Wednesday, July 21, 2021

H C Verma solutions, MAGNETIC FIELD, Chapter-34, Questions for Short Answer, Concepts of Physics, Part-II

Magnetic Field


Questions for Short Answer


    1.  Suppose a charged particle moves with a velocity v near a wire carrying an electric current. A magnetic force, therefore, acts on it. If the same particle is seen from a frame moving with velocity v in the same direction, the charge will be found at rest. Will the magnetic force become zero in this frame? Will the magnetic field become zero in this frame?  



ANSWER: Since the charge is at rest in the second frame, the magnetic force is zero in this frame.    

    In the second frame, the magnetic field appears to be zero but it will appear as an electric field or a combination of both because electric and magnetic fields are not basically independent.




    2.  Can a charged particle be accelerated by a magnetic field? Can its speed be increased?



ANSWER: A uniform magnetic field exerts a centripetal force on a charged particle, hence can accelerate centripetally. 

     Since the magnetic force on the charged particle is always perpendicular to its direction, its speed can not be increased.   




    3.  Will a current loop placed in a magnetic field always experience a zero force?  



ANSWER: The current loop will always experience a zero force if placed in a uniform magnetic field. But not if the magnetic field is non-uniform. 




    4.  The free electrons in a conducting wire are in constant thermal motion. If such a wire carrying no current, is placed in a magnetic field, is there a magnetic force on each free electron? On the wire?  



ANSWER: Yes, there will be a magnetic force on each free electron. Since the electrons are in random motions, the net magnetic force on the wire is zero.  




    5.  Assume that the magnetic field is uniform in a cubical region and is zero outside. Can you project a charged particle from outside into the field so that the particle describes a complete circle in the field?  



ANSWER: No. Suppose the charged particle is projected in a plane perpendicular to the uniform magnetic field to describe a circle. As soon as the charged particle enters the cubical area, a magnetic force perpendicular to the direction of velocity starts acting on it and it moves in a circular path. But the path can not be a complete circle as the direction of velocity at the entry point is tangent to the circular path being described and the particle can not return to this point inside the cube. See the diagram below.  
Diagram for Q-5




    6.  An electron beam projected along the positive X-axis deflects along the positive Y-axis. If this deflection is caused by a magnetic field, what is the direction of the field? Can we conclude that the field is parallel to the Z-axis?  





ANSWER: The direction of the component of the magnetic field causing the deflection is along the Z-axis. Another component of the magnetic field may be along the X-axis which will have no effect on the charged particle.  

      It can not be concluded that the resultant field is parallel to Z-axis. 




    7.  Is it possible for a current loop to stay without rotating in a uniform magnetic field? If yes what should be the orientation of the loop?  



ANSWER: Yes, a current loop can stay without rotating in a uniform field. 

    The plane of the loop should be perpendicular to the direction of magnetic field.   




    8.  The net charge in a current-carrying wire is zero. Then, why does a magnetic field exert a force on it? 



ANSWER: The net charge in a current-carrying wire is zero, but the moving charge in it is only electrons, that also in the same direction. Hence a magnetic field exerts a force on a current-carrying wire through the moving electrons. 




    9.  The torque on a current loop is zero if the angle between the positive normal and the magnetic field is either θ =0 or θ =180°. In which of the two orientations, the equilibrium is stable? 



ANSWER: The equilibrium is stable in the case of minimum potential energy. Let us calculate the potential energy of a loop when deflected from θ =0° to θ.  

   Torque on the loop,

Γ =iABsinθ

   Where i = current in the loop, A =area of the loop, B = magnitude of the uniform magnetic field. Let us rotate the loop by a very small angle dθ against the torque very slowly. 

    The work done one the loop against the magnetic field = The potential energy stored =Γ dθ.

→dU = iAB sinθdθ

On integration,

U =iAB [-cosθ ]

    Putting the limit of integration between 0° to θ, we get,

U =iAB(-cosθ +1)

→U =iAB(1 -cosθ)

   Now for the two zero torque positions, let us check the potential energy.

   For θ = 0°, U = 0. 

But for θ =180°, U =2iAB.

   Clearly, the potential energy is minimum for θ =0°, hence the equilibrium is stable for θ =0°.   




    10.  Verify that the units weber and volt-second are the same.   



ANSWER: When a loop of one tern is rotated in a magnetic field, the electromotive force produced in it is, 

v =dφ/dt, i.e. rate of change of magnetic flux. Thus, 

dφ =v dt 

Unit of φ, magnetic flux is weber. On the right-hand side, the same is volt-second. So both are the same.    

 

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Links to the Chapters



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".

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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)

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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


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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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