CBSE Questions for Class 12 Medical Physics Alternating Current Quiz 15 - MCQExams.com

The equation of an alternating voltage is $$V = 220\sin \left [\omega t + \dfrac {\pi}{6}\right ]$$ and the equation for current is $$I = 10\sin \left (\omega t + \dfrac {\pi}{6}\right )$$. The impedance (in ohm) of the circuit is
  • $$11$$
  • $$44$$
  • $$20$$
  • $$22$$
Find the resonant frequency and $$Q-factor$$ of a series $$LCR$$ circuit with $$L = 3.0\ H, C = 27\mu F$$ and $$R = 7.4\Omega$$.
  • $$111\ rad/s, 45$$
  • $$111\ rad/s, 90$$
  • $$55.5\ rad/s, 45$$
  • $$55.5\ rad/s, 90$$
In an inductor, the current l varies with time t as i=5A+16 (A/s) t. If induced emf in the inductor is 5 mV, the self inductance of the inductor is 
  • $$3.75\times 10^{-3}H$$
  • $$3.75\times 10^{-4}H$$
  • $$3.125\times 10^{-3}H$$
  • $$3.125\times 10^{-4}H$$
A magnet is moving towards the coil  along the axis and the emf induced tin the coil is 'e' if the coil also starts moving towards the magent with same speed.  the induced emf will be 
  • $$\frac{e}{2}$$
  • e
  • 2e
  • 4e
A condenser of capacity $$C$$ is charged to a potential difference of $$V_1$$. The plates of the condenser are then connected to an ideal inductor of inductance $$L$$. The current through the inductors when then potential difference across the condenser reduces to $$V_2$$
  • $$ \frac{\mathrm{C}\left(\mathrm{V}_{1}^{2}-\mathrm{V}_{2}^{2}\right)}{\mathrm{L}} $$
  • $$ \frac{C\left(V_{1}^{2}+V_{2}^{2}\right)}{L} $$
  • $$ \left(\frac{C\left(V_{1}^{2}-V_{2}^{2}\right)}{L}\right)^{1 / 2} $$
  • $$ \left(\frac{a v_{1}-v_{2} p^{2}}{L}\right)^{1 / 2} $$
For an alternating current of frequency $$\dfrac { 5 }{ \pi } Hz$$ in $$L-C-R$$ series circuit with $$L-1H,C-1{ \mu }{ F }_{ R }-100 { \Omega }$$ impedance is :-
  • $${ 100 }{ \Omega }$$
  • $${ 100 }{\sqrt \pi \Omega }$$
  • $${ 100 }{\sqrt 2\pi \Omega }$$
  • $${ 100 }{ z\Omega }$$
yf the frequency of the source e.m.f. in an AC circuit is $$n$$ , the power varies wath a frequency 
  • $$n$$
  • 2$$n$$
  • $$n / 2$$
  • zero
For a series LCR circuit at resonance, the statement which  is not true:-
  • Peak energy stored by a capacitor $$=$$ peak
    energy stored by an inductor
  • Average power $$=$$ apparent power
  • Wattless current is zero
  • Power factor is one
An alternating source of frequency $$\nu$$ is connected to a series LCR-circuit. The graph that best represents the variation of the current $$I$$ with the frequency of the applied ac is
Two circular coils can be arranged in any of the three situations shown in the figure. Their mutual inductance will be
1452523_be7c533c658344f699a0fc6e277d286b.png
  • maximum in situation (a)
  • maximum in situation (b)
  • maximum in situation (c)
  • the same in all situation
An LCR circuit as shown in the figure is connected to a voltage soruce $$V_{ac}$$ whose frequency one be varied.
The frequency, at which the voltage across the resistor is maximum, is :
1441369_1db689a2f4ed4a1789dfa78107043116.png
  • $$902 Hz$$
  • $$143 Hz$$
  • $$23 Hz$$
  • $$345 Hz$$
As the frequency of an alternating current increases, the impedance of the circuit
  • increase continuously
  • decreases continuously
  • remains constant
  • None of these
Two inductors $${L}_{1}$$ and $${L}_{2}$$ are connected in parallel and a time varying current $$i$$ flows as shown. The ratio of currents $${i}_{1}/{i}_{2}$$ at any time $$t$$ is
1547027_92b37b6b62554fdf86b1bacd3ef59f58.png
  • $${L}_{1}/{L}_{2}$$
  • $${L}_{2}/{L}_{1}$$
  • $$\cfrac { { L }_{ 1 }^{ 2 } }{ { \left( { L }_{ 1 }+{ L }_{ 2 } \right) }^{ 2 } } $$
  • $$\cfrac { { L }_{ 2 }^{ 2 } }{ { \left( { L }_{ 1 }+{ L }_{ 2 } \right) }^{ 2 } } $$
In a  $$R - L - C$$  series circuit shown, the readings of voltmeters  $$V _ { 1 }$$  and  $$V _ { 2 }$$  are  $$100 V$$  and  $$120 V .$$  Choose the correct statement  $$( s ).$$ 
1544598_50360299e49a4034a0969c46e834061a.png
  • Voltage across resistor, inductor and capacitor are $$50{ V },86.6{ V }$$ and $$206.6{ V }$$ respectively.
  • Voltage across resistor, inductor and capacitor are $$10 V , 90 V$$ and $$30 V$$ respectively.
  • Power factor of the circuit is $$\dfrac { 5 } { 13 }$$
  • None of these
The resistance of a coil for dc is $$10 $$ ohm. When ac is sent through it, its resistance would be
  • $$A= 10 ohm$$
  • $$A< 10 ohm$$
  • $$A > 10 ohm$$
  • $$A= 5 ohm$$
On decreasing the angular frequency of A.C. source used in $$L-C-R$$ series circuit, the capacitive reactance____________ and inductive reactance _________ respectively.
  • Increases, Decreases
  • Increases, Increases
  • Decreases, Increases
  • Decreases, Decreases
Two long straight wires equal cross-sectional radii a are located parallel to each other in air .The distance between their axes equal b. Find the mutual capacitance of the wire per unit length under the condition b>>a
  • $$C=\dfrac { { 2\pi \varepsilon }_{ 0 } }{ ln\left( b/a \right) } $$
  • $$C=\dfrac { { \pi \varepsilon }_{ 0 } }{ ln\left( a/b \right) } $$
  • $$C=\pi /{ \varepsilon }_{ 0 }In\left( b/a \right) $$
  • None of these
An alternating emf is applied across a parallel combination of a resistance  $$R,$$  capacitance  $$C$$  and an inductance  $$L.$$  If  $$I _ { R } ,  I _ { L } , I _ { C }$$  are the currents through  $$R ,$$   $$L$$   and  $$C ,$$  respectively, then the diagram which correctly represents, the phase relationship among  $$I _ { R } ,  I _ { L } , I _ { C }$$  and source emf  $$E,$$  is given by
The equation P of an ac is represented by I=0.6 sin 100 $$\pi t$$. The frequency of ac is
  • $$50 \pi$$
  • $$50$$
  • $$100 \pi$$
  • $$100$$
In a series LCR circuit $$R=200\Omega $$ and the voltage and the frequency of the main supply is 220 V and 50 Hz respectively. On talking out the capacities from the circuit the current lags behind the voltage by $${ 30 }^{ \circ  }$$ On taking out the induct or from the circuit the current leads the voltage by $${ 30 }^{ \circ  }$$ The power dissipated in the LCR circuit is  
  • 305 W
  • 210 W
  • 0 W
  • 242 W
In an A.C circuit, a resistance of R ohm is connected in series with an indutance L.If phase angle between voltage and current be$${ 45 }^{ 0 }$$, the value of inductive reacatance  will be  
  • R/4
  • R/2
  • R
  • R/5
The average power of A.C. lost per cycle is given by?
  • $$\dfrac{1}{2}E_0i_0\sin\phi$$
  • $$\dfrac{1}{2}E_0i_0\cos\phi$$
  • $$\dfrac{1}{2}E_0i_0\tan\phi$$
  • $$\dfrac{1}{2}E_0i_0\phi$$

In the R-C series A.C. circuit, current is by $$\delta $$ than voltage. 
  • $$ahead,Where\;\delta = {\tan ^{ - 1}}\left( {\dfrac{1}{{\omega CR}}} \right)$$
  • $$behind,Where\;\delta = {\tan ^{ - 1}}\left( {\dfrac{1}{{\omega CR}}} \right)$$
  • $$ahead,Where\;\delta = {\tan ^{ - 1}}\left( {\dfrac{{\omega C}}{R}} \right)$$
  • $$behind,Where\;\delta = {\tan ^{ - 1}}\left( {\dfrac{{\omega C}}{R}} \right)$$
A choke coil is preferred over a resistor in ac circuit because 
  • it is cheaper than a resistor
  • it lasts longer than a resistor
  • the resistor may over heat and melt
  • it brings about a voltage drop with negligible power consumption
The resonance frequency of a certain RLC series circuit is $$ \omega_o $$. A source of angular frequency  $$ 2 \omega_o $$ is inserted into the circuit.After transients die out, the angular frequency of current oscillation is
  • $$ \dfrac {\omega_o}{2} $$
  • $$ \omega_o $$
  • $$ 2 \omega_o $$
  • $$ 1.5 \omega_o $$
An A.C. circuit consists of a resistance and a choke in series. The resistance is of $$220$$ $$\Omega$$ and choke is of $$0.7$$ henry. The power absorbed from $$220$$ volts and $$50$$ Hz, source connected with the circuit, is?
  • $$120.08$$ watt
  • $$109.97$$ watt
  • $$100.08$$ watt
  • $$98.08$$ watt
In the circuit diagram shown, $$X_c= 100\Omega$$, $$X_L= 200\Omega$$ and $$R= 100\Omega$$. The effective current through the source is
1578634_302d2bdf56a34b79bd0c37f108ab1a15.png
  • $$2A$$
  • $$\sqrt2 $$ A
  • $$0.5 $$ A
  • $$2\sqrt2$$ A
An ac source of frequency $$ \omega $$ when fed into an RC series circuit, current is recorded to be I. If now frequency is charged to $$ \frac {\omega}{3} $$ (keeping voltage same) the circuit is found to be I/2.The ration of reactance to resistance at original frequency $$ \omega $$ is
  • 2
  • $$ \frac {1}{2} $$
  • $$ \frac { 1 }{ \sqrt { 2 } } $$
  • $$ \sqrt {3\5}$$
The growth of current in two series LR circuits $$A$$ and $$B$$ is shown in $$C$$. Let $$L_1, R_1$$ and $$R_2$$ be the corresponding inductor, resistor values in the two circuits. Then
1567994_4ce77f6ddd804970be8ebfd2c9f6d879.1567994-Q
  • $$R_1 > R_2$$
  • $$R_1 = R_2$$
  • $$L_1 > L_2$$
  • $$L_1 < L_2$$
In a series $$LCR$$ circuit which is connected to an $$AC$$ voltage source, choose the incorrect statement 
  • Algebraic sum of instantaneous voltage across $$L, C, R$$ is a variable
  • $$V_{Linst} + V_{Cinst} + V_{Rinst} = V_{source inst}$$
  • Voltage across inductor, capacitor, resistance behave as a vector
  • Current is same in inductor, capacitor and resistance
In fig. $$10.43$$, which voltmeter reads zero, when $$\omega$$ is equal to the resonant frequency of series $$LCR$$ circuit : 
1574769_7f07e1165b7e442f9e2c74d3b57ac8ea.png
  • $$V_1$$
  • $$V_2$$
  • $$V_3$$
  • None of these
In the series LCR circuit (Fig. 7.33)  , the voltmeter and ammeter readings are : 
1749340_e68fae2d7e284ebda42d20aa8f3b8678.jpg
  • $$ V = 100 \,V , I = 2A $$
  • $$ V = 100 \,V , I = 5A $$
  • $$ V = 1000 \,V , I = 2A $$
  • $$ V = 300 \,V , I = 1A $$
In an AC series circuit, the instantaneous current is zero when the instantaneous voltage is maximum. Connected to the source may be a 
  • pure inductor
  • pure capacitor
  • pure resistor
  • combination of an inductor and a capacitor
In the series LCR circuit, the voltmeter and ammeter readings are respectively.
1705013_d6e426adabca488c8df19264bf2fe561.png
  • $$V=250V, I=4A$$
  • $$V=150V, I=2A$$
  • $$V=1000V, I=5A$$
  • $$V=100V, I=2A$$
The alternating current is given by the equation $$i = 10\sin \left ( 100 \pi t + \dfrac{\pi}{6} \right ).$$ The current attains its first maximum at t is :
  • $$\dfrac{1}{600}s$$
  • $$\dfrac{1}{50}s$$
  • $$\dfrac{1}{100}s$$
  • $$\dfrac{1}{300}s$$
In a radio receiver, the short wave and medium wave station are tuned by using the same capacitor but coils of difference inductance $$L_s$$ and $$L_m$$ respectively then 
  • $$L_s > L_m$$
  • $$L_s < L_m$$
  • $$L_s = L_m$$
  • $$None\ of\ these$$
How can you decrease current in an alternating current circuit without any loss of power?
  • By using pure inductor
  • By using pure resistance
  • By using resistance and inductor
  • By using resistance and capacitor
In an A.C. circuit, a resistance of $$3 \Omega $$, an inductance coil of $$4 \Omega $$ and a condenser of $$ 8 \Omega $$ are connected in series with an A.C. source of $$50$$ volts (R.M.S). The average power loss in the circuit will be
  • $$600$$ watt
  • $$500$$ watt
  • $$400$$ watt
  • $$300$$ watt
In an LCR series circuit , at resonance
  • the current and voltage are in phase
  • the impedance is maximum
  • the current is minimum
  • the quality factor is independent of $$R$$
  • the current leads the voltage by $$ \dfrac{\pi}{2} $$
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