Conductors and Insulators

A positively charged rod attracts electrons. When the nearby conductor is connected to Earth, Earth acts as a very large reservoir of mobile charge, so electrons can flow from Earth into the conductor toward the positive rod. This gives the conductor an excess of electrons while the external rod remains separate. Therefore option A is correct; electrons do not turn into protons or disappear, and their direction is opposite to conventional current.

The governing idea is repulsion between like charges. A negatively charged rod pushes the conductor’s mobile electrons away from the rod. If the conductor is earthed, these repelled electrons can continue through the connection and flow from the conductor into Earth. Hence option A is correct. They do not flow into the isolated rod, and earthing means that movement is possible rather than impossible; conventional current would be opposite to electron flow.

In the induction sequence, the nearby charged object first maintains separation of charges while earthing allows electrons to enter or leave. The earth connection must be removed while the external object is still present; otherwise the charge can flow back and neutralise when the inducing object is removed. After isolation from Earth, removing the external object only removes the polarising field, so the conductor retains its net induced charge. Option A is correct.

Successful induction charging depends on the order of operations. While the external charged object is present, it separates charge in the conductor. If that object is removed first while the conductor is still earthed, the separating influence disappears but the conducting path to Earth remains. Electrons can then flow until the conductor is neutral or reaches the earth-potential condition, so the desired net charge may not be retained. Option A is correct.

The governing concept is polarisation of a dielectric. In an insulator, bound positive and negative charges shift slightly in opposite directions, but no net charge is created. When an external charge is nearby, the induced opposite-sign charge effect lies closer to it than the like-sign effect on the farther side. Since electrostatic force increases as separation decreases, the nearer attraction is stronger and a net attraction appears. Option A is correct.

The governing concept is the mobility of charge carriers. In electrostatic induction, free electrons in a conductor can travel through the material and collect on different regions, although the total charge is conserved. In polarisation, charges are bound to atoms or molecules, so their centres shift only slightly or dipoles orient. Therefore B is correct; A and C violate charge conservation, while D is not necessary.

Charging by rubbing transfers electrons from one material to another; it does not destroy charge. The governing distinction is that an insulator has no large population of mobile charge carriers. Consequently, the transferred electrons remain near the rubbed region instead of redistributing over the whole surface. Thus A is correct. B is contradictory, C violates conservation of charge, and D is a property associated with electrostatic conductors, not all insulators.

The electric field is determined by the spatial distribution of charge. Since charge on an insulator can remain concentrated in a limited region, the field strength and direction can vary from point to point around that region; it is therefore local and non-uniform. A is correct. A zero field is not implied by an insulator, uniformity requires special symmetry, and conductor boundary conditions cannot automatically be transferred to an insulator.

An electrostatic conductor becomes equipotential because its mobile charges redistribute until no tangential electric field remains. In an insulator, charges are largely bound and cannot move freely over the body, so an uneven potential can persist. Hence B gives the governing reason. An insulator can contain charge, it certainly contains atoms or molecules, and it is not necessarily metallic; therefore A, C and D are false.

A metal has mobile electrons, so excess charge can move readily through it. When the object is held, the human body provides a conducting path with some resistance to the surrounding Earth; leakage therefore removes the charge, especially in humid conditions. A is correct. Metals do contain electrons, charge does not turn into colour, and metals are conductors rather than insulators, so B, C and D are invalid.

A charged metal object retains charge only when an easy leakage path is prevented. An insulating stand separates it from Earth and from other conductors, so its mobile electrons cannot readily escape. Therefore A is correct. A metal wire deliberately provides an earthing path, holding it connects it through the body, and a wet cloth generally increases surface conduction; B, C and D therefore favour discharge rather than retention.

When identical conducting spheres touch, free charge flows between them until electrostatic equilibrium is reached, meaning their potentials are equal. Identical spheres have equal capacitance, and Q = CV; with the same final potential and the same C, their final charges are equal. Thus A is correct. Charge is conserved, the spheres are conductors, and gravity is not the mechanism, so B, C and D are incorrect.

Conductors in contact exchange free charge until their potentials become equal, not necessarily until their charges become equal. Using Q = CV, unequal conductors generally have different capacitances, so at a common final potential their charges satisfy Q1/Q2 = C1/C2 and need not be equal. A is correct. Charge can enter either conductor, both contain electrons, and total charge is conserved; therefore B, C and D are false.

The zero-field result applies to the interior of a conductor in electrostatic equilibrium: free charges have redistributed until the internal force is cancelled. It does not say that the conductor carries no net charge. Any excess charge resides on its surface, while the field inside remains zero. Hence B is correct. A and C confuse field with charge, and D incorrectly places excess charge at the centre.

In electrostatic equilibrium, free charges in a conductor are mobile and repel one another. They continue moving until the electric field inside the conducting material is zero; consequently, the volume charge density in the interior is zero and any excess charge appears on the surface. A is correct. It is not confined to the centre, does not fill the volume, and cannot transform into mass, so B, C and D are rejected.

The governing concept is electrostatic equilibrium in a conductor. Mobile charges rearrange under electric forces until the conductor reaches a minimum-energy stable arrangement. At equilibrium, the electric field inside the conductor is zero and the potential is uniform, so there is no net force to drive continued charge motion. Therefore, option A is correct; charges do not necessarily collect at the centre, and charge is neither destroyed nor converted into mass.

Electric-field energy density is given by u = 1/2 εE² in a linear medium. If the electric field E inside the closed conductor is zero, this local field-energy density is also zero. Hence no electric-field energy is stored in that field-free interior region, making option A correct. The other choices contradict the relation, introduce an unrelated form of energy, or incorrectly claim that charge becomes mass.

This is the principle of electrostatic shielding. A metal has mobile electrons, so an external electric field causes charges to redistribute over its outer and inner surfaces as required. Their induced field opposes the applied field inside the closed conductor; in electrostatic equilibrium the net interior field becomes zero, provided no internal charge is present. Thus option A is correct, while the other choices describe false or irrelevant ideas.

The relevant principle is electrostatic shielding. The conducting metal body of a closed car allows excess charge from an external electrical discharge to redistribute mainly over its outer surface. In the ideal electrostatic picture, the electric field inside the enclosure is greatly reduced or zero, so the occupant is relatively protected. Option A is therefore correct; sound reflection, light refraction, and thermal expansion do not explain this electrical protection.

A lightning rod works through conductivity and earthing. Metal has low resistance, so it can carry a large transient current without behaving like an insulating barrier. Its connection to the Earth provides a broad charge reservoir and a comparatively safe path for the discharge to spread away from the protected structure. Therefore option A is correct. The rod does not destroy charge; charge is transferred, and colour has no physical role.