# Extra Questions For Class 12 Chapter 1 Electric Charges and Fields

#### Extra Questions For Class 12 Physics Chapter 1 Electric Charges and Fields

We are providing chapter-wise extra questions for class 12 physics. Students can practice these questions to score better marks in their exam.

Q.1. Give two points of distinction between charge and mass.
Ans. Charge can be positive, negative or zero. But mass is a positive quantity. Further, electric charge is always conserved. But mass is not conserved as it can be changed into energy and vice-versa.

Q.2. Does Coulomb’s law of electric force obey Newton’s third law of motion ?
Ans. Yes, it obeys. Forces exerted by two charges on each other are always equal and opposite.

Q.3. Is the electric force between two electrons greater than the gravitational force between them ? If so, by what factor ?
Ans. Yes, electric force between two electrons, is greater than gravitational force between them by a factor of 1042.

Q.4. What is the importance of Coulomb’s law of electric force in vector form?
Ans. Coulomb’s law in vector form establishes that forces between any two point charges are equal and opposite.
These forces act along the line joining the charges. Therefore, electrostatic forces are central forces.

Q.5. What is the cause of charging ?
Ans. The cause of charging is actual transfer of electrons from one body to the other.

Q.6. An isolated conducting sphere is given a positive charge. Does its mass increase, decrease or remain
the same ?

Ans. Its mass decreases slightly as it loses some electrons.

Q.7. An ebonite rod is rubbed with fur or wool. What type of charges do they acquire ?
Ans. The ebonite rod acquires negative charge and fur/wool acquires an equal positive charge.

Q.8. What is meant by quantization of charge ?
Ans. Charge on any body or particle can be integral multiple of charge on an electron (-e), i.e., q= ne, where
n= 1, 2, 3,

Q.9. Consider three charged bodies P, Q and R. If P and Q repel each other and P attracts R, what is the nature of force between Q and R?
Ans. Attractive, because Q and R will carry opposite charges.

Q.10. Why does an ebonite rod get negatively charged on rubbing with fur ?
Ans. This is because electrons in fur are less tightly bound than electrons in ebonite rod.

Q.11. A glass rod rubbed with silk acquires a charge + 1.6 x 10-12 C. What is the charge on the silk ?
Ans. Charge on silk is equal and opposite to charge on glass rod, i.e., q = -16 x 10-12 C.

Q.12. Does motion of a body affect its charge ?
Ans. No, charge on a body does not change with motion of the body.

Q.13. Name any two basic properties of electric charge.
Ans. (i) Quantization of charge
(ii) Conservation of charge.

Q.14. What is the value of charge on an electron ? Is a charge less than this value possible ?
Ans. e = -1.6 10-19 C. No, charge less than this value does not exist.

Q.15. What is the basic cause of quantization of charge ?
Ans. Only integral number of electrons can be transferred from one body to the other.

Q.16. Electrostatic forces are much stronger than gravitational forces. Give one example.
Ans. A charged glass rod can lift a piece of paper against the gravitational pull of earth on this piece.

Q.17. What is the dimensional formula for εo ?
Ans. M-1L-3 T4 A2.

Q.18. What is the relevance of large value of K (= 81) for water ?
Ans. It makes water a great solvent. This is because binding force of attraction between oppositely charged ions of the substance in water becomes 1/81 of the force between these ions in air.

Q.19. Does the coulomb force that one charge exerts on another change if other charges are brought nearby?
Ans. No, the coulomb force due to one charge on another charge is not changed.

Q.20. Force of attraction between two point charges placed at a distance d is F. What distance apart
should they be kept in the same medium so that force between them is F/3 ?
Ans. As F α1/r2 :. New distance = √3r.

Q.21. Can ever photons have a charge ? If not why?
Ans. No, photons can never have a charge. This is because charge cannot exist without rest mass.

Q.22. Which of the following quantities depends on state of rest or motion of a body : (a) mass (b) length
(c) time (d) charge (e) charge density and (f) phase ?

Ans. Mass, length, time and charge density depend on state of rest or motion of a body. However, charge and phase are invariant.

Q.23. In coulomb’s law, on what factors the value of electrostatic force constant K depends ?
Ans. The value of K depends on nature of medium separating the charges and on the system of units.

Q.24. Define electric dipole moment. Write its SI unit.
Ans. Electric dipole moment is equal to product of magnitude of either charge and distance between them. Its SI unit is C-m.

Q.25. In which orientation, a dipole placed in a uniform field is in (i) stable (i) unstable equilibrium ?
Ans. If is angle between P and É , then 0 = 0° for stable equilibrium, 0 = 180° for unstable equilibrium.

Q.26. Name any four vector fields.
Ans. Electric field, magnetic field, gravitational field and flow field (of a liquid) are vector fields.

Q.27. How does a free electron at rest move in an electric field ?
Ans. A free electron at rest will move in a direction opposite to electric field.

Q.28. Four charges of same magnitude and same sign are placed at the corners of a square, of each side
0-1 m. What is electric field intensity at the centre of the square ?

Ans. Electric field intensity at the centre of the square will be zero.

Q.29. Force experienced by an electron in an electric field Ē is F newton. What will be the force experienced by a proton in the same field ? Take mass of proton 1836 times the mass of an electron.
Ans. The proton will experience the same force, F newton, but in the opposite direction.

Q.30. What is the use of the concept of electric field intensity ?
Ans. It enables us to find force on any charge in the field, i.e., F = qE .

Q.31. Name the physical quantity whose SI unit is NC-1?
Ans. Electric field intensity.

Q.32. Why is electric field intensity inside a charged conductor zero ?
Ans. This is because there are no electric lines of force inside the body of a conductor.

Q.33. Draw electric lines of force due to an electric dipole.
Ans. The electric lines of force in respect of an electric dipole are as shown in Fig. 1(b).9.
II. What is the net force on a dipole in a uniform electric field ?
Ans. Zero

Q.34. How does a torque affect the dipole in an electric field ?
Ans. Torque tries to align the dipole along the field.

Q.35. Two point charges of +3 μC each are 100 cm apart. At what point on the line joining the charges will the electric intensity be zero ?
Ans. At the centre.

Q.36. Which rule gives you the direction of torque ?
Ans. The direction of torque is given by right handed screw rule.

Q.37. What happens when an electric dipole is held in a non uniform electric field ?
Ans. It experiences some net force and some net torque.

Q.38. What is nature of symmetry of field due to a point charge ?
Ans. The field due to point charge has spherical symmetry.

Q.39. When is an electric line of force straight ?
Ans. Electric line of force is straight in the field of a single charge.

Q.40. When is an electric line of force curved ?
Ans. Electric line of force is curved in the field of more than one point charges.

Q.41. At what points, dipole field intensity is parallel to the line joining the charges ?
Ans. At any point on axial line or equatorial line of dipole.

Q.42. Fill in the blanks:

(i) The process of sharing the charges with the earth is called ______________.

Ans. grounding or earthing

(ii) The value of the permittivity of free space (e0) in SI unit is ______________.

Ans. 8.854 × 10–12 C2 N–1m–2

(iii) A simple apparatus to detect charge on a body is the ______________.

Ans. gold-leaf electroscope

(iv) The quantization of charge was experimentally demonstrated by ______________ in 1912.

Ans. Millikan

(v) Two point charges, one coulomb each are separated by vacuum and placed I meter apart from each other. The force acting between them is ______________.

Ans. 9 × 109 N

(vi) Direction of electric field intensity due to a dipole on equatorial point is ______________to the
direction of dipole moment.

Ans. opposite

(vii) The concept of field was first introduced by ______________and is now among the central
concepts in physics.

(viii) Two point charges are separated by some distance inside vacuum. When space between the
charges is filled by some dielectric, the force between two point charges ______________.

Ans. decreases

(ix) A proton at rest has a charge e. When it moves with high speed v, its charge is ______________.

Ans. e

(x) Two equal and opposite charges of magnitude 0.2 × 10–6 C are 15 cm apart, the magnitude and direction of the resultant electric intensity E at a point midway between the charge is ______________.

Ans. 6.4 × 105 N/C, towards the –ve charge

Q.43. Figure shows a point charge +Q, located at a distance R/2 from the centre of a spherical metal shell. Draw the electric field lines for the given system.

Ans.

Q.44. Sketch the electric field lines for a uniformly charged hollow cylinder shown in figure.

Ans.

Q.45. 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.

Q.46. What is the nature of electrostatic force between two point electric charges q1 and q2 if (a) q1 + q2>0? (b) q1 + q2<0?

Ans. (a) If both q1 and q2 are positive, the electrostatic force between these will be repulsive. However, if one of these charges is positive and is greater than the other negative charge, the electrostatic force between them will be attractive.
Thus, the nature of force between them can be repulsive or attractive.

(b) If both q1 and q2 are –ve, the force between these will be repulsive.
However, if one of them is –ve and it is greater in magnitude than the second +ve charge, the force between them will be attractive.
Thus, the nature of force between them can be repulsive or attractive.

Q.47. Two metallic spheres A and B kept on insulating stands are in contact with each other. A positively charged rod P is brought near the sphere A as shown in the figure. The two spheres are separated from each other, and the rod P is removed. What will be the nature of charges on spheres A and B?

Ans.

• Sphere A will be negatively charged.
• Sphere B will be positively charged.

Explanation: If positively charged rod P is brought near metallic sphere A due to induction negative charge starts building up at the left surface of A and positive charge on the right surface of B.

If the two spheres are separated from each other, the two spheres are found to be oppositely
charged. If rod P is removed, the charges on spheres rearrange themselves and get uniformly
distributed over them.

Q.48. An arbitrary surface encloses a dipole. What is the electric flux through this surface?

Ans. Net charge on a dipole = – q + q = 0. So according to Gauss’s theorem, electric flux through the surface, will be zero.

Q.49. 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.

Q.50. Two equal balls having equal positive charge ‘q’ coulombs are suspended by two insulating strings of equal length. What would be the effect on the force when a plastic sheet is inserted between the two?

Ans. Force will decrease.

Q.51. . Plot a graph showing the variation of coulomb force (F) versus (1/r2), where r is the distance between the two charges of each pair of charges: (1 µC, 2 µC) and (2 µC, – 3 µC). Interpret the graphs obtained.

Ans.

Q.52. Two identical point charges, q each, are kept 2 m apart in air. A third point charge Q of unknown magnitude and sign is placed on the line joining the charges such that the system remains in equilibrium. Find the position and nature of Q.

Ans. System is in equilibrium therefore net force on each charge of system will be zero.

For the total force on ‘Q’ to be zero

⇒ x = 2 – x

⇒ 2x = 2

⇒ x = 1 m

For the equilibrium of charge “q” the nature of charge Q must be opposite to the nature of charge q.

Q.53. Represent graphically the variation of electric field with distance, for a uniformly charged plane sheet.

Ans. Electric field due to a uniformly charged plane sheet is independent of distance.

E= σ/∈0

So, it represents a straight line parallel to distance axis.

Q.54. A simple pendulum consists of a small sphere of mass m suspended by a thread of length L. The sphere carries a positive charge q. The pendulum is placed in a uniform electric field of strength E directed vertically downwards. Find the period of oscillation of the pendulum due to the electrostatic force acting on the sphere, neglecting the effect of the gravitational force.

Ans.

Q.55. A hollow cylindrical box of length 1 m and area of cross-section 25 cm2 is placed in a three dimensional coordinate system as shown in the figure. The electric field in the region is given by E = 50x i where E is in NC–1 and x is in metres.
Find
(i) net flux through the cylinder.
(ii) charge enclosed by the cylinder.

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