ac, the resistance
  • will remains same
  • will increase
  • will decrease
  • will be zero
An alternating current of frequency ' f ' is flowing in a circuit containing a resistance R and a choke L in series. The impedance of this circuit is
  • $$ R + 2 \pi \, fL $$
  • $$ \sqrt{R^2 + 4 \, \pi^2 f^2 L^2} $$
  • $$ \sqrt{R^2 + L^2} $$
  • $$ \sqrt{R^2 + 2 \pi \, fL }$$
Power delivered by the source of the circuit becomes maximum, when
  • $$ \omega L = \omega C $$
  • $$ \omega L = \dfrac{1}{\omega C} $$
  • $$ \omega L = - \left ( \dfrac{1}{\omega C} \right )^2 $$
  • $$ \omega L = \sqrt{\omega C} $$
The natural frequency of a $$ L-C $$  circuit is equal to
  • $$ \dfrac{1}{2 \pi} \sqrt{LC} $$
  • $$ \dfrac{1}{2 \pi \sqrt{LC}} $$
  • $$ \dfrac{1}{2 \pi} \sqrt{\dfrac{L}{C}} $$
  • $$ \dfrac{1}{2 \pi} \sqrt{\dfrac{C}{L}} $$
An alternating e.m.f. is applied to purely capacitive circuit. The phase relation between e.m.f. and current flowing in the circuit is or In a circuit containing capacitance only
  • e.m.f is ahead of current by $$ \dfrac{\pi}{2} $$
  • Current is ahead of e.m.f by $$ \dfrac{\pi}{2} $$
  • Current lags behind e.m.f by $$ \pi $$
  • Current is ahead of e.m.f by $$ \pi $$
A series ac circuit consist of an inductor and a capacitor. The inductance and capacitance is respectively $$1$$ henry and $$ 25\mu F.$$ If the current is maximum in circuit then angular frequency will be
  • $$ 200 $$
  • $$ 100 $$
  • $$ 50 $$
  • $$ 200 / 2\pi $$
In a series LCR circuit, operated with an ac of angular frequency $$ \omega $$  ,the total impedance is
  • $$ [R^2 + (L \omega - C \omega)^2]^{1/2} $$
  • $$ \left [ R^2 + \left (L \omega - \dfrac{1}{C \omega} \right )^2 \right ]^{1/2} $$
  • $$ \left [R^2 + \left (L \omega - \dfrac{1}{C \omega} \right )^2 \right ]^{-1/2} $$
  • $$ \left [(R \omega)^2 + \left (L \omega - \dfrac{1}{C \omega} \right )^2 \right ]^{1/2} $$
When $$ 100 $$ volt dc is applied across a coil, a current of $$1$$ amp flows through it. When $$ 100$$  volt ac at $$ 50$$ cycle $$s^{-1} $$ is applied to the same coil, only $$0.5$$  ampere current flows. The impedance of the coil is
  • $$ 100 \Omega $$
  • $$ 200 \Omega $$
  • $$ 300 \Omega $$
  • $$ 400 \Omega $$
If resistance of $$ 100 \,\Omega $$  inductance of $$ 0.5 $$  henry and capacitance of $$ 10 \times 10^{-6} F $$  are connected in series through $$ 50 $$ Hz ac supply, then impedance is
  • $$ 1.876 $$
  • $$ 18.76 $$
  • $$ 189.72 $$
  • $$ 101.3 $$
In a LCR circuit having $$ L = 8.0 $$  henry, $$ C = 0.5 \mu F $$ and $$ R = 100$$ ohm in series. The resonance frequency in per second is
  • $$ 600 $$ radian
  • $$ 600 $$ Hz
  • $$ 500 $$ radian
  • $$ 500 $$ Hz
The frequency for which a $$ 5 \mu F $$ capacitor has a reactance of $$ \dfrac{1}{1000} $$ ohm is given by
  • $$ \dfrac{100}{\pi} \, MHz $$
  • $$ \dfrac{1000}{\pi} \, Hz $$
  • $$ \dfrac{1}{1000} \,Hz $$
  • $$ 1000 \, Hz $$
If an $$ 8\, \Omega $$  resistance and $$ 6 \, \Omega $$  reactance are present in an ac series circuit then the impedance of the circuit will be
  • $$ 20 \, ohm $$
  • $$ 5 \, ohm $$
  • $$ 10 \, ohm $$
  • $$ 14 \sqrt{2} \, ohm $$
Which of the following components of a LCR circuit with ac supply dissipates energy
  • L
  • R
  • C
  • All of these
In series $$LCR$$ circuit, in the condition of resonance, if $$C=1\mu F;L=1H$$ then frequency will be:
  • $${ 10 }^{ 6 }Hz$$
  • $$2\pi \times { 10 }^{ 6 }Hz$$
  • $$\cfrac { { 10 }^{ 6 } }{ 2\pi } Hz$$
  • $$2\pi \times { 10 }^{ -6 }Hz$$
An alternating current circuit is in resonance at $$10kHz$$ frequency. If frequency increases to $$12kHz$$, then what will be effect on impedance?
  • remains unchanged
  • increases $$1.2$$ times
  • increases and becomes capacitive
  • increases and becomes inductive
Which of the following is used in a circuit which shows that the current lead the voltage at the phase:
  • pure resistance
  • pure inductance
  • pure capacitor
  • none of these
In pure inductive circuit, the curves between frequency f and reciprocal of inductive reactance $$ 1/ X_L $$ is
An oscillator circuit consists of an inductance of $$0.5 \,mH$$ and a capacitor of $$20 \mu F$$. The resonant frequency of the circuit is nearly
  • $$15.92 \,Hz$$
  • $$159.2 \,Hz$$
  • $$1592 \,Hz$$
  • $$15910 \,Hz$$
A resistance of $$ 40 $$ ohm and an inductance of $$ 95.5 $$ millihenry are connected in series in a $$ 50 $$ cycles/second ac circuit. The impedance of this combination is very nearly 
  • $$ 30 \,ohm $$
  • $$ 40 \, ohm $$
  • $$ 50 \, ohm $$
  • $$ 60 \, ohm $$
Which of the following curves correctly represents the variation of capacitive reactance $$ X_C $$  with frequency f
What is the rms voltage?
  • $$\sqrt{2}\Delta V_{max}$$
  • $$\Delta V_{max}$$
  • $$\Delta V_{max}/\sqrt{2}$$
  • $$\Delta V_{max}/2$$
  • $$\Delta V_{max}/4$$
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