# Class 12 Physics Ch-01 Electric Charges & Fields (MCQ)

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## Class 12 Physics Chapter 1 Electric Charges and Fields MCQ

Q.1. Charge is the property associated with matter due to which it produces and experiences
(a) electric effects only
(b) magnetic effects only
(c) both electric and magnetic effects
(d) None of these

Q.2. Charge is
(a) transferable (b) associated with mass
(c) conserved (d) All of these

Q.3. A body is positively charged, it implies that
(a) there is only positive charge in the body.
(b) there is positive as well as negative charge in the body but the positive charge is more than negative charge
(c) there is equal positive and negative charge in the body but the positive charge lies in the outer regions
(d) negative charge is displaced from its position

Answer Answer: (b) When we say that a body is charged, we always mean that the body is having excess of electrons (negatively charged) or is of deficient of electrons (positively charged).

Q.4. On rubbing, when one body gets positively charged and other negatively charged, the electrons transferred from positively charged body to negatively charged body are
(a) valence electrons only
(b) electrons of inner shells
(c) both valence electrons and electrons of inner shell
(d) yet to be established

Answer Answer: (a) Valence electrons are outermost electrons these can get transferred on rubbing.

Q.5. Which of the following is the best insulator?
(a) Carbon (b) Paper
(c) Graphite (d) Ebonite

Q.6. If a body is negatively charged, then it has
(a) excess of electrons (b) excess of protons
(c) deficiency of electrons (d) deficiency of neutrons

Q.7. When a body is charged by induction, then the body
(a) becomes neutral
(b) does not lose any charge
(c) loses whole of the charge on it
(d) loses part of the charge on it

Q.8. On charging by conduction, mass of a body may
(a) increase (b) decreases
(c) increase or decrease (d) None of these

Q.9. Quantisation of charge implies
(a) charge cannot be destroyed
(b) charge exists on particles
(c) there is a minimum permissible charge on a particle
(d) charge, which is a fraction of a coulomb is not possible.

Q.10. If an object possesses an electric charge, it is said to be electrified or … A … When it has no charge, it is said to be … B … Here, A and B refer to
(a) charged, neutral (b) neutral, charged
(c) discharged, charged (d) active, reactive

Q.11. A positively charged rod is brought near an uncharged conductor. If the rod is then suddenly withdrawn, the charge left on the conductor will be
(a) positive (b) negative
(c) zero (d) cannot say

Q.12. Two spheres A and B of exactly same mass are given equal positive and negative charges respectively. Their masses after charging
(a) remains unaffected (b) mass of A > mass of B
(c) mass of A < mass of B (d) Nothing can be said

Q.13. When a comb rubbed with dry hair attracts pieces of paper. This is because the
(a) comb polarizes the piece of paper
(b) comb induces a net dipole moment opposite to the direction of field
(c) electric field due to the comb is uniform
(d) comb induces a net dipole moment perpendicular to the direction of field

Answer Answer: (a) Comb induces charge on paper due to which paper is attracted towards the comb.

Q.14. When some charge is transferred to …A… it readily gets distributed over the entire surface of … A… If some charge is put on … B…, it stays at the same place. Here, A and B refer to
(a) insulator, conductor (b) conductor, insulator
(c) insulator, insulator (d) conductor, conductor

Answer Answer: (b) When some charge is given to conductor it spreads on its surface. When some charge is given to insulator, it remains there, it do not spread, Free charges in conductor interact with added charge, so added charge spreads on surface to be in equilibrium.

Q.15. Quantisation of charge was experimentally demonstrated by
(a) Einstein’s photoelectric effect
(b) Frank-Hertz experiment
(c) Davisson and Germer experiment
(d) Millikan’s oil drop experiment

Q.16. In annihilation process, in which an electron and a positron transform into two gamma rays, which property of electric charge is displayed?
(b) Quantisation of charge
(c) Conservation of charge
(d) Attraction and repulsion

Q.17. The law, governing the force between electric charges is known as
(a) Ampere’s law (b) Ohm’s law
(c) Faraday’s law (d) Coulomb’s law

Q.18. The value of electric permittivity of free space is
(a) 9 × 109 NC2/m2 (b) 8.85 × 10–12 Nm2/C2 sec
(c)8.85 × 10–12 C2/Nm2 (d) 9 × 109 C2/Nm2

Q.19. Coulomb’s law is true for
(a) atomic distances (= 10–11 m)
(b) nuclear distances (= 10–15 m)
(c) charged as well as uncharged particles
(d) all the distances

Q.20. What happens when some charge is placed on a soap bubble?
(c) The bubble collapses (d) None of these

Q.21. When a glass rod is rubbed with silk, it
(a) gains electrons from silk.
(b) gives electrons to silk.
(c) gains protons from silk.
(d) gives protons to silk.

Explanation: (b) On rubbing a glass rod with silk, excess electrons are transferred from glass to silk. So glass rod becomes positive and silk becomes negative.

Q.22. In general, metallic ropes are suspended on the carriers taking inflammable materials. The reason is
(a) to control the speed of the carrier.
(b) to keep the centre of gravity of the carrier nearer to the earth.
(c) to keep the body of the carrier in contact with the earth.
(d) none of these.

Explanation: (c) For providing a path to the charge induced on the surface of the carriers.

Q.23. Two charges q1 and q2 are placed in vacuum at a distance d and the force acting between them is F. If a medium of dielectric constant 4 is introduced around them, the force now will be (a) F (b) F/2 (c) F/4 (d) 4F

Explanation: F/4 In the presence of medium, force becomes 1/K times.

Q.24. When 1014 electrons are removed from a neutral metal sphere, the charge on the sphere becomes ______ .

Explanation: 16 µC, Q = ne = 1014 x 1.6 × 10-19 or Q = 1.6 × 10-5 C = 16 µC As electrons are removed, so charge will be positive.

Q.25. Two similar spheres having +Q and -Q charges are kept at a certain distance. F force acts between the two. If at the middle of two spheres, another similar sphere having +Q charge is kept, then it experiences a force in magnitude and direction as
(a) zero having no direction.
(b) 8F towards +Q charge.
(c) 8F towards -Q charge.
(d) 4F towards +Q charge.

Explanation: Initially, force between A and C, When a similar sphere B having charge +Q is kept at the mid-point of line joining A and C, then net force on B is The direction is shown in figure.

Q.26. A charge Q is divided into two parts of q and Q – q. If the coulomb repulsion between them when they are separated is to be maximum, the ratio of Q/q should be
(a) 2:1
(b) 1/2
(c) 4:1
(d) 1/4

Explanation: (a) Let separation between two parts be r, then F = k.q(Q – q)/r² , For F to be maximum dF/dq = 0 then Q/q = 2/1 = 2 : 1

Q.27. Four equal charges q are placed at the four comers A, B, C, D of a square of length a. The magnitude of the force on the charge at B will be Explanation:  Q.28. Dielectric constant for metal ______ .

Q.29. Two charges of equal magnitudes kept at a distance r exert a force F on each other. If the charges are halved and distance between them is doubled, then the new force acting on each charge is Explanation:

(d) F = $\frac{k \cdot Q^{2}}{r^{2}}$. If Q is halved, r is doubled then F = $\frac{1}{16}$ time

Q.30. The electric field inside a spherical shell of uniform surface charge density is
(a) zero.
(b) constant, less than zero.
(c) directly proportional to the distance from the centre.
(d) none of the these

Explanation: All charges reside on the outer surface of the shell so according to Gauss’s law, electric field inside the shell is zero.

Q.31. A cylinder of radius R and length L is placed in a uniform electric field E parallel to the cylinder axis. The total flux for the surface of the cylinder is given by Explanation: Q.32. Electric field at a point varies as r° for
(a) an electric dipole
(b) a point charge
(c) a plane infinite sheet of charge
(d) a line charge of infinite length

Q.33. An electric charge q is placed at the centre of a cube of side a. The electric flux on one of its faces will be Explanation: Using Gauss’s theorem

Q.34. Total electric flux coming out of a unit positive charge kept in air is Explanation: Total flux coming out from the unit charge is Q.35. The electric field intensity due to an infinite cylinder of radius R and having charge q per unit length at a distance air r (r > R) from its axis is
(a) directly proportional to r².
(b) directly proportional to r3.
(c) inversely proportional to r.
(d) inversely proportional to r².

Explanation: Q.36. A point charge q is placed at a distance a/2 directly above the centre of a square of side a. The electric flux through the square is Explanation: An imaginary cube can be made by considering charge q at the centre and given square is one of its face. So flux through the given square (i.e. one face) Q.37. Which of the following graphs shows the variation of electric field E due to a hollow spherical conductor of radius R as a function of distance from the centre of the sphere? Explanation: (a) Electric field due to a hollow spherical conductor is governed by equations E = 0, for r < R …(i)
and $E=Q / 4 \pi \varepsilon_{0} r^{2}$ for r ≥ R ….(ii)
i.e. inside the conductor, electric field will be zero and outside the conductor it will vary according to E oc 1/r².

Q.38. The magnitude of electric field intensity E is such that, an electron placed in it would experience an electrical force equal to its weight is given by
(a) mge
(b) mg/e
(c) e/mg
(d) e²g/m²

Explanation: According to the question, eE = mg or E = mg/e

Q.39. In Fig. (i) two positive charges q2 and q3 fixed along the y-axis, exert a net electric force in the +x direction on a charge q1 fixed along the x-axis. If a positive charge Q is added at (x, 0) in figure (ii), the force on q1 is [NCERT Exemplar] (a) shall increase along the positive x-axis.
(b) shall decrease along the positive x-axis.
(c) shall point along the negative x-axis.
(d) shall increase but the direction changes because of the intersection of Q with q2 and qy

The net electrostatic force on the charge q1 by the charges q2 and q3 is along the positive x-direction. Hence the nature of force between qu q2 and qx, q3 should be attractive. It means qx should be negative.

Q.40. Which of the following statement is correct? The electric field at a point is [NCERT Exemplar]
(a) always continuous.
(b) continuous if there is a charge at that point.
(c) discontinuous only if there is a negative charge at that point.
(d) discontinuous if there is a charge at that point.

The electric field due to any charge will be continuous, if there is no other charge in the medium. It will be discontinuous if there is a charge at the point under consideration.

Q.41. A point charge +q is placed at a distance d from an isolated conducting plane. The field at a point P on the other side of the plane is [NCERT Exemplar]
(a) directed perpendicular to the plane and away from the plane.
(b) directed perpendicular to the plane but towards the plane.
(c) directed radially away from the point charge.
(d) directed radially towards the point charge.

The electric field lines are away from positive charge and perpendicular to the surface. Hence the field at a point P on the other side of the plane is directed perpendicular to the plane and away from the plane.

Q.42. Gauss’s law will be invalid if
(a) there is magnetic monopole.
(b) the inverse square law is not exactly true.
(c) the velocity of light is not a universal constant.
(d) none of these.

The electric field lines are away from positive charge and perpendicular to the surface. Hence the field at a point P on the other side of the plane is directed perpendicular to the plane and away from the plane.

Q.43. SI unit of permittivity of free space is
(b) Weber
(c) C2N-1 m-2
(d) C2N-1 m-2

Q.44. A charge Q is placed at the centre of the line joining two point charges +q and +q as shown in the figure. The ratio of charges Q and q is (a) 4
(b) 1/4
(c) -4
(d) -1/4

Q.45. The force per unit charge is known as
(a) electric flux
(b) electric field
(c) electric potential
(d) electric current

Q.46. Electric field lines provide information about
(a) field strength
(b) direction
(c) nature of charge
(d) all of these

Q.47. Which of the following figures represent the electric field lines due to a single negative charge? Q.48. The SI unit of electric flux is
(a) N C-1 m-2
(b) N C m-2
(c) N C-2 m2
(d) N C-1 m2

Q.49. The unit of electric dipole moment is
(a) newton
(b) coulomb
(d) debye

Q.50. Consider a region inside which, there are various types of charges but the total charge is zero. At points outside the region
(a) the electric field is necessarily zero.
(b) the electric field is due to the dipole moment of the charge distribution only.
(c) the dominant electric field is inversely proportional to r3, for large r (distance from origin).
(d) the work done to move a charged particle along a closed path, away from the region will not be zero.

Q.51. The surface considered for Gauss’s law is called
(a) Closed surface
(b) Spherical surface
(c) Gaussian surface
(d) Plane surface

Q.52. The total flux through the faces of the cube with side of length a if a charge q is placed at corner A of the cube is  Q.53. Which of the following statements is not true about Gauss’s law?
(a) Gauss’s law is true for any closed surface.
(b) The term q on the right side side of Gauss’s law includes the sum of all charges enclosed by the surface.
(c) Gauss’s law is not much useful in calculating electrostatic field when the system has some symmetry.
(d) Gauss’s law is based on the inverse square dependence on distance contained in the coulomb’s law

Q.54. Four charges are arranged at the comers of a square ABCD, as shown. The force on the charge kept at the centre O is (a) zero
(b) along the diagonal AC
(c) along the diagonal BD
(d) perpendicular to side AB

Place a unit positive charge at O. Resultant force due to the charges placed at A and C is zero and resultant charge due to B and D is towards D along the diagonal BD.

Q.55. Calculate the flux through the cuboid shown in the figure for E = E0 y j :-

(a) E0abc
(b) E0abd
(c) E0abcd
(d) Zero

Q.56. An electron is rotating around an infinite positive linear charge in a circle of radius 0.1 m, if the linear charge density is 1 μC/m, then the velocity of electron in m/s will be
(a) 0.562 × 107
(b) 0.562 x 106
(c) 562 x 107
(d) 0.0562 x 107

Q.57. The force between two small charged spheres having charges of 1 × 10–7 C and 2 ×10–7 C placed 20 cm apart in air is
(a) 4.5 × 10–2 N
(b) 4.5 × 10–3 N
(c) 5.4 × 10–2 N
(d) 5.4 × 10–3 N

Q.58. Two insulated charged metallic sphere P and Q have their centres separated by a distance of 60 cm. The radii of P and Q are negligible compared to the distance of separation. The mutual force of electrostatic repulsion if the charge on each is 3.2 × 10–7 C is
(a) 5.2 × 10–4 N
(b) 2.5 × 10–3 N
(c) 1.5 × 10–3N
(d) 3.5 × 10–4 N

Q.59. An electric dipole is placed at an angle of 30° with an electric field of intensity 2 × 105 NC–1, It experiences a torque of 4 Nm. Calculate the charge on the dipole if the dipole length is 2 cm.
(a) 8 mC
(b) 4 mC
(c) 8 μC
(d) 2 mC

Q.60. If the dipole of moment 2.57 × 10–17 cm is placed into an electric field of magnitude 3.0 × 104 N/C such that the fields lines are aligned at 30° with the line joining P to the dipole, what torque acts on the dipole?
(a) 7.7 × 10–13 Nm
(b) 3.855 × 10–13 Nm
(c) 3.855 × 10–15 Nm
(d) 7.7 × 10–15 Nm

## Quiz on Electrostatics

General Instructions:

(1) Total number of question is 5.

(2) Each question has 4 choices, out of which ONLY ONE CHOICE is correct.

1.
Consider three point objects P, Q and R. P and Q repel each other, while P and R attract. What is the nature of force between Q and R?

2.
An electric dipole placed in a uniform electric field experiences maximum moment of couple when the dipole is placed

3.
Figure shows electric lines of forces due to charges Q1 and Q2. Hence 4.
An uncharged sphere of metal is placed in a uniform electric field produced by two oppositely charged plates. The lines of force will appear as

5.
A charge Q is kept at the corner of a cube. Electric flux passing through one of those faces not touching that charge is

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NCERT Class 12 Physics Solutions

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