Electric Charges and Fields Class 12 Physics MCQs Pdf
1. 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.
Answer/Explanation
Answer: b
Explaination:
(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.
2. 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.
Answer/Explanation
Answer: c
Explaination:
(c) For providing a path to the charge induced on the surface of the carriers.
3. 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 ______ .
Answer/Explanation
Answer:
Explaination: \(\frac{F}{4}\).In the presence of medium, force becomes
\(\frac{1}{K}\) time
4. When 1014 electrons are removed from a neutral metal sphere, the charge on the sphere becomes ______ .
Answer/Explanation
Answer:
Explaination:
16 µC, Q = ne= 1014 x 1.6 × 10-19 or 0=1.6 ×
10-5 C = 16 µC
As electrons are removed, so charge will be positive.
5. 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.
Answer/Explanation
Answer: c
Explaination:
(c) 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.
6. 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
Answer/Explanation
Answer: a
Explaination:
(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
7. 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
Answer/Explanation
Answer: c
Explaination:
8. Dielectric constant for metal ______ .
Answer/Explanation
Answer:
Explaination:
Infinite [Dielectric constant K = \(\frac{\varepsilon}{\varepsilon_{0}}\)
Permittivity of metals (\(\E \)) is assumed to be very high.]
9. 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
Answer/Explanation
Answer: d
Explaination:
(d) F = \(\frac{k \cdot Q^{2}}{r^{2}}\). If Q is halved, r is doubled then F =
\(\frac{1}{16}\) time
10. 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
Answer/Explanation
Answer: a
Explaination:
(a) All charges reside on the outer surface of the shell so according to Gauss’s
law, electric field inside the shell is zero.
11. 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
Answer/Explanation
Answer: d
Explaination:
12. 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
Answer
Answer: c