CBSE Questions for Class 12 Medical Physics Alternating Current Quiz 6 - MCQExams.com

In an $$AC$$ circuit, the impedance is $$\sqrt 3$$ times the reactance, then the phase angle is
  • $$60^o$$
  • $$30^o$$
  • zero
  • none of these
In a L-C-R circuit, as the frequency of an alternating current increases the impedance of the circuit
  • increases continuously.
  • decreases continuously.
  • remains constant.
  • None of these.
A circuit contains resistance $$R$$ and an inductance $$L$$ in series. An alternating voltage $$V=V_0\sin \omega t$$ is applied across it. The currents in $$R$$ and $$L$$ respectively will be

219012.png
  • $$I_R=I_0\cos \omega t, I_L=I_0\cos \omega t$$
  • $$I_R=-I_0\sin \omega t, I_L=I_0\cos \omega t$$
  • $$I_R=I_0\sin \omega t, I_L=-I_0\cos \omega t$$
  • None of the above.
An $$AC$$ voltage is applied across a series combination of $$L$$ and $$R$$. If the voltage drop across the resistor and inductor are $$20V$$ and $$15V$$ respectively, then applied peak voltage is :
  • $$25V$$
  • $$35V$$
  • $$25\sqrt 2V$$
  • $$5\sqrt 7V$$
The impedance of a series $$L-C-R$$ circuit in an $$AC$$ circuit is
  • $$\displaystyle \sqrt{R+(X_L-X_C)}$$
  • $$\displaystyle \sqrt{R^2+(X^2_L-X^2_C)}$$
  • $$R$$
  • None of these
In the $$AC$$ network shown in figure, the rms current flowing through the inductor and capacitor are $$0.6A$$ and $$0.8A$$ respectively. Then the current coming out of the source is

222155_b2cb721195f6471984a3d163b5f88943.png
  • $$1.0A$$
  • $$1.4A$$
  • $$0.2A$$
  • None of the above
In a parallel $$L-C-R$$ circuit as shown in figure if $$I_R, I_L, I_C$$ and $$I$$ represents the rms values of current flowing through resistor, inductor, capacitor and the source, then choose the appropriate correct answer

222144.JPG
  • $$\displaystyle I=I_R+I_L+I_C$$
  • $$\displaystyle I=I_R+I_L-I_C$$
  • $$I_L$$ or $$I_C$$ may be greater than $$I$$
  • None of the above
In a series $$LC$$ circuit, the applied voltage is $$V_0$$. If $$\omega$$ is very low, then the voltage drop across the inductor $$V_L$$ and capacitor $$V_C$$ are

222187_17f17d57e257462cb6e457da2b0cabea.png
  • $$\displaystyle V_L=\frac{V_0}{2}; V_C=\frac{V_0}{2}$$
  • $$\displaystyle V_L=0; V_C=V_0$$
  • $$\displaystyle V_L=V_0; V_C=0$$
  • $$\displaystyle V_L=-V_C=\frac{V_0}{2}$$
If the rms current through a $$6.8 k \Omega$$ resistor is $$8 mA$$, the rms voltage drop across the resistor is
  • $$5.44 V$$
  • $$54.4 V$$
  • $$7.07 V$$
  • $$8 V$$
In a series $$L-C-R$$ circuit, current in the circuit is $$11A$$ when the applied voltage is $$220V$$. Voltage across the capacitor is $$200V$$. If value of resistor $$20\Omega$$, then the voltage across the unknown inductor is
  • Zero
  • $$200V$$
  • $$20V$$
  • None of these
The adjoining figure shows an $$AC$$ circuit with resistance $$R$$, inductance $$L$$ and source voltage $$V_s$$. Then

222117_d8743c5b5c144b5d9645ef8ca139a0d1.png
  • the source voltage $$V_s=72.8V$$.
  • the phase angle between current and source voltage is $$\tan^{-1}(7/2)$$.
  • Both $$(a)$$ and $$(b)$$ are correct.
  • Both $$(a)$$ and $$(b)$$ are wrong.
In series $$L-C-R$$ circuit voltage drop across resistance is $$8V$$, across inductor is $$6V$$ and across capacitor is $$12V$$. Then
  • Voltage of the source will be leading in the circuit.
  • Voltage drop across each element will be less than the applied voltage.
  • Power factor of the circuit will be $$3/4$$.
  • None of the above.
In the circuit shown in figure, the $$AC$$ source gives a voltage $$V=20\cos (2000t)$$. Neglecting source resistance, the voltmeter and ammeter readings will be

219024.png
  • $$0 V, 2.0A$$
  • $$0V, 1.4A$$
  • $$5.6V, 1.4A$$
  • $$8V, 2.0A$$
An $$AC$$ voltage source $$V=V_0\sin \omega t$$ is connected across resistance $$R$$ and capacitance $$C$$ as shown in the figure. It is given that $$R=1/\omega C$$ and the peak current is $$I_0$$. If the angular frequency of the voltage source is changed to $$\omega/\sqrt 3$$ then the new peak current in the circuit is

222195.png
  • $$\displaystyle \frac{I_0}{2}$$
  • $$\displaystyle \frac{I_0}{\sqrt 2}$$
  • $$\displaystyle \frac{I_0}{\sqrt 3}$$
  • $$\displaystyle \dfrac{I_0}{3}$$
A resistor and an inductor are connected to an ac supply of $$120\space V$$ and $$50\space Hz$$. The current in the circuit is $$3\space A$$. If the power consumed in the circuit is $$108\space W$$, then the resistance in the circuit is
  • $$12\space\Omega$$
  • $$40\space\Omega$$
  • $$\sqrt{(52\times28)}\space\Omega$$
  • $$360\space\Omega$$
When $$100\space V$$ dc is applied across a solenoid, a current of $$1.0\space A$$ flows in it. When $$100\space V$$ ac is applied across the same coil, the current drops to $$0.5\space A$$. If the frequency of the ac source is $$50\space Hz$$, the impedance and inductance of the solenoid are
  • $$200\Omega\space and\space 0.55\space H$$
  • $$100\Omega\space and\space 0.86\space H$$
  • $$200\Omega\space and\space 1.0\space H$$
  • $$200\Omega\space and\space 0.93\space H$$
An inductive coil has resistance of $$100\Omega$$. When an ac signal of frequency $$1000\space Hz$$ is fed to the coil, the applied voltage leads the current by $$45^{\small\circ}$$. What is the inductance of the coil?
  • $$2\space mH$$
  • $$3.3\space mH$$
  • $$16\space mH$$
  • $$\sqrt5\space mH$$
A $$220V$$, $$50\space Hz$$ AC generator is connected to an inductor and a $$50\space \Omega$$ resistance in series. The current in the circuit is $$1.0\space A$$. What is the Potential difference across inductor?
  • $$102.2\space V$$
  • $$186.4\space V$$
  • $$214\space V$$
  • $$170\space V$$
An alternating voltage $$E = 50\sqrt2\sin(100t)\space V$$ is connected to a $$1\space \mu F$$ capacitor through an ac ammeter. What will be the reading of the ammeter?
  • $$10\space mA$$
  • $$5\space mA$$
  • $$5\sqrt2\space mA$$
  • $$10\sqrt2\space mA$$
A $$50\space W$$, $$100\space V$$ lamp is to be connected to an AC mains of $$200\space V, \space 50\space Hz$$. What capacitor is essential to be put in series with the lamp?
  • $$\displaystyle\frac{25}{\sqrt2}\mu F$$
  • $$\displaystyle\frac{50}{\pi\sqrt3}\mu F$$
  • $$\displaystyle\frac{50}{\sqrt2}\mu F$$
  • $$\displaystyle\frac{100}{\pi\sqrt3}\mu F$$
A coil has an inductance of $$0.7\space H$$ and is joined in series with a resistance of $$220\Omega$$. When an alternating emf of $$220\space V$$ at $$50\space cps$$ is applied to it, then the wattless component of the current in the circuit is (take $$0.7\pi = 2.2)$$
  • $$5\space A$$
  • $$0.5\space A$$
  • $$0.7\space A$$
  • $$7\space A$$
A current source sends a current $$I - i_0\cos(\omega t)$$. when connected across an unknown load, it gives a voltage output of $$v = v_0\sin[\omega t + (\pi/4)]$$ across that load. then the voltage across the current source may be brought in phase with the current through it by
  • Connecting an inductor in series with the load
  • Connecting a capacitor in series with the load
  • Connecting an inductor in parallel with the load
  • Connecting a capacitor in parallel with the load
The maximum value of a.c. voltage in a circuit is 707V. Its r.m.s. value is
  • 70.7 V
  • 100 V
  • 500 V
  • 707 V
For an $$LCR$$ series circuit with an A.C. source of angular frequency $$\omega$$, which statement is correct?
  • Circuit will be capacitive if $$\omega > \displaystyle\frac{1}{\sqrt{LC}}$$
  • Circuit will be capacitive if $$\omega = \displaystyle\frac{1}{\sqrt{LC}}$$
  • Power factor of circuit will be unity if capacitive reactance equals inductive reactance
  • Current will be leading voltage if $$\omega > \displaystyle\frac{1}{\sqrt{LC}}$$
In $$LCR$$ series $$AC$$ circuit
  • If $$R$$ is increased current will decrease.
  • If $$L$$ is increased current will decrease.
  • If $$C$$ is increased current will increase.
  • If $$C$$ is increased current will decrease.
Current in an ac circuit is given by $$I = 3\sin\omega t + 4\cos\omega t$$, then
  • RMS value of current is $$5\space A$$
  • Mean value of this current in any one half period will be $$6\pi$$
  • If voltage applied is $$V = V_m\sin\omega t$$, then the circuit may be containing resistance and capacitance only
  • If voltage applied is $$V = V_m\sin\omega t$$, the circuit may contain resistance and inductance only
In an ac circuit, the potential differences across an inductance and resistance joined in series are, respectively, $$16\space V$$ and $$20\space V$$. The total potential difference across the circuit is
  • $$20\space V$$
  • $$25.6\space V$$
  • $$31.9\space V$$
  • $$53.5\space V$$
In an ideal parallel LC circuit, the capacitor is charged by connecting it to a DC source which is then disconnected. The current in the circuit
  • becomes zero instantaneously.
  • grows monotonically.
  • decays monotonically.
  • oscillates instantaneously.
What is the value of inductance L for which the current is a maximum in a series LCR circuit with $$C=10 \mu F $$ and $$\omega  = 1000s^{-1}$$ ?
  • $$10 mH$$
  • $$100mH$$
  • $$1 mH$$
  • cannot be calculated unless R is known
Voltage across each elements of a series LCR circuit are given by $$V_L= 60V, V_C = 20V, V_R= 30V$$ Find out source voltage
  • 50V
  • 100V
  • 150V
  • 200V
A coil has resistance $$30   ohm$$ and inductive reactance $$20   ohm$$ at $$50   Hz$$ frequency. If an $$ac$$ source of $$200   volt,  100   Hz$$, is connected across the coil, the current in the coil will be
  • $$4.0 A$$
  • $$8.0 A$$
  • $$\displaystyle\frac{20}{\sqrt{13}} A$$
  • $$2.0 A$$
A coil of self-inductance L is connected in series with a bulb B and an AC source. Brightness of the bulb decreases when
  • Frequency of the AC source is decreased
  • Number of turns in the coil is reduced
  • A capacitance of reactance $$X_{C}=X_{L}$$ is included in the same circuit
  • An iron rod is inserted in the coil
An a.c. supply of 100 volts is applied to a capacitor of capacitance 20 $$\mu$$ F. If the current in the circuit is 0.628 A, the frequency of a.c. must be
  • 50 Hz
  • 60 Hz
  • 25 Hz
  • 40 Hz
In a circuit L, C and R are connected in series with an alternating voltage source of frequency f. The current leads the voltage by 45$$^o$$. The value of C is
  • $$\displaystyle \frac{1}{2 \pi f (2 \pi f L - R)}$$
  • $$\displaystyle \frac{1}{2 \pi f ( 2\pi f L + R)} $$
  • $$\displaystyle \frac{1}{ \pi f (2 \pi f L - R)}$$
  • $$\displaystyle \frac{1}{ \pi f (2 \pi f L + R)}$$
In an A.C. circuit, the current flowing in inductance is $$\displaystyle I=5\sin { \left( 100t-{ \pi  }/{ 2 } \right)  } $$ ampers and the potential difference is V = 200 sin (100 t) volts. The power consumption is equal to 
  • 1000 watt
  • 40 watt
  • 20 watt
  • Zero
Assertion: A capacitor blocks direct current in the steady state. 
Reason : The capacitive reactance of the capacitor is inversely proportional to frequency f of the source of emf. 
  • If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.
  • If both Assertion and Reason are true but the Reason is not the correct explanation of the Assertion.
  • If Assertion is true statement but Reason is false.
  • If both Assertion and Reason are false statements.
If a cell of $$200V$$ is connected across an inductor, the current is found to be $$5A$$. When cell is replaced by $$200 V - 100 rad/s$$ supply, r.m.s. value of current becomes $$4A$$. The inductance of inductor is-
  • $$0.2$$ henry
  • $$0.3$$ henry
  • $$0.4$$ henry
  • $$0.5$$ henry
Assertion: In the purely resistive element of a series LCR, AC circuit the maximum value of rms current increases with increase in the angular frequency of the applied e.m.f.
Reason: $$\displaystyle { I }_{ max }=\frac { { \varepsilon  }_{ max } }{ z } ,z=\sqrt { { R }^{ 2 }+{ \left( \omega L-\frac { I }{ \omega C }  \right)  }^{ 2 } } $$
where $$\displaystyle { I }_{ max }$$ is the peak current in a cycle. 
  • If both Assertion and Reason are correct and Reason is the correct explanation of Assertion.
  • If both Assertion and Reason are correct, but Reason is not the correct explanation of Assertion.
  • If Assertion is correct but Reason is incorrect.
  • If Assertion is incorrect but Reason is correct.
L, C, R represent physical quantities inductance, capacitance and resistance respectively. The combinations which have the dimensions of frequency are 
  • $$\displaystyle { 1 }/{ RC }$$
  • $$\displaystyle { R }/{ L }$$
  • $$\displaystyle { 1 }/{ \sqrt { LC } }$$
  • $$\displaystyle { C }/{ L }$$
The Current in resistance R at resonance is 
431641_8341e94cee3a41bd8a7789b1ce0ea4de.png
  • Zero
  • Minimum but finite
  • Maximum but finite
  • Infinite
A lamp is connected in series with a capacitor and an AC source. What happens if the capacity of the capacitor is reduced?
  • The lamp shines more brightly
  • The lamp shines less brigthly
  • There is no change in the brightness of the lamp
  • Brightness may increase or decrease depending on the frequency of the AC
If the inductance and capacitance are both doubled in L-C-R circuit, the resonant frequency of the circuit will :
  • Decrease to one-half of the original value
  • Decrease to one-fourth of the original value
  • Increase to twice the original value
  • Decrease to twice the original value
Resonance frequency of a circuit is $$f$$. If the capacitance is made $$4$$ times the initial value, then the resonance frequency will become
  • $$\dfrac{f}{2}$$
  • $$2f$$
  • $$f$$
  • $$\dfrac{f}{4}$$
The resonant frequency of an L-C circuit is
  • $$\dfrac {1}{2\pi \sqrt {LC}}$$
  • $$\dfrac {1}{2\pi} \sqrt {\dfrac {L}{C}}$$
  • $$\dfrac {1}{4\pi} \sqrt {\dfrac {L}{C}}$$
  • $$\dfrac {1}{2\pi} \sqrt {\dfrac {C}{L}}$$
An alternating emf given by equation $$E = 300 sin [(100\pi )t]$$ $$volt$$ is applied to a resistance $$100\ ohms$$. The rms current through the circuit is (in $$amperes$$):
  • $$\displaystyle \frac {3}{\sqrt 2}$$
  • $$\displaystyle \frac {9}{\sqrt 2}$$
  • $$3$$
  • $$\displaystyle \frac {6}{\sqrt 2}$$
220 V, 50 Hz, AC source is connected to an inductance of 0.2 H and a resistance of 20 $$\Omega$$ in series. What is the current in the circuit?
  • 3.33 A
  • 33.3 A
  • 5 A
  • 10 A
In L-C-R circuit power of 3 mH inductance and $$4\Omega$$ resistance, EMF $$E=4\,cos\,1000t$$ volt is applied. The amplitude of current is
  • $$0.8\mathring{A}$$
  • $$\dfrac{4}{7}\mathring{A}$$
  • $$1.0\mathring{A}$$
  • $$\dfrac{4}{\sqrt{7}}\mathring{A}$$
The impedance of a circuit, when a resistance $$R$$ and an inductor of inductance $$L$$ are connected in series in an AC circuit of frequency $$f$$, is
  • $$\sqrt { R+2{ \pi }^{ 2 }{ f }^{ 2 }{ L }^{ 2 } } $$
  • $$\sqrt { R+4{ \pi }^{ 2 }{ f }^{ 2 }{ L }^{ 2 } } $$
  • $$\sqrt { { R }^{ 2 }+4{ \pi }^{ 2 }{ f }^{ 2 }{ L }^{ 2 } } $$
  • $$\sqrt { { R }^{ 2 }+2{ \pi }^{ 2 }{ f }^{ 2 }{ L }^{ 2 } } $$
In a series LCR circuit, the voltage across the resistance, capacitance and inductance is $$10\ V$$ each. If the capacitance is short circuited the voltage across the inductance will be :
  • $$10\ V$$
  • $$10\sqrt{2}\ V$$
  • $$\dfrac{10}{\sqrt{2}} \ V$$
  • $$20\ V$$
In an LCR circuit inductance is changed from $$L$$ to $$\dfrac{L}{2}$$. To keep the same resonance frequency, $$C$$ should be changed to
  • $$2C$$
  • $$\dfrac{C}{2}$$
  • $$4C$$
  • $$\dfrac{C}{4}$$
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