Alternating Current
EXERCISE
MULTIPLE CHOICE QUESTIONS
Choose the best possible answer:
1. The r.m.s value of a sine wave is 100 A. Its peak value is
A. 70.7 A
B. 141.4 A
C. 150 A
D. 282.8 A
2.A sine wave has a frequency of 50 Hz. Its angular frequency is..... rad/s.
A. 100л
B. 50л
C. 25π
d. 5π
3. The power factor of a purely resistive circuit is
A. zero
B. unity
C. lagging
D. leading
4. A capacitor is perfectly insulator for
A. Direct current
B. Alternating current
C. Direct as well as alternating current
D. Induced e.m.f
5. The peak value of alternating current is 5√2A. The mean square value of current will be
A. 5 A
B. 2.5 A
C. 52A D.5²A
6. In choke coil the reactance X, and resistance R are related as
A. XL = R
B. XL<<R
C. XL >> R
D. X =∞
7. The phase difference between the current and voltage at resonance is
A. 0°
B. π
C. -π
D. π/2
8. In AC system we generate sine wave form because
A. It can be easily draw
B. It produces least disturbance in electrical circuits
C. It is nature standard
D. Other waves cannot be produced easily
9. An alternating voltage is given by 20sin 157t. The frequency of alternating voltage is
A. 50 Hz
B. 25 Hz
C. 100 Hz
D. 75 Hz
10. In LR circuit which one of the following statement is correct?
A. Land R opposes each other
B. R value increases with frequency
C. The inductive reactance increases with frequency
D. The inductive reactance decreases with frequency
11. An alternating quantity (voltage or current) is completely known if we know It's
A. maximum value
B. frequency and phase
C. effective value
D. both (a) and (b)
12. For electromagnetic waves, Maxwell generalized
A. Gauss's law for magnetism
B. Gauss's law for electricity
C. Faraday's law
D. Ampere's law
13. An electromagnetic wave goes from air to glass which of the following does not change?
A. Radio waves
B. X-rays
C. Ultraviolet radiation
D. Ultrasound waves
14. The circuit in which current and voltage are in phase, the power factor is
A. Zero
B. 1
C.-1
D. 2
15. with increase in frequency of an AC supply, the impedance of an RLC series
circuit
A. remains constant
B. increases
c. decreases
D. capacitive reactance Xc decreases only
CONCEPTUAL QUESTIONS
1. Sketch a graph of e.m.f induced in an inductive coil against rate of change of current. What is the significant of the gradient?
Ans. When an alternating current is flowing through an inductor, or the amount of current flowing through the inductor changes, an e.m.f is induced in the induc tor. It is found that the e.m.f and change in the current are directly proportional to one another.
Thus, if we draw a graph b/w emf Ɛ and Al/At, we get a straight line as shown in the figure. The gradient of
the graph is given by
Mathematically:
Gradient = Ay
Δ.Χ.
In case of rate of change of current and induce e.m.f in the inductor, the gradient is;
Gradient
ΔΙ At
Now consider the proportionality of induced e.m.f and rate of change of current
in the conductor;
⇒ 8=14
ΔΙ
AV
Therefore, by comparison; Gradient L
At Significance of Gradient: The significance of the gradient is that it gives us the value of self-inductance of the coil. Larger the gradient, greater will be the induct ance of the coil and vice versa.
2. Current and voltage provided by an AC generator are sometimes negative and sometimes positive. Why for, an AC generator connected to a resistor, power can never be negative?
Ans. When a resistor of resistance R is connected across an A.C. generator, the
instantaneous sinusoidal voltage and instantaneous sinusoidal current across R is
given by;
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