Conductors and Insulators

The governing conditions are charge conservation and equal potential. When identical conducting spheres touch, free charge flows between them until both reach the same potential. Since their radii, and hence their capacitances, are equal, equal potential implies equal final charges. If the initial total charge is Q, each sphere finally carries Q/2 in the ideal isolated case. Therefore B is correct; charge is not destroyed, and gravity or insulation is not responsible.

The governing relation is Q = CV, together with the requirement that connected conductors reach a common potential. Unequal conductors generally have different capacitances, so at the same final potential their charges satisfy Q1/Q2 = C1/C2 rather than being equal. Thus C is correct. Charge can flow to both conductors, the smaller one still has electrons, and total charge is conserved; therefore A, B, and D are incorrect.

The governing concept is electrostatic equilibrium, not absence of charge. In a conductor at equilibrium, free charges rearrange so that the electric field within the conducting material is zero. This does not prevent excess charge from residing on the outer surface, where it produces fields outside the conductor. Hence D is correct. A and B confuse zero internal field with zero charge, while C incorrectly places excess charge necessarily at the centre.

The governing principle is electrostatic equilibrium in a conductor. Because free charges can move, any excess charge in the bulk would create an electric field and continue driving charge motion. The equilibrium condition therefore leaves zero volume charge density, with excess charge residing on the conductor’s surface; for an isolated solid conductor this is the outer surface. Thus A is correct, while B, C, and D conflict with charge mobility and conservation.

In a conductor, free charges initially move because an internal electric field exerts force on them. Their redistribution continues until electrostatic equilibrium is reached. At that stage, the electric field inside the conducting material is zero, the potential is uniform, and no net charge motion is driven by an internal electric force. Therefore, option B correctly describes the equilibrium state; charge is not destroyed, and the conductor does not become an insulator.

The energy density of an electrostatic field in vacuum is u = ½ε₀E²; in a linear material it is similarly proportional to E². Inside a conductor in electrostatic equilibrium, E = 0, so the local energy density associated with that electric field is zero. Hence option C is correct. Zero field does not imply infinite energy, sound energy, or conversion of charge into mass.

A metal contains mobile electrons. When an external electrostatic field is applied, these charges redistribute over the outer surface of the cage. Their induced electric field opposes the applied field within the enclosed region. In electrostatic equilibrium, the resultant field inside the conducting enclosure is zero, provided there is no charge placed inside the cavity. Thus option D gives the principle of electrostatic shielding.

The conducting metal body of a car behaves approximately as an enclosing conductor. During a lightning event, charge and current preferentially remain on or travel along the exterior conducting surface, so the electric field inside the passenger space can be much smaller. This is the principle of electrostatic shielding, making option A correct. The effect is not caused by sound reflection, light refraction, or thermal expansion; occupants must still avoid touching exposed metal.

A lightning rod is made of metal because metal has high electrical conductivity and can carry a large transient current. Connecting it to earth provides a low-resistance path to the ground, which acts as a huge charge reservoir and helps keep the protected structure near earth potential. Thus option B is correct. The rod does not destroy charge, make itself an insulator, or change colour; it safely conducts and disperses charge.

On a conductor, charge density is not uniform when the surface has different curvatures. At a sharp tip, the radius of curvature is small, so charge tends to concentrate there and the local surface charge density becomes large. Since the electric field just outside a conductor is related to surface charge density by E = σ/ε₀, the field is strong near the tip. Therefore option C is correct; a tip does not destroy charge or reduce mass.

In an insulator, electrons are strongly bound to atoms or molecules, so charge cannot move freely through the entire material. If charge is localized, touching one point to earth may remove or alter charge only near that contact; the rest may remain charged because there is no continuous low-resistance path. Hence option D is correct. Earth can accept charge, but the insulating material prevents rapid redistribution throughout the object.

An external electric field usually cannot make the bound electrons in an insulator flow freely through the material. However, it can produce a small relative displacement between the positive nucleus and the surrounding negative charge cloud, or orient existing molecular dipoles. This microscopic separation creates polarisation and an induced dipole moment. Therefore option A is correct; the insulator need not become a conductor and no charge is destroyed.

Paper is an insulator, but its molecules can be polarised by the electric field of a charged comb. The positive and negative charge centres shift slightly, so the side nearer the comb acquires an induced charge effect opposite to the comb’s charge. Because this opposite effect is closer, its attractive force is stronger than the repulsive force from the farther side, producing a net attraction. Thus option B is correct without requiring net charge transfer.

Net charge is the algebraic sum of all charge on the conductor, so equal positive and negative induced charges can give a total of zero. An external charged object or electric field can cause the conductor’s mobile electrons to redistribute: one surface region becomes electron-rich and negative, while another becomes electron-deficient and positive. This is electrostatic induction, not charge destruction or a change into an insulator. Therefore option C is correct.

The governing principle is electrostatic equilibrium in a conductor. Free charges move until the electric field inside the conducting material becomes zero. This does not mean that the conductor has no charge or that the external field vanishes. Excess charge resides on the surface and its electric field can exist outside the conductor. Therefore option D is correct; A, B and C contradict the properties of a conductor.

The governing concept is electrostatic shielding. In a closed conducting enclosure, external charges cause free charges on the enclosure to rearrange so that the electric field within the conducting material is zero; under suitable shielding conditions, the enclosed region is protected from external electrostatic influence. Thus option A is correct. Rubber and coloured glass do not provide conducting shielding, while a bare wire alone is not a complete enclosure.

Electrostatic equilibrium means that free charges in a conductor have no net motion. If a nonzero electric field existed inside, these charges would continue to move, so the internal field must be zero. Since the field inside is zero, the potential is constant throughout the conductor and on its surface. Any excess charge settles on the surface. Hence option B combines all three correct conditions.

An insulator contains very few mobile charge carriers; most of its charges are bound to atoms or molecules. Consequently, an added charge can remain localized instead of spreading freely over the surface. An external electric field can slightly displace bound positive and negative charges, producing polarisation. Therefore option C gives the correct deeper description. Options A and D describe conductor-like behaviour, while B incorrectly denies polarisation.

The fundamental basis is the availability and mobility of charge carriers. In a conductor, electrons or other carriers can move comparatively freely through the material, allowing charge redistribution and current. In an insulator, carriers are tightly bound and cannot move appreciably over macroscopic distances, although slight displacement and polarisation may occur. Thus option D is correct; colour, name and sound do not determine electrical conduction.

When a material has very few free charge carriers, charges cannot travel easily across it. An introduced charge may therefore remain localized. However, bound positive and negative charges can undergo a small relative displacement in an external electric field, producing polarisation. This is typical insulating behaviour, so option A is correct. Options B, C and D are characteristic of ideal conductor behaviour and are not generally valid for an insulator.