Conductors and Insulators

The governing concept is surface charge distribution on a conductor in electrostatic equilibrium. Excess charge stays on the outer surface, but its surface density depends on geometry. At regions with smaller radius of curvature, especially sharp points, charges crowd more strongly, producing a larger surface charge density and electric field. Option A is correct; colour, absence of charge and absence of electrons are not physical explanations.

The governing concept is electrostatic equilibrium in a conductor. If two points inside the conductor had different potentials, a potential gradient and hence an electric field would exist between them. Free charges would then move until this field vanished. Therefore the electric field inside becomes zero and the entire conductor, including its surface, has one potential. Metallic colour, weight, and temperature do not establish the equipotential condition.

When two metal spheres touch, they form one conducting system, so mobile charges can move between them. The transfer continues until both spheres reach the same electric potential. Because the spheres are identical, equal potential also means equal capacitance and therefore equal final charge. If the initial total charge is Q, each sphere finally carries Q/2, provided there is no charge leakage to the surroundings. Charge is conserved; it is not destroyed.

Conductors in contact exchange charge until their potentials become equal, not until their charges become equal. For an isolated conductor, Q = CV. Unequal-sized conductors generally have different capacitances, so the same final potential gives different charges: Q1 = C1V and Q2 = C2V. Equal sharing occurs only in special cases such as identical spheres. Charge is conserved and can enter either conductor; size does not block electron motion.

At electrostatic equilibrium, the surface of a conductor is equipotential, but the surface charge density need not be uniform. Charge crowds more strongly in regions of small radius of curvature, such as a sharp tip. Just outside the conductor, the normal field is related to surface charge density by E = σ/ε0; hence a larger σ produces a stronger local field. A tip is not an insulator, and charge is not destroyed there.

For a conductor in electrostatic equilibrium, the electric field immediately outside the surface is normal to the surface and has magnitude Eout = σ/ε0, where σ is the local surface charge density. Thus, if σ increases at a particular region, the nearby external field increases in magnitude there, with its direction set by the sign of the surface charge. The field is not confined inside the conductor, where it is zero in equilibrium, and it does not convert into mass.

A negatively charged rod repels the mobile electrons in the nearby neutral conductor. When the conductor is connected to Earth, Earth acts as a very large charge reservoir, so the repelled electrons can flow from the conductor into the Earth through the grounding connection. The rod remains nearby during this process. The electrons do not move toward the negative rod, become fixed at the centre, or transform into mass; the effect is charging by induction with earthing.

The zero-field result applies to the conducting material in electrostatic equilibrium, because any internal field would drive free charges and cause motion. It does not imply that the conductor produces no field outside. Excess charge resides on the surface and its electric field extends into the surrounding space. In fact, just outside a charged conductor the normal field is E = σ/ε0. Thus internal equilibrium and an external field are perfectly consistent.

An equipotential conductor is established by the redistribution of mobile free charges: they move until the internal electric field vanishes. In an insulator, charge carriers are strongly bound to atoms or molecules, so charge cannot generally redistribute freely throughout the material. Treating it as an equipotential conductor therefore incorrectly assumes free charge mobility and zero internal field everywhere. Polarization may occur in an insulator, but that is not the same as conductor-like equipotential behavior.

The external charge changes the electric potential and redistributes mobile charges in the earthed conductor. A positive external charge attracts electrons from Earth into the conductor, whereas a negative external charge repels electrons from the conductor toward Earth. Therefore the direction of electron flow depends primarily on the sign of the external charge, with the exact amount also affected by geometry and distance. Colour, name, and mass do not determine the direction.

The governing concept is electrostatic induction in conductors. Free charges inside a conductor can move, and a nearby external charge attracts or repels them, producing a non-uniform surface distribution. Thus the presence, sign, and position of nearby charged objects can change the distribution even without contact. The conductor’s name, book colour, and sound intensity have no role in this electrostatic effect, so option A is correct.

The governing principle is conservation of charge for an isolated conductor. An external charge exerts forces and causes the conductor’s free charges to redistribute, so positions, local surface effects, and surface density can change. However, without contact or an earthing path, no charge enters or leaves the conductor. Therefore its algebraic net charge remains constant, making option A correct; induction changes distribution, not total charge.

The governing concept is the limited mobility of charge in an insulator. Since its charge carriers are bound rather than freely moving throughout the material, charge can remain concentrated in a particular region. The electric field produced by that localized distribution generally varies with position and may be especially strong nearby. It is not always zero or uniform, and it need not imitate a conductor’s field, so option A is correct.

In electrostatic equilibrium, the electric field inside a conductor must be zero; otherwise its free charges would continue accelerating and equilibrium would not exist. The mobile charges therefore move until their arrangement cancels the internal field. Mutual repulsion also drives excess charge toward the outer surface, where it remains for an isolated conductor. Charge is not destroyed, and colour or gravity is irrelevant, so option A gives both reasons.

Electrostatic shielding follows from the behavior of free charges in a conductor. When an external electric field is applied, charges on a closed conducting shell redistribute over its surfaces so that the net field within the conducting material is zero and, under suitable closed-shell conditions, the protected interior is shielded from external electrostatic influence. The effect is unrelated to sound, light amplification, or conversion of charge into mass; therefore A is correct.

The governing concept is electrostatic shielding. An external charge produces an electric field, but free charges in the conducting material redistribute on its surfaces. Their induced field opposes the external field, making the resultant electrostatic field inside the conducting region zero; for a properly closed conductor, the cavity is thereby protected from external influence. Charge is not converted into mass, gravity is not stopped, and insulation is not the cause, so A is correct.

The fundamental distinction is the mobility of charge carriers. In a conductor, electrons or other carriers can move relatively freely through the material, allowing current and rapid charge redistribution. In an insulator, charge carriers are strongly bound, so charge does not move freely over the whole body and may remain localized. Length, shine, and the object’s name do not determine this electrical classification; therefore option A is correct.

Earthing removes charge efficiently only when charge carriers can move through a conducting path. In a conductor, mobile carriers redistribute and flow to Earth, bringing the potential toward ground. In an insulator, carriers are bound and the charge may remain trapped in localized regions, so contact with Earth does not immediately remove all of it. Insulators can hold charge, and Earth can accept charge; hence option A is correct.

For a conductor in electrostatic equilibrium, the electric field just outside its surface is normal to the surface and has magnitude E⊥ = σ/ε₀, where σ is the local surface charge density. Therefore, if σ is zero at a particular point, the normal field component there can also be zero. It is not necessarily infinite or maximum; option D is physically unrelated.

When a charged body is brought near an insulating object, its bound charges shift slightly, producing polarisation. The induced opposite charge appears closer to the external charge, while the like charge is farther away. Since electrostatic force varies as 1/r², the nearer attraction is stronger than the farther repulsion, giving a net attraction. Thus option A is correct; the other statements are false.