Wednesday, May 25, 2022

H C Verma solutions, ALTERNATING CURRENT, Chapter-39, OBJECTIVE-II, Concepts of Physics, Part-II

Alternating Current


Objective-II


     1.  An inductor, a resistor, and a capacitor are joined in series with an AC source. As the frequency of the source is slightly increased from a very low value, the reactance

(a) of the inductor increases

(b) of the resistor increases

(c) of the capacitor increases

(d) of the circuit increases. 


ANSWER: (a).  

 

EXPLANATION: The reactance is for the capacitor and inductor. Hence option (b) is not correct. Reactance for the capacitor is 

Xc =1/⍵C

and for inductor

Xi =⍵L

With the increase in frequency, ⍵ increases. Thus Xi will increase and Xc will decrease. Option (a) is correct, (c) is not correct.

The reactance of the circuit,

X =⍵L-1/⍵C

When ⍵ is increased from a very low value, the numerical value of X decreases. Hence option (d) is not correct. 





     2.  The reactance of a circuit is zero. It is possible that the circuit contains

(a) an inductor and a capacitor

(b) an inductor but no capacitor

(c) a capacitor but no inductor

(d) neither an inductor nor a capacitor.  


ANSWER: (a), (d).  

 

EXPLANATION: Reactance of a circuit,

X =Xc -Xi 

   =⍵L -1/⍵C

In case, ⍵L =1/⍵C

X =0.

That is even if there is a capacitor and an inductor, the reactance of the circuit may be zero. Option (a) is correct.

If there is only a capacitor or only an inductor, the reactance will have a certain value. Option (b) and (c) are not correct.

If there is neither a capacitor nor an inductor, the reactance will be zero. Option (d) is correct. 




     3.  In an AC series circuit, the instantaneous current is zero when the instantaneous voltage is maximum. Connected to the source may be a

(a) pure inductor 

(b) pure capacitor

(c) pure resistor

(d) combination of an inductor and a capacitor.     


ANSWER: (a), (b), (d).  

 

EXPLANATION: In a resistor, current and voltage are in the same phase when connected to an AC source. Hence option (c) is not correct. 

In an inductor, the current lags behind the emf by 90° while in a capacitor the current leads the emf by 90°, hence the given condition may occur here. Options (a) and (b) are correct. 

In a combination of inductor and capacitor, the current may lead or lag behind the emf depending upon the comparative value of their reactances. Hence the given condition may occur here. Option (d) is correct.    




     4.  An inductor coil having some resistance is connected to an AC source. Which of the following quantities have zero average value over a cycle?

(a) current

(b) induced emf in the inductor

(c) Joule heat

(d) magnetic energy stored in the inductor. 


ANSWER: (a), (b).  

 

EXPLANATION: The plot of current and induced emf w.r.t. to time are sinusoidal waves with some phase difference. Hence for each half-cycle, their values will be just opposite of the next half-cycle. Thus over a cycle average value of current or induced emf will be zero. Options (a) and (b) are correct. 

   Since Joule heat or magnetic energy stored in an inductor are proportional to the square of rms current, they will be always positive over a cycle, not zero. Options (c) and (d) are not correct.    




     5.  The AC voltage across a resistance can be measured using 

(a) a potentiometer

(b) a hotwire voltmeter

(c) a moving coil galvanometer

(d) a moving magnet galvanometer.      


ANSWER: (b).  

 

EXPLANATION: The AC voltage across a resistance changes sinusoidally with respect to time and so is the current. The principle behind a potentiometer, a moving coil galvanometer, and a moving magnet galvanometer use a function that is directly proportional to the current. So they can not be used to measure AC voltage. Options (a), (c) and (d) are not correct. 

In a hot wire voltmeter principle of Joule heating is used that is proportional to the square of the rms current. Hence it is used to measure AC voltage. Option (b) is correct.     




     6.  To convert mechanical energy into electrical energy, one can use 

(a) DC dynamo 

(b) AC dynamo 

(c) motor

(d) transformer.     


ANSWER: (a), (b).  

 

EXPLANATION: With the help of mechanical energy an inducting coil is rotated in a magnetic field to generate electrical energy. This is done either in a DC dynamo or AC dynamo. Options (a) and (b) are correct. 

   A motor works on the opposite principle. It is used to convert electrical energy into mechanical energy. Option (c) is not correct.  

    A transformer is used either to step up the available AC voltage or step down. It does not convert the form of energy. Option (d) is not correct.   




     7.  An AC source rated 100 V (rms) supplies a current of 10 A (rms) to a circuit. The average power delivered by the source

(a) must be 1000 W

(b) maybe 1000 W

(c) maybe greater than 1000 W

(d) maybe less than 100 W. 


ANSWER: (b), (d).  

 

EXPLANATION: Average power delivered by an AC source is given as, 

P =VᵣₘₛIᵣₘₛCos φ

With given data,

P =100*10*Cos φ

   =1000.Cos φ

Since the value of φ has a range of (-π/2 to π/2), so the cos φ remains between 0 to 1. Thus the average power supplied in this AC circuit is between 0 to 1000 W.

  Options (b) and (d) are correct.  

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Links to the Chapters




CHAPTER- 39- Alternating Current


CHAPTER- 34- Magnetic Field

CHAPTER- 29- Electric Field and Potential











CHAPTER- 28- Heat Transfer

OBJECTIVE -I







EXERCISES - Q51 to Q55


CHAPTER- 27-Specific Heat Capacities of Gases

CHAPTER- 26-Laws of Thermodynamics


CHAPTER- 25-CALORIMETRY

Questions for Short Answer

OBJECTIVE-I

OBJECTIVE-II


EXERCISES - Q-11 to Q-18


CHAPTER- 24-Kinetic Theory of Gases







CHAPTER- 23 - Heat and Temperature






CHAPTER- 17 - Light Waves




CHAPTER- 14 - Fluid Mechanics



CHAPTER- 13 - Fluid Mechanics


CHAPTER- 12 - Simple Harmonic Motion








CHAPTER- 11 - Gravitation




CHAPTER- 10 - Rotational Mechanics




CHAPTER- 9 - Center of Mass, Linear Momentum, Collision


CHAPTER- 8 - Work and Energy

Click here for → Question for Short Answers

Click here for → OBJECTIVE-I

Click here for → OBJECTIVE-II

Click here for → Exercises (1-10)

Click here for → Exercises (11-20)

Click here for → Exercises (21-30)

Click here for → Exercises (31-42)

Click here for → Exercise(43-54)

CHAPTER- 7 - Circular Motion

Click here for → Questions for Short Answer 

Click here for → OBJECTIVE-I

Click here for → OBJECTIVE-II

Click here for → EXERCISES (1-10)

Click here for → EXERCISES (11-20)

Click here for → EXERCISES (21-30)

CHAPTER- 6 - Friction

Click here for → Questions for Short Answer

Click here for → OBJECTIVE-I

Click here for → Friction - OBJECTIVE-II

Click here for → EXERCISES (1-10)

Click here for → Exercises (11-20)

Click here for → EXERCISES (21-31)

For more practice on problems on friction solve these- "New Questions on Friction".

---------------------------------------------------------------------------------

CHAPTER- 5 - Newton's Laws of Motion


Click here for → QUESTIONS FOR SHORT ANSWER

Click here for→Newton's Laws of Motion,Exercises(Q.No. 13 to 27)

-------------------------------------------------------------------------------

CHAPTER- 4 - The Forces

The Forces-

"Questions for short Answers"    


Click here for "The Forces" - OBJECTIVE-I


Click here for "The Forces" - OBJECTIVE-II


Click here for "The Forces" - Exercises


--------------------------------------------------------------------------------------------------------------

CHAPTER- 3 - Kinematics - Rest and Motion

Click here for "Questions for short Answers"


Click here for "OBJECTIVE-I"


Click here for EXERCISES (Question number 1 to 10)


Click here for EXERCISES (Question number 11 to 20)


Click here for EXERCISES (Question number 21 to 30)


Click here for EXERCISES (Question number 31 to 40)


Click here for EXERCISES (Question number 41 to 52)


CHAPTER- 2 - "Physics and Mathematics"

Click here for "Questions for Short Answers"


Click here for "OBJECTIVE-II"

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