Conductors and Insulators

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.

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.

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.

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.

The governing concept is charge mobility in materials. Rubbing transfers electrons between surfaces, but an insulator contains very few mobile charge carriers. Consequently, the transferred charge remains near the rubbed region instead of redistributing over the entire surface. Option A is correct. Rubbing does not destroy charge, an insulator is not necessarily equipotential, and the presence of metal is not the reason; options B, C, and D are therefore incorrect.

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.

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.

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.

A metal allows excess charge to move easily, so charge is retained only when leakage paths are minimised. Placing the object on an insulating stand separates it from the earth and reduces conduction through supports or the body. Thus option C is correct. Earthing, holding the object, or using a wet cloth provides conducting paths and causes faster discharge rather than charge retention.

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.

On contact, two conductors exchange free charge until their potentials become equal, not until their charges become equal. For an isolated conductor, Q = CV; unequal size or shape generally gives unequal capacitance. At the same final potential, the conductor with larger capacitance can hold more charge. Therefore B is correct, while the other options contradict charge conservation or conductor properties.

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.

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.

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.

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.

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.

The conducting metal body of a car provides a partially enclosed path for charge. During a lightning event, charge tends to travel mainly over the exterior surface, so the electric field inside the passenger space can be greatly reduced, illustrating electrostatic shielding. This protection is not because the car is a perfect insulator or because lightning is destroyed. Safety still requires avoiding contact with metal parts and following proper lightning precautions.

A lightning rod must conduct a very large and rapidly changing charge, so metal is selected because it has many mobile charge carriers and low resistance. Connecting the rod to Earth provides a comparatively safe, low-impedance path through which charge can flow into the vast Earth, reducing the chance that it will pass through the building. The rod does not make charge disappear or convert it into light; it controls the discharge path.

In electrostatic equilibrium, excess charge on a conductor resides on its surface and is more concentrated where the radius of curvature is small. A sharp tip therefore has a relatively high surface charge density σ, and the field just outside the surface is approximately E = σ/ε₀. The stronger local field can promote ionisation of nearby air and a controlled discharge. A tip does not destroy charge, act as an insulator, or reduce mass.

In an insulator, electrons are strongly bound to atoms or molecules, so the material has very few mobile charge carriers. A charge placed at one location therefore remains largely localized instead of spreading rapidly through the entire object. Earthing that point may remove or alter nearby charge, but the rest may not reach Earth quickly. This contrasts with a conductor, where free charges redistribute readily throughout the surface and discharge more effectively.