Bihar Board - Class 12 Physics - Chapter 4: Moving Charges and Magnetism Short Answer Question

For Class 12 Bihar Board, the chapter Moving Charges and Magnetism in Physics explains the interaction between moving electric charges and magnetic fields. This chapter also covering topics like Lorentz force, Ampere's law, and the magnetic field due to a current. You can find short-question answers that provide a clear understanding of these important concepts, aiding in better preparation for exams on Vidyakul.

Short Answer Type Questions 

1. In which orientation, a dipole placed in a uniform electric field is in

• stable,

• unstable equilibrium ?

Answer:

• For stable equilibrium, a dipole is placed parallel to the electric field.

• For unstable equilibrium, a dipole is placed antiparallel to the electric field.

2. Name the physical quantity whose S.I. unit is JC-1. Is it a scalar or a vector quantity? 

Answer:

• Physical quantity whose S.I. unit is JC-1 is Electric potential.

• It is a Scalar quantity.

3. Define electric dipole moment. Write its S.I. unit. 

Answer:
Electric dipole moment of an electric dipole is defined as the product of the magnitude of either charge and dipole length.

                              P =q(2l)

                             
S.I. unit of dipole p is coulomb metre (Cm).

4. Why should electrostatic field be zero inside a conductor?

Answer:
Electrostatic field inside a conductor should be zero because of the absence of charge. As in a static condition, charge remains only on the surface.

5. Why must electrostatic field be normal to the surface at every point of a charged conductor? 

Answer:

So that tangent on charged conductor gives the direction of the electric field at that point.

6. Depict the direction of the magnetic field lines due to a circular current carrying loop. 

Answer:

Direction of the magnetic field lines is given by right hand thumb rule.

7. Why do the electric field lines not form closed loops?

Answer:
Electric field lines do not form closed loops because the direction of an electric field is from positive to negative charge. So one can regard a line of force starting from a positive charge and ending on a negative charge. This indicates that electric field lines do not form closed loops.

8. Is the electric field due to a charge configuration with total charge zero, necessarily zero? Justify. 

Answer:
No, it is not necessarily zero. If the electric field due to a charge configuration with total charge is zero because the electric field due to an electric dipole is non-zero.

9.Why do the electrostatic field lines not form closed loops? 

Answer:
Electric field lines do not form closed loops because the direction of an electric field is from positive to negative charge. So one can regard a line of force starting from a positive charge and ending on a negative charge. This indicates that electric field . lines do not form closed loops.

10. Why do the electric field lines never cross each other?

Answer:
The electric lines of force give the direction of the electric field. In case, two lines of force intersect, there will be two directions of the electric field at the point of intersection, which is not possible.

11. What is the electric flux through a cube of side 1 cm which encloses an electric dipole?

Answer:
Zero because the net charge of an electric dipole (+ q and – q) is zero.

12. Why are electric field lines perpendicular at a point on an equipotential surface of a conductor? 

Answer:
If the electric field lines were not normal to the equipotential surface, it would have a non-zero component along the surface. To move a unit test charge against the direction of the component of the field, work would have to be done which means this surface cannot be equipotential surface.
Hence, electric field lines are perpendicular at a point on an equipotential surface of a conductor.

13.A point charge +Q is placed at point O as shown in the figure.
                                         
Is the potential difference VA – VB positive, negative or zero? 

Answer:
The potential difference is positive.

14. How does the electric flux due to a point charge enclosed by a spherical Gaussian surface get affected when its radius is increased?

Answer:
The electric flux due to a point charge enclosed by a spherical gaussian surface remains ‘unaffected’ when its radius is increased.

15. Show on a plot the nature of variation of the Electric field (E) and potential (V), of a (small) electric dipole with the distance (r) of the field point from the centre of the dipole.

Answer:
                                           

16. Does the charge given to a metallic sphere depend on whether it is hollow or solid? Give reason for your answer.

Answer:
No, it does not, because the charge resides only on the surface of the conductor.

17. Draw a plot showing variation of electric field with distance from the centre of a solid conducting sphere of radius R, having a charge of +Q on its surface.

Answer:
Plot between E and r
                                                     

18. Explain the meaning of the statement ‘electric charge of a body is quantised’.

Answer:

Electric charge of a body is quantized. This means that only integral (1, 2, …., n) number of electrons can be transferred from one body to the other. Charges are not transferred in fraction. Hence, a body possesses total charge only in integral multiples of electric charge.

19.  Why can one ignore quantisation of electric charge when dealing with macroscopic i.e., large scale charges?

Answer:

 In macroscopic or large scale charges, the charges used are huge as compared to the magnitude of electric charge. Hence, quantization of electric charge is of no use on macroscopic scale. Therefore, it is ignored and it is considered that electric charge is continuous.

20. When a glass rod is rubbed with a silk cloth, charges appear on both. A similar phenomenon is observed with many other pairs of bodies. Explain how this observation is consistent with the law of conservation of charge.

Answer:

Rubbing produces charges of equal magnitude but of opposite nature on the two bodies because charges are created in pairs. This phenomenon of charging is called charging by friction. The net charge on the system of two rubbed bodies is zero. This is because equal amount of opposite charges annihilate each other. When a glass rod is rubbed with a silk cloth, opposite natured charges appear on both the bodies. This phenomenon is in consistence with the law of conservation of energy. A similar phenomenon is observed with many other pairs of bodies.

21. (a) An electrostatic field line is a continuous curve. That is, a field line cannot have sudden breaks. Why not?

(b) Explain why two field lines never cross each other at any point?

Answer:

(a) An electrostatic field line is a continuous curve because a charge experiences a continuous force when traced in an electrostatic field. The field line cannot have sudden breaks because the charge moves continuously and does not jump from one point to the other.

(b) If two field lines cross each other at a point, then electric field intensity will show two directions at that point. This is not possible. Hence, two field lines never cross each other.

22. Four point charges qA = + 2 μC, qB = -5μC, qC = + 2 μC and qD = -5 μC are located at the corners of a square ABCD of side 10 cm. What is the force on a charge of 1 μC placed at the centre of the square?

Answer:

Forces of repulsion on 1 μC charge at O due to 2 μC charge, at A and C are equal and opposite. Therefore, they cancel. Similarly, forces of attraction on 1 μC charge at O, due to -5 μC charges at B and at D are also equal and opposite. Therefore, these also cancel. Hence, the net force on the charge of 1 μC at O is zero.

23. Figure shows tracks of three charged particles in a uniform electrostatic field. Give the signs of the three charges. Which particle has the highest charge to mass ratio?
                                 

Answer:

Particles 1 and 2 are negatively charged as they experience forces in direction opposite to that of electric field E, whereas particle 3 is positively charged as it experience force in the direction of electric field E.
                                         

Particle-3 has the highest charge to mass ratio, as it shows maximum deflection in the electric field.

24. What is the net flux of the uniform electric field in previous question through a cube of side 20 cm oriented so that its faces are parallel to the coordinate planes?

Answer:

net=0,  As the net electric flux with closed surface like cube in uniform electric

field is equal to zero, because the number of lines entering the cube is the same as the number of lines leaving the cube

25. The dimensions of an atom are of the order of an Angstrom. Thus there must be large electric fields between the protons and electrons. Why, then is the electrostatic field inside a conductor zero?

Ans. The electric fields bind the atoms to neutral entity. Fields are caused by excess charges. There can be no excess charge on the inner surface of an isolated conductor. So, the electrostatic field inside a conductor is zero.