CBSE Class 12 Physics Electrostatic Potential and Capacitance MCQs Set E

Assertion (A) & Reason(R)

For question numbers 1 to 20, two statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below.
a) Both A and R are true and R is the correct explanation of A
b) Both A and R are true but R is NOT the correct explanation of A
c) A is true but R is false
d) A is false and R is also false

Question. Assertion(A): The Coulomb force between two points charges depend upon the dielectric constant of the intervening medium.
Reason(R): Coulomb’s force varies inversely with the dielectric constant of medium.
Answer: a

Question. Assertion (A): The electrostatics force increases with decrease the distance between the charges.
Reason (R): The electrostatic force of attraction or repulsion between any two stationary point charges is inversely proportional to the square of the distance between them.
Answer: a

Question. Assertion (A): A comb run through one’s dry hair attracts small bits of paper.
Reason(R): Molecules in the paper gets polarized by the charged comb resulting in net force of attraction.
Answer: a

Question. Assertion(A): The charge given to a metallic sphere does not depend on whether it is hollow or solid
Reason(R): The charge resides only at the surface of conductor.
Answer: a

Question. Assertion(A): Electric field at the surface of a charged conductor is always normal to the surface at every point.
Reason(R): Electric field gives the magnitude & direction of electric force
Answer: b

Question. Assertion(A): A proton is placed in a uniform electric field, it tend to move along the direction of electric field.
Reason(R): A proton is placed in a uniform electric field it experiences a force.
Answer: b

Question. Assertion(A): The potential inside a hollow spherical charged conductor is zero.
Reason(R): Inside the hollow spherical conductor electric field is constant.
Answer: d

Question. Assertion(A): Electric filed lines not form closed loops.
Reason(R): Electric filed lines are always normal to the surface of a conductor.
Answer: b

Question. Assertion(A): No work is done in moving a point charge 𝑄 around a circular arc of radius ′𝑟′at the Centre of which another point charge ′𝑞′ is located.
Reason(R): No work is done in moving a test charge from one point to another over an equipotential surface.
Answer: a

Question. Assertion(A): No work is done in moving a test charge from one point to another over an equipotential surface.
Reason(R): Electric field is always normal to the equipotential surface at every point
Answer: b
 

Case Study Based Questions

1. For electrostatics, the concept of electric field is convenient, but not really necessary. Electric field is an elegant way of characterizing the electrical environment of a system of charges. Electric field at a point in the space around a system of charges tells you the force a unit positive test charge would experience if placed at that point (without disturbing the system). Electric field is a characteristic of the system of charges and is independent of the test charge that you place at a point to determine the field. The term field in physics generally refers to a quantity that is defined at every point in space and may vary from point to point. Electric field is a vector field, since force is a vector quantity.

Question. Which of the following statement is correct? The electric field at a point is
(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.
Answer: b

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

Question. The SI unit of electric field is
(a) N/m
(b) N-m
(c) N/C
(d) N/C2
Answer: c

Question. 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
Answer: b

Question. At a particular point, Electric field depends upon
(a) Source charge Q only
(b) Test Charge q0 only.
(c) Both q and q0
(d) Neither Q nor q0
Answer: a

2. Dielectric with polar molecules also develops a net dipole moment in an external field, but for a different reason. In the absence of any external field, the different permanent dipoles are oriented randomly due to thermal agitation; so the total dipole moment is zero. When an external field is applied, the individual dipole moments tend to align with the field. When summed overall the molecules, there is then a net dipole moment in the direction of the external field, i.e., the dielectric is polarised. The extent of polarisation depends on the relative strength of two factors: the dipole potential energy in the external field tending to align the dipoles mutually opposite with the field and thermal energy tending to disrupt the alignment. There may be, in addition, the ‘induced dipole moment’ effect as for non-polar molecules, but generally the alignment effect is more important for polar molecules. Thus in either case, whether polar or non-polar, a dielectric develops a net dipole moment in the presence of an external field. The dipole moment per unit volume is called polarization.

Question. The best definition of polarisation is
(a) Orientation of dipoles in random direction
(b) Electric dipole moment per unit volume
(c) Orientation of dipole moments
(d) Change in polarity of every dipole
Answer: b

Question. Calculate the polarisation vector of the material which has 100 dipoles per unit volume in a volume of 2 units.
(a) 200
(b) 50
(c) 0.02
(d) 100
Answer: a

Question. The total polarisation of a material is the
(a) Product of all types of polarisation
(b) Sum of all types of polarisation
(c) Orientation directions of the dipoles
(d) Total dipole moments in the material
Answer: b

Question. Dipoles are created when dielectric is placed in
(a) Magnetic Field
(b) Electric field
(c) Vacuum
(d) Inert Environment
Answer: b

Question. Identify which type of polarisation depends on temperature.
(a) Electronic
(b) Ionic
(c) Orientational
(d) Interfacial
Answer: c

3. Figure (a) shows an uncharged metallic sphere on an insulating metal stand. If we Bring a negatively charged rod close to the metallic sphere, as shown in Fig. (b). As the rod is brought close to the sphere, the free electrons in the sphere move away due to repulsion and start piling up at the farther end. The near end becomes positively charged due to deficit of electrons. This process of charge distribution stops when the net force on the free electrons inside the metal is zero. Now if we Connect the sphere to the ground by a conducting wire. The electrons will flow to the ground while the positive charges at the near end will remain held there due to the attractive force of the negative charges on the rod, as shown in Fig. (c). Disconnect the sphere from the ground. The positive charge continues to be held at the near end Fig.(d). if we remove the electrified rod. The positive charge will spread uniformly over the sphere as shown in Fig. (e). In this experiment, the metal sphere gets charged by the process of induction and the rod does not lose any of its charge.

Question. What do you call the process of charging a conductor by bringing it near another Charged object?
(a) Induction
(b) Polarisation
(c) neutralization
(d) conduction
Answer: a

Question. Transferring a charge without touching is
(a) Conduction
(b) Induction
(c) Grounding
(d) Newtons 3rd law
Answer: b

Question. If we bring charged plastic rod near-neutral aluminum rod, then rods will
(a) Repel each other
(b) Attract each other
(c) Remain their position
(d) Exchange charges
Answer: b

Question. Due to electrostatic induction in aluminum rod due to charged plastic rod, the total charge on the aluminum rod is
(a) Zero
(b) Positive
(c) Negative
(d) Dual
Answer: a