Conductors and Insulators

Correct answer: A. Because free charges would move again if a field remained

Explanation: The governing principle is electrostatic equilibrium in a conductor. A conductor contains mobile free charges, and an electric field exerts force F = qE on each charge. If E were non-zero anywhere inside, those charges would drift and the conductor would not remain at rest. They redistribute on the surface until the internal field becomes zero. Hence A is correct; the other choices make false claims about particles, neutrality, or insulators.

Correct answer: B. There will be electric field inside and free charges will move

Explanation: In electrostatic equilibrium, the entire conductor is an equipotential body, so any two internal points must have the same potential. A potential difference means the electric potential varies within the conductor; consequently an electric field exists, since E is related to the spatial rate of change of potential. That field exerts force on free charges and makes them move. Therefore B is correct, while A, C, and D contradict conductor behaviour.

Correct answer: C. Free charges reach the outer surface through repulsion and rearrangement

Explanation: The governing concept is the redistribution of excess charge in a conductor. Like charges repel one another, and the mobile charges can move through the conducting material. They continue rearranging until electrostatic equilibrium is reached, when the electric field inside the conducting material is zero and excess charge resides on the outer surface. Thus C is correct. A is irrelevant, B violates charge conservation, and D falsely describes conductors.

Correct answer: D. Equal negative charge

Explanation: The governing concept is electrostatic induction and the zero electric field inside conducting material. If the charge in the cavity is +q, a Gaussian surface drawn within the conductor must enclose zero net charge, so the inner surface induces −q. This charge is equal in magnitude and opposite in sign; the conductor’s outer surface may carry compensating charge depending on its total charge. Therefore option D is correct, not zero or a partial charge.

Correct answer: A. Equal negative charge

Explanation: Let the charge inside the cavity be −q. Electrostatic equilibrium requires +q on the inner surface, because the electric field within the conductor must be zero. Since the conductor was initially neutral and has not been grounded, its total charge remains zero: (+q) on the inner surface must be balanced by −q on the outer surface. Thus option A is correct; zero would violate charge conservation, while infinity is physically meaningless.

Correct answer: A. Because it would move surface charges

Explanation: In electrostatic equilibrium, free charges in a conductor must have no net tangential force. If a parallel, or tangential, electric-field component existed at the surface, the force qE_parallel would drive mobile surface charges along the conductor. Their redistribution would continue until that component became zero. The remaining field can be normal to the surface. Therefore option A gives the physical reason; the other choices are unrelated.

Correct answer: C. Perpendicular to the surface

Explanation: At electrostatic equilibrium, the tangential component of the electric field at a conductor’s surface is zero; otherwise free charges would move along the surface. Consequently, the field immediately outside has only the normal component and is perpendicular to the surface. For a positively charged surface it points outward, while for a negatively charged surface it points inward. Thus option C states the direction correctly; it is not generally parallel or irregular.

Correct answer: C. Because there is complete symmetry in all directions

Explanation: An isolated spherical conductor has complete rotational symmetry: no point on its surface is physically distinguished from another. Mobile charges redistribute until electrostatic equilibrium is reached, and the symmetry then requires the surface charge density to be the same everywhere. The sphere need not be earthed, and a conductor does contain mobile charges; charge resides on the surface rather than only at the centre. Therefore option C is correct.

Correct answer: A. The external charge affects free charges non-uniformly

Explanation: A conductor contains mobile free charges. When an external charge is brought near it, the electric force is stronger on the nearer side and weaker on the farther side, so charges redistribute over the surface. This is electrostatic induction. The conductor remains a conductor, and its total charge is conserved if it is isolated; only its distribution changes. Therefore option A is correct, while the other options violate basic charge and material principles.

Correct answer: A. Providing a path for charge exchange

Explanation: The governing concept is charging by induction. A nearby charged body first separates charges in the conductor. Earthing then connects the conductor to Earth, a vast charge reservoir, so electrons can flow into or out of it in response to the external field. After the earth connection is removed in the correct sequence, the conductor retains a net charge. Thus option A is correct; the other choices describe no physical function of earthing.

Correct answer: B. From Earth to the conductor

Explanation: The governing idea is induction with an earth connection. A positive rod attracts electrons toward the conductor’s near surface, making the conductor electron-deficient unless electrons are supplied. Because the conductor is earthed, electrons can flow from Earth, which acts as a large reservoir, into the conductor. Electrons do not travel from the insulating gap through the rod, and charge is not destroyed. Therefore option B is correct; A gives the opposite direction.

Correct answer: B. From the conductor to Earth

Explanation: The governing principle is electrostatic induction combined with earthing. A negatively charged rod repels the conductor’s mobile electrons. Since the conductor is connected to Earth, those repelled electrons can leave the conductor and flow into the Earth, a very large reservoir. They cannot normally cross the gap into the rod, and earthing means a path is available, so “nowhere” is incorrect. Hence option B is correct; option A describes the usual direction for a positive external rod.

Correct answer: A. So that the final charge can be retained

Explanation: The governing concept is the correct sequence of charging by induction. While the external charged object is nearby, it maintains charge separation and determines whether electrons enter or leave through the earth connection. Disconnecting the earth first isolates the transferred charge on the conductor. Only after that should the external object be removed; the separated charge then redistributes over the conductor without escaping to Earth. Therefore option A is correct. The other options describe unrelated or opposite effects.

Correct answer: A. The desired charge may not remain

Explanation: The governing concept is the sequence required for induction charging. The external charged object maintains the electric field and charge separation while earthing permits charge exchange. If the object is removed first, the separating field disappears while the conductor is still connected to Earth. Electrons may then flow back or away until the conductor approaches the earth’s potential, so the intended net charge may be lost. Thus option A is correct; the conductor is not necessarily positive or negative in every setup.

Correct answer: A. The nearer opposite charge effect is stronger because its distance is smaller

Explanation: The governing concept is polarisation of an insulator and the dependence of electric force on distance. A nearby charged body slightly displaces bound positive and negative charges in the insulator, creating an induced dipole while keeping the net charge unchanged. The opposite sign is usually closer to the external charge, so its attraction is stronger than the repulsion from the farther like sign. The resultant force is therefore attractive. Option A is correct; no net charging, metallisation, or zero far-side effect is required.

Correct answer: C. Free charges rearrange in a conductor and bound charges shift slightly in an insulator

Explanation: Electrostatic induction in a conductor occurs because its mobile free charges redistribute under the influence of an external electric field; the conductor may remain neutral overall. In an insulator, electrons and nuclei are bound, so they undergo only a small relative displacement, producing polarisation. Thus option C correctly distinguishes charge mobility; charge is neither destroyed nor necessarily transferred to earth.

Correct answer: B. Local and non-uniform

Explanation: An insulator does not allow its bound charges to spread freely across its surface. Therefore, if excess charge remains concentrated in one region, the electric field is strongest and changes most rapidly near that region, so it is localised and non-uniform. Option B is correct. A zero field is characteristic of the interior of an ideal conductor in electrostatic equilibrium, not of a charged insulator.

Correct answer: A. Assuming that free charges can move far in it

Explanation: A conductor becomes equipotential in electrostatic equilibrium because its free charges move until the internal electric field vanishes and no potential difference remains within it. An insulator has charges that are largely bound, so they cannot freely redistribute over the material to equalise potential. Hence option A identifies the basic mistake; the other statements are unrelated to equipotential behaviour.

Correct answer: C. Charge can flow through the body and earth

Explanation: Metals contain mobile electrons, so a charged metal object can lose charge through any conducting path. When it is held by a person, the body provides a path with finite resistance to the surroundings and usually to the earth, allowing charge to flow away or arrive until electrical equilibrium is reached. Therefore C is correct. Metals do contain electrons, and neither colour nor insulation explains the loss.

Correct answer: A. Because they are identical conductors and become equipotential

Explanation: When identical conducting spheres touch, free electrons can move between them until both spheres reach the same electric potential. For identical spheres, equal potential corresponds to equal charge because their capacitances are equal, so the total charge is shared equally: each finally has half the conserved total charge, Q_final = (Q1 + Q2)/2. Therefore A is correct; charge is not destroyed.

Correct answer: B. No, because excess charge can be on the surface

Explanation: In electrostatic equilibrium, free charges in a conductor rearrange themselves so that the electric field within the conducting material becomes zero. This condition does not mean that the conductor has no net charge. Any excess charge resides on its outer surface, where it can produce an external field. Hence B is correct; zero internal field expresses equilibrium, not absence of charge or concentration at the centre.

Correct answer: A. On the outer surface

Explanation: In electrostatic equilibrium, free charges in a conductor cannot remain distributed through its bulk because their mutual repulsion and the electric forces drive them until the internal field is zero. Consequently, the volume charge density inside the conducting material is zero, while any excess charge is represented by a surface charge density on the outer surface. Therefore option A is correct; it is not confined to the centre or converted into mass.

Correct answer: A. A state where no further electric force drives them

Explanation: Free charges in a conductor respond to electric forces and redistribute themselves while an internal electric field exists. They finally reach electrostatic equilibrium when the internal field is zero, the conductor is at uniform potential, and no further electric force causes systematic charge motion. Thus A correctly describes the final state. Charge is not destroyed, charges are not confined to the centre, and the material does not become an insulator.

Correct answer: B. Such electric-field energy is not present inside

Explanation: The energy density of an electrostatic field is u = ½ε₀E² in vacuum, or more generally u = ½εE² in a material. Since the electric field inside the closed conductor is E = 0, this field-energy density is also zero. Therefore, no energy associated with an electric field is stored in that interior region under the stated electrostatic condition. The other options are unrelated to electrostatic field energy.

Correct answer: C. Surface charges rearrange and can make the internal electric field zero

Explanation: A metal contains mobile conduction electrons. When an external electrostatic field is applied, these charges redistribute over the cage’s outer surface until electrostatic equilibrium is reached. Their induced field opposes the applied field within the enclosed region, making the net electric field inside zero for a closed conductor, provided no charge is placed inside the cavity. This phenomenon is called electrostatic shielding or the Faraday-cage effect.