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Alternating Current

116 Questions Start Allen Test
All Exams Allen
2000 Q1 Allen 6. ANALYTICAL QUESTIONS MCQ
Resonance occurs in a series LCR circuit when the frequency of the applied emf is 1000 Hz. Then
(a) when frequency = 900 Hz, then the current through the voltage source will be ahead of e.m.f. of the source
(b) the impedance of the circuit is minimum at f = 1000 Hz
(c) at resonance the voltage across L and current in C differ in phase by 180°
(d) If the value of C is doubled resonance occurs at f = 2000 Hz
A.
only a & b are correct
B.
a, b, c are correct
C.
c & d are correct
D.
all are correct
Q2 Allen 1. PEAK, AVERAGE AND RMS VALUE MCQ
What is the r.m.s. value of an alternating current which when passed through a resistor produces heat which is thrice of that produced by a direct current of 2 amperes in the same resistor :-
A.
6 amp
B.
2 amp
C.
3.46 amp
D.
0.66 amp
Q3 Allen 1. PEAK, AVERAGE AND RMS VALUE MCQ
The peak value of an alternating e.m.f. which is given by $E = E_{0} \cos\omega t$ is 10 volts and its frequency is 50 Hz. At time $t = \frac{1}{600}s$ , the instantaneous e.m.f. is
A.
10 V
B.
$5\sqrt{3}V$
C.
5 V
D.
1V
Q4 Allen 1. PEAK, AVERAGE AND RMS VALUE MCQ
The phase difference between current and voltage in an AC circuit is $\frac{\pi}{4}$ radian. If the frequency of AC is 50 Hz, then the phase difference is equivalent to the time difference:
A.
0.78 s
B.
15.7 ms
C.
2.5 s
D.
2.5 ms
Q5 Allen 1. PEAK, AVERAGE AND RMS VALUE MCQ
A current in circuit is given by $i = 3 + 4 \sin \omega t$ . Then the effective value of current is:
A.
5
B.
$\sqrt{7}$
C.
$\sqrt{17}$
D.
$\sqrt{10}$
Q6 Allen 1. PEAK, AVERAGE AND RMS VALUE MCQ
Incorrect statement are:
(a) A.C. meters can measure D.C also
(b) If A.C. meter measures D.C. there scale must be linear and uniform
(c) A.C. and D.C. meters are based on heating effect of current
(d) A.C. meter reads rms value of current
A.
a, b
B.
b, c
C.
c, d
D.
d, a
Q7 Allen 1. PEAK, AVERAGE AND RMS VALUE MCQ
The r.m.s. value of current for a variable current $i = i_{1} \cos \omega t + i_{2} \sin \omega t : -$
A.
$\frac{1}{\sqrt{2}}(i_{1} + i_{2})$
B.
$\frac{1}{\sqrt{2}}(i_{1} + i_{2})^{2}$
C.
$\frac{1}{\sqrt{2}}(i_{1}^{2} + i_{2}^{2})^{1/2}$
D.
$\frac{1}{2}(i_{1}^{2} + i_{2}^{2})^{1/2}$
Q8 Allen 1. PEAK, AVERAGE AND RMS VALUE MCQ
The relation between an A.C. voltage source and time in SI units is: $V = 120 \sin (100 \pi t) \cos (100 \pi t) \text{ volt}$ value of peak voltage and frequency will be respectively:-
A.
120 volt and 100 Hz
B.
$\frac{120}{\sqrt{2}}$ volt and 100 Hz
C.
60 volt and 200 Hz
D.
60 volt and 100 Hz
Q9 Allen 1. PEAK, AVERAGE AND RMS VALUE MCQ
If an A.C. main supply is given to be 220 V. What would be the average e.m.f. during a positive half cycle:-
A.
198 V
B.
386 V
C.
256 V
D.
None of these
Q10 Allen 1. PEAK, AVERAGE AND RMS VALUE MCQ
The hot wire ammeter measures :-
A.
D.C. current
B.
A.C. current
C.
None of above
D.
both (1) & (2)
Q11 Allen 1. PEAK, AVERAGE AND RMS VALUE MCQ
Frequency of A.C. in India is –
A.
45 Hz
B.
60 Hz
C.
50 Hz
D.
None of the above
Q12 Allen 1. PEAK, AVERAGE AND RMS VALUE MCQ
For an alternating current $I = I_{0} \cos \omega t$ , the rms value and peak value of current is:
A.
$I_{0}$ , $\frac{I_{0}}{\sqrt{2}}$
B.
$\frac{I_{0}}{\sqrt{2}}$ , $I_{0}$
C.
$I_{0}$ , $\frac{I_{0}}{2}$
D.
$2I_{0}$ , $\frac{I_{0}}{\sqrt{2}}$
Q13 Allen 1. PEAK, AVERAGE AND RMS VALUE MCQ
If a step-up transformer have turn ratio 5, frequency 50 Hz root mean square value of potential difference on primary 100 volts and the resistance of the secondary winding is 500 Ω then the peak value of voltage in secondary winding will be (the efficiency of the transformer is hundred percent)
A.
$500\sqrt{2}$
B.
$10\sqrt{2}$
C.
$50\sqrt{2}$
D.
$20\sqrt{2}$
Q14 Allen 2. SIMPLE AC CIRCUIT MCQ
A resonant A.C. circuit contains a capacitor of capacitance $10^{-6}$ F and an inductor of $10^{-4}$ H. The frequency of electrical oscillations will be :
A.
$10^{5}$ Hz
B.
10 Hz
C.
$\frac{10^{5}}{2\pi}$ Hz
D.
$\frac{10}{2\pi}$ Hz
Q15 Allen 2. SIMPLE AC CIRCUIT MCQ
A resistance of $300\Omega$ and an inductance of $\frac{1}{\pi}$ henry are connected in series to a A.C. voltage of 20 volts and 200 Hz frequency. The phase angle between the voltage and current is:-
A.
$\tan^{-1}\left(\frac{4}{3}\right)$
B.
$\tan^{-1}\left(\frac{3}{4}\right)$
C.
$\tan^{-1}\left(\frac{3}{2}\right)$
D.
$\tan^{-1}\left(\frac{2}{3}\right)$
Q16 Allen 2. SIMPLE AC CIRCUIT MCQ
The graphs given below depict the dependence of two reactive impedances $X_{1}$ and $X_{2}$ on the frequency of the alternating e.m.f. applied individually to them. We can then say that :
A.
$X_{1}$ is an inductor and $X_{2}$ is a capacitor
B.
$X_{1}$ is a resistor and $X_{2}$ is a capacitor
C.
$X_{1}$ is a capacitor and $X_{2}$ is an inductor
D.
$X_{1}$ is an inductor and $X_{2}$ is a resistor
Q17 Allen 2. SIMPLE AC CIRCUIT MCQ
A 12 ohm resistor and a 0.21 henry inductor are connected in series to an AC source operating at 20 volts, 50 cycle/second. The phase angle between the current and the source voltage is:
A.
$30^{\circ}$
B.
$40^{\circ}$
C.
$80^{\circ}$
D.
$90^{\circ}$
Q18 Allen 2. SIMPLE AC CIRCUIT MCQ
A 110 V, 60 W lamp is run from a 220 V AC mains using a capacitor in series with the lamp, instead of a resistor then the voltage across the capacitor is about:-
A.
110 V
B.
190 V
C.
220 V
D.
311 V
Q19 Allen 2. SIMPLE AC CIRCUIT MCQ
The resistance that must be connected in series with inductance of 0.2 H in order that the phase difference between current and e.m.f. may be $45^{\circ}$ when the frequency is 50 Hz, is:-
A.
6.28 ohm.
B.
62.8 ohm.
C.
628 ohm.
D.
31.4 ohm.
Q20 Allen 2. SIMPLE AC CIRCUIT MCQ
A student connects a long air cored - coil of manganin wire to a 100 V D.C. supply and records a current of 25 amp. When the same coil is connected across 100 V. 50 Hz a.c. the current reduces to 20 A, the reactance of the coil is :-
A.
4 Ω
B.
3 Ω
C.
5 Ω
D.
None
Q21 Allen 2. SIMPLE AC CIRCUIT MCQ
The graph shows variation of I with f for a series R-L-C network. Keeping L and C constant, if R decreases :

(a) Maximum current ( $I_{m}$ ) increases
(b) Sharpness of the graph increases
(c) Quality factor increases
(d) Band width increases
A.
a, b, c
B.
b, c, d
C.
c, d, a
D.
All
Q22 Allen 2. SIMPLE AC CIRCUIT MCQ
Alternating current is flowing in inductance L and resistance R. The frequency of source is $\omega / 2\pi$ . Which of the following statement is correct:
A.
For low frequency the limiting value of impedance is L.
B.
For high frequency the limiting value of impedance is $\omega$ L.
C.
For high frequency the limiting value of impedance is R.
D.
For low frequency the limiting value of impedance is $\omega$ L.
Q23 Allen 2. SIMPLE AC CIRCUIT MCQ
A bulb and a capacitor are connected in series to a source of alternating current. If its frequency is increased, while keeping the voltage of the source constant, then
A.
Bulb will give more intense light.
B.
Bulb will give less intense light.
C.
Bulb will give light of same intensity as before
D.
Bulb will stop radiating light.
Q24 Allen 2. SIMPLE AC CIRCUIT MCQ
In an A.C. circuit resistance and inductance are connected in series. The potential and current in inductance is:
A.
$V_{0} \sin \omega t$ , $\frac{V_{0}}{\omega L} \sin \omega t$
B.
$V_{0} \sin \omega t$ , $\frac{V_{0}}{\omega L} \sin (\omega t + \pi / 2)$
C.
$V_{0} \sin (\omega t + \pi / 2)$ , $\frac{V_{0}}{\omega L} \sin \omega t$
D.
$V_{0} \sin (\omega t + \pi / 2)$ , $\frac{V_{0}}{\omega L} \sin (\omega t - \pi / 2)$
Q25 Allen 2. SIMPLE AC CIRCUIT MCQ
An a.c. source of voltage V and of frequency 50 Hz is connected to an inductor of 2H and negligible resistance. A current of r.m.s value I flows in the coil. When the frequency of the voltage is changed to 400 Hz keeping the magnitude of V the same, the current is now :-
A.
8I in phase with V
B.
4I and leading by 90° from V
C.
$\frac{I}{4}$ and lagging by 90° from V
D.
$\frac{I}{8}$ and lagging by 90° from V