Wednesday, September 28, 2022

H C Verma solutions, ELECTRIC CURRENT THROUGH GASES, Chapter-41, OBJECTIVE-II, Concepts of Physics, Part-II

Electric Current Through Gases


OBJECTIVE-II


     1.  Electric conduction takes place in a discharge tube due to the movement of

(a) positive ions

(b) negative ions

(c) electrons

(d) protons.  


ANSWER: (a), (b), (c).   


EXPLANATION: Due to cosmic rays, some ions are always present in a gas. When a potential difference is applied across a discharge tube, the ions are accelerated and collide with other molecules and strike the outer loose electrons. Thus the molecules become positive ions and the released electrons after going through some distance may attach to some neutral molecule making it a negative ion. Thus positive ions, negative ions, and electrons are present in the discharge tube that moves under the effect of the electric field and conduction takes place. No free proton is present in the discharge tube. Options (a), (b) and (c) are correct.  





     2.  Which of the following is true for cathode rays?

(a) It travels along straight lines.

(b) It emits an X-ray when strikes a metal

(c) It is an electromagnetic wave.

(d) It is not deflected by a magnetic field. 


ANSWER: (a), (b).   


EXPLANATION: Cathode rays are actually a stream of fast-moving electrons. They travel in a straight line and when they strike a metal X-ray is emitted. Options (a) and (b) are true. It is not an electromagnetic wave and due to the moving charge, it is deflected by a magnetic field. The other two options are not correct.




     3.  Because of the space charge in a diode valve,

(a) the plate current decreases

(b) the plate voltage increases

(c) the rate of emission of thermions increases

(d) the saturation current increases. 


ANSWER: (a).   


EXPLANATION: The space between the cathode and anode contains electrons and hence, is negatively charged. It is called the space charge. New electrons coming out of the cathode are repelled back by this space charge which reduces the plate current. Option (a) is correct.

   It does not affect the plate voltage, rate of emission of thermions, and saturation current. 

 




     4.  The saturation current in a triode valve can be changed by changing

(a) the grid voltage

(b) the plate voltage

(c) the separation between the grid and the cathode

(d) the temperature of the cathode. 


ANSWER: (d).   


EXPLANATION: When all the electrons emitted by the cathode are collected by the anode, the saturation current occurs. At this point, whether we increase the plate voltage or grid voltage, the number of electrons is not going to increase, and hence the saturation current is constant. The only way to increase the number of electrons emitted by the cathode is to increase the temperature of the cathode so that the number of thermions increases. Hence only option (d) is correct.


 




     5.  Mark the correct options.

(a) a diode valve can be used as a rectifier.

(b) a triode valve can be used as a rectifier.

(c) A diode valve can be used as an amplifier.

(d) A triode valve can be used as an amplifier.  


ANSWER: (a), (b), (d).   


EXPLANATION: Since a diode valve and a triode valve allows the current to flow in one direction only, these two can be used as a rectifier. Options (a) and (b) are correct. 

     Due to the presence of a grid in a triode valve, the output signal is controlled in proportion to the input signal. Hence it can be used as an amplifier. In a diode valve, there is no grid and hence the output signal cannot be controlled. Option (d) is correct, and option (c) is not correct.  



 




     6.  The plate current in a diode is zero. It is possible that

(a) the plate voltage is zero

(b) the plate voltage is slightly negative

(c) the plate voltage is slightly positive

(d) the temperature of the filament is low. 


ANSWER: All.   


EXPLANATION: If the plate voltage is zero, then there will be no current in the diode. Hence option (a) is possible. 

   If the plate voltage is negative, then there will be no plate current because the diode does not allow current in the reverse direction. Option (b) is possible. 

   When the plate voltage is slightly positive, it may not be able to overcome the effects of space charge. In this case, also, the plate current will be zero. Option (c) is possible. 

   When the temperature of the filament is low, there will not be sufficient thermions emission and the plate current will be zero. Option (d) is also possible.

  So all options are correct. 


  




     7.  The plate current in a triode valve is zero. the temperature of the filament is high. It is possible that

(a) Vᵨ>0, Vₚ>0

(b) Vᵨ>0, Vₚ<0

(c) Vᵨ<0, Vₚ>0

(d) Vᵨ<0, Vₚ<0.  


ANSWER: (b), (c), (d).   


EXPLANATION: When the temperature of the filament is high and both the plate voltage and the grid voltage are positive, the electrons will travel from cathode to anode and the current will not be zero. Option (a) is not correct. 

    If the plate voltage is negative but the grid voltage is positive or vice versa, then the electrons will not be able to reach the anode and the plate current will be zero. Options (b) and (c) are correct. 

    If both the plate voltage and the grid voltage are negative, then the thermions from the filament will be repelled towards the cathode itself and no plate current will appear. Option (d) is correct. 



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Links to the Chapters




CHAPTER- 41- Electric Current Through Gases



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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