Conductors and Insulators

Rubbing can transfer electrons and charge the plastic comb. The paper is initially neutral, but the comb's electric field slightly separates charge within it, producing polarisation. Opposite charge is effectively induced on the side nearer the comb, and because it is closer, its attractive force is stronger than the repulsion from the farther like charge. Hence the paper is attracted to the comb.

In electrostatic equilibrium, free charges in the conductor redistribute so that the electric field within the conducting material is zero. If the closed cavity contains no charge, its inner surface is an equipotential surface, and the electric field throughout the cavity is also zero. Thus, an external electrostatic field does not penetrate the cavity; this is electrostatic shielding. Option A is incorrect because charge redistribution on the conductor cancels the effect of the external field inside the cavity.

In electrostatic equilibrium, the electric field within the conducting material is zero. If excess charge remained in the interior volume, it would produce an internal electric field and charges would continue to move until equilibrium was reached. Therefore, excess charge resides on the outer surface, not throughout the volume or at the centre.

In an insulator, electrons and positive charges are generally bound to atoms or molecules. When an external electric field is applied, these bound charges shift by very small amounts in opposite directions, producing electric dipoles. This process is called dielectric polarisation. Since charges do not move freely through the material, no sustained current flows. In contrast, conduction involves the bulk motion of free charges.

The negatively charged object repels the conductor’s free electrons toward the far side. When the conductor is earthed, these electrons flow to Earth. If the earth connection is removed before the nearby object is taken away, the electrons cannot return; the conductor is left with a deficiency of electrons and hence becomes positively charged. This is charging by induction; charging by conduction would require direct contact between the charged object and the conductor.

Rubbing can transfer electrons from one material to another, thereby charging an object. In an insulator, electrons are tightly bound to atoms or molecules, so there are very few freely mobile charge carriers. Hence, the transferred charge remains localized near the rubbed region. In contrast, free electrons in a conductor allow charge to spread relatively quickly.

The key difference is the mobility of charge carriers. In a conductor, free electrons can move through the material and redistribute charge, usually placing excess charge on the surface in electrostatic equilibrium. In an insulator, charges are mainly bound to atoms or molecules and remain localized, with only limited polarization. Thus option A gives the governing reason; colour, shape, mass and sound do not determine this distribution.

In electrostatic equilibrium, free charges redistribute until the electric field within the conducting material becomes zero. If a non-zero field existed inside, it would exert a force on free charges and they would continue to move, so equilibrium would not be possible. Even a uniform non-zero field as stated in option B would cause charge motion.

Electrostatics distinguishes materials by how easily their charge carriers move. In conductors, mobile electrons redistribute charge rapidly, whereas in insulators charges remain largely bound and may only shift slightly during polarization. This proves that material nature affects charge motion and distribution, so option A is correct. Colour has no governing role, materials do not conduct equally, and electric fields arise from charge rather than only from mass.

In electrostatic equilibrium, free charges in the conductor rearrange to oppose the external electric field. Since there is no charge in the cavity of the closed conductor, the cavity boundary is an equipotential surface and the resultant electric field inside the cavity is zero. Thus, the external field does not penetrate the cavity. Unlike option B, no non-zero uniform field is produced inside the cavity.

A tangential electric field exerts a force parallel to the conductor’s surface on free charges, so they begin to move. The charges redistribute until electrostatic equilibrium is established. In electrostatic equilibrium, the tangential component of electric field at a conductor’s surface is zero and the surface is equipotential. Thus, option D is not true initially; it describes the situation after equilibrium has been reached.

Free charges in a metal can move. They redistribute until electrostatic equilibrium is reached, making the electric field inside the conductor zero. Therefore, the excess positive charge on an isolated metal sphere resides on its outer surface. Option C is incorrect because excess static charge does not remain distributed throughout the volume of a conductor.

In polarisation, the centres of bound positive and negative charges in an insulator are displaced slightly. This separates charge and may produce bound charges on its surfaces, but charge is neither created nor destroyed. Therefore, if no charge is transferred from outside, the insulator’s net charge remains unchanged; this is different from the net charge becoming positive.

The negatively charged rod repels the conductor’s free electrons. The electrons redistribute toward the end farther from the rod, so that end acquires a negative induced charge. The nearer end becomes positive because it has a deficiency of electrons. The conductor’s total charge remains zero; only the charge distribution changes.

The positively charged rod attracts the conductor’s free electrons toward the nearer end. The farther end is therefore deficient in electrons and develops an induced positive charge. This is charge separation, so the isolated conductor still has zero net charge overall. Induced negative charge appears at the end nearer the rod, not at the far end.

In electrostatic equilibrium, the electric field inside a conductor is \,\(\vec{E}=0\). Hence \(\Delta V=-\int \vec{E}\cdot d\vec{l}=0\). Therefore, all points inside the conductor are at the same potential. The conductor may have a non-zero potential due to its charge, but the potential difference between any two internal points is zero.

For a charged conductor, excess charge accumulates more densely at regions of small radius of curvature, such as a sharp tip. The electric field just outside the tip therefore becomes very strong. If it exceeds the breakdown strength of the surrounding air, ionisation and electric discharge can occur. Hence option A is correct. The effect does not increase mass, cancel magnetism or destroy charge; it concerns field strength and discharge.

Earth acts as an extremely large conductor and charge reservoir. When the sphere is grounded, charge flows until the potential difference between the sphere and Earth becomes zero. Hence, in electrostatic equilibrium the sphere is at Earth's potential, conventionally taken as zero. Option A is incorrect because the initial potential does not remain unchanged.

Electrostatic induction separates the existing positive and negative charges inside a neutral conductor; it does not create charge. If the conductor is isolated and there is no earthing, no charge can enter or leave, so conservation of charge requires its net charge to remain zero. Thus option A is correct. The conductor may have opposite charges on different regions, but their algebraic sum remains zero; it is not automatically positive, negative or infinite.

A metal is a conductor, so its free electrons and any excess charge can move easily. When the rod is held, the hand and body provide a path for charge to flow to the Earth, especially when the person is in contact with the ground. Thus, the charge is not destroyed; it leaks from the rod to the Earth, leaving the rod nearly neutral. Option C is incorrect because friction does not destroy charge.