CBSE Class 12 Physics Electric Charges and Fields MCQs Set B

Question. A charge Q is enclosed by a Gaussian spherical surface of radius R. If the radius is doubled, then the outward electric flux will
(a) increase four times
(b) be reduced to half
(c) remain the same
(d) be doubled

Answer : C

Question. Suppose the charge of a proton and an electron differ slightly. One of them is –e, the other is (e + Δ). If the net of electrostatic force and gravitational force between two hydrogen atoms placed at a distance d (much greater than atomic size) apart is zero, then Δ is of the order of
[Given: mass of hydrogen mh = 1.67 × 10^–27 kg]
(a) 10^–23 C
(b) 10^–37 C
(c) 10^–47 C
(d) 10^–20 C

Answer : B

Question. Two identical charged spheres suspended from a common point by two massless strings of lengths l, are initially at a distance d (d < < l) apart because of their mutual repulsion. The charges begin to leak from both the spheres at a constant rate. As a result, the spheres approach each other with a velocity v. Then v varies as a function of the distance x between the spheres, as
(a) v ∝ x–1/2
(b) v ∝ x–1
(c) v ∝ x1/2
(d) v ∝ x

Answer : A

Question. A hollow metal sphere of radius R is uniformly charged. The electric field due to the sphere at a distance r from the centre
(a) decreases as r increases for r < R and for r > R
(b) increases as r increases for r < R and for r > R
(c) zero as r increases for r < R, decreases as r increases for r > R
(d) zero as r increases for r < R, increases as r increases for r > R

Answer : C

Question: An electric dipole is placed at an angle of 30° with an electric field intensity 2 × 10⁵ N C^–1. It experiences a torque equal to 4 N m. The charge on the dipole, if the dipole length is 2 cm, is
(a) 8 mC
(b) 2 mC
(c) 5 mC
(d) 7 mC

Answer : B

Question: A toy car with charge q moves on a frictionless horizontal plane surface under the influence of a uniform electric field E. Due to the force qE, its velocity increases from 0 to 6 m ^s–1 in one second duration. At that instant the direction of the field is reversed. The car continues to move for two more seconds under the influence of this field. The average velocity and the average speed of the toy car between 0 to 3 seconds are respectively
(a) 2 m s^–1, 4 m ^s–1
(b) 1 m s^–1, 3 m s^–1
(c) 1 m ^s–1, 3.5 m s–1
(d) 1.5 m s^–1, 3 m s^–1

Answer : B

Question: A particle of mass m and charge q is placed at rest in a uniform electric field E and then released. The kinetic energy attained by the particle after moving a distance y is
(a) qEy
(b) qE²y
(c) qEy ²
(d) q²Ey

Answer : A

Question. A thin conducting ring of radius R is given a charge +Q. The electric field at the centre O of the ring due to the charge on the part AKB of the ring is E. The electric field at the centre due to the charge on the part ACDB of the ring is
(a) E along KO
(b) 3E along OK
(c) 3E along KO
(d) E along OK

Answer : C

Question. A point Q lies on the perpendicular bisector of an electrical dipole of dipole moment p. If the distance of Q from the dipole is r (much larger than the size of the dipole), then the electric field at Q is proportional to
(a) p² and r^–3
(b) p and r^–2
(c) p^–1 and r^–2
(d) p and r^–3

Answer : D

Question. From a point charge, there is a fixed point A. At A, there is an electric field of 500 V/m and potential difference of 3000 V. Distance between point charge and A will be
(a) 6 m
(b) 12 m
(c) 16 m
(d) 24 m

Answer : A

Question. If electric field in a region is radially outward with magnitude E = Ar, the charge contained in a sphere of radius r centred at the origin is

Answer : B

Question. There is an electric field E in X-direction. If work done in moving a charge 0.2 C through a distance of 2m along a line making an angle of 60 degree with X-axis is 4.0 joule, what is the value of E?
(a) √3 newton per coulomb
(b) 4 netwon per coulomb
(c) 5 newton per coulomb
(d) None of these

Answer : D

Question. Two particle of equal mass m and charge q are placed at a distance of 16 cm. They do not experience any force. The value of q/m is

Answer : C

Question. An electric dipole is placed along the x-axis at the origin O. A point P is at a distance of 20 cm from this origin such that OP makes an angle π/3 with the x-axis. If the electric field at P makes an angle Θ with the x-axis, the value of Θ would be

Answer : B

Question. A and B are two identically spherical charged bodies which repel each other with force F, kept at a finite distance. A third uncharged sphere of the same size is brought in contact with sphere B and removed. It is then kept at mid point of A and B. Find the magnitude of force on C.
(a) F/2
(b) F/8
(c) F
(d) Zero

Answer : C

Question. A simple pendulum has a length l , mass of bob m. The bob is given a charge q. The pendulum is suspended between the vertical plates of the charged parallel plate capacitor. If E is the field strength between the plates, then time period T equal to

Answer : D

Question. A soap bubble is given negative charge, its radius will
(a) increase
(b) decrease
(c) remain unchanged
(d) fluctuate

Answer : A

Question. Positive and negative point charges of equal magnitude are kept at (0,0,(a/2)) and (0,0, (-a/2)) respectively. The work done by the electric field when another positive point charge is moved from (–a, 0, 0) to (0, a, 0) is
(a) positive
(b) negative
(c) zero
(d) depends on the path connecting the initial and final positions

Answer : C

Question. The electric dipole is situated is an electric field as shown in fig 1. The dipole and electric field are both in the plane of the paper. The dipole is rotated about an axis perpendicular to plane of paper passing through A in anticlockwise direction. If the angle of rotation (q) is measured with respect to the direction of electric field, then the torque (t) experienced by the dipole for different values of the angle of rotation q will be represented in fig. 2, by

(a) curve (1)
(b) curve (2)
(c) curve (3)
(d) curve (4)

Answer : B

Question: The given figure gives electric lines of force due to two charges q₁ and q₂. What are the signs of the two charges?​​​
(a) q₁ is positive but q₂ is negative.
(b) q₁ is negative but q₂ is positive.
(c) both are negative.
(d) both are positive.

Answer : C

Question. Point charges +4q, –q and +4q are kept on the X-axis at point x = 0, x = a and x = 2a respectively. Then
(a) only –q is in stable equilibrium
(b) all the charges are in stable equilibrium
(c) all of the charges are in unstable equilibrium
(d) none of the charges is in equilibrium.

Answer : C

Question. An electron falls from rest through a vertical distance h in a uniform and vertically upward directed electric field E. The direction of electric field is now reversed, keeping its magnitude the same. A proton is allowed to fall from rest in it through the same vertical distance h. The time of fall of the electron, in comparison to the time of fall of the proton is
(a) smaller
(b) 5 times greater
(c) 10 times greater
(d) equal

Answer : A

Question. A hollow insulated conduction sphere is given a positive charge of 10 μc. What will be the electric field at the centre of the sphere if its radius is 2 metres?
(a) Zero
(b) 5 mCm–2
(c) 20 mCm–2
(d) 8 mCm–2

Answer : A

Question. 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

Answer : B

Question. A drop of oil of density r and radius r carries a charge q when placed in an electric field E, it moves upwards with a velocity v. If r0 is the density of air, h be the viscosity of the air, then which of the following force is directed upwards?
(a) q E
(b) 6π ηr v
(c) 4/3 πr3 (P – P 0)g
(d) None of these

Answer : A

Question. 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 μc
(b) 4 μc
(c) 8 μc
(d) 2 μc

Answer : D

Question. An electric dipole has the magnitude of its charge as q and its dipole moment is p. It is placed in uniform electric field E. If its dipole moment is along the direction of the field, the force on it and its potential energy are respectively.
(a) q.E and max.
(b) 2 q.E and min.
(c) q.E and min
(d) zero and min.

Answer : D

Question. Charge Q is distributed to two different metallic spheres having radii R and 2R such that both spheres have equal surface charge density, then charge on large sphere is
(a) 4Q/5
(b) Q/5
(c) 3Q/5
(d) 5Q/4

Answer : A

Question. Two point charges q1 = 4 μc and q2 = 9 μc are placed 20 cm apart. The electric field due to them will be zero on the line joining them at a distance of
(a) 8 cm from q1
(b) 8 cm form q2
(c) 80/13 cm from q1
(d) 80/13 cm from q2

Answer : A

Question. Among two discs A and B, first have radius 10 cm and charge 10–6 μc and second have radius 30 cm and charge 10–5 μC. When they are touched, charge on both qA and qB respectively will, be
(a) qA = 2.75μC, qB=3.15μC
(b) qA = 1.09μC, qB = 1.53μC
(c) qA = qB = 5.5μC
(d) None of these

Answer : A

Question. Three charge q, Q and 4q are placed in a straight line of length l at points distant 0, 1/2 and l respectively from one end. In order to make the net froce on q zero, the charge Q must be equal t o
(a) –q
(b) – 2q
(c) -q /2
(d) q

Answer : A

Question. The spatial distribution of electric field due to charges (A, B) is shown in figure. Which one of the following statements is correct ?

(a) A is +ve and B –ve, |A| > |B|
(b) A is –ve and B +ve, |A| = |B|
(c) Both are +ve but A > B
(d) Both are –ve but A > B

Answer : A

Question. Force between two identical charges placed at a distance of r in vacuum is F. Now a slab of dielectric of dielectric contrant 4 is inserted between these two charges. If the thickness of the slab is r/2, then the force between the charges will become
(a) F
(b) (3/5)F
(c) (4/9)F
(d) F/2

Answer : C

Question. ABC is an equilateral triangle. Charges +q are placed at each corner as shown as fig. The electric intensity at centre O will be

Answer : D

Question. A charge +q is at a distance L/2 above a square of side L. Then what is the flux linked with the surface?
(a) q/4∈₀
(b) 2q/3∈₀
(c) q/6∈₀
(d) 6q/∈₀

Answer : C