Conductors and Insulators

An insulator normally does not permit sustained conduction current because its charge carriers are bound. However, an external charged body can slightly displace positive and negative charges within its molecules, producing polarisation. The closer induced opposite charge experiences a stronger force, so attraction occurs without charge flowing through the insulator. Therefore A is correct; B changes the material, while C and D are physically impossible descriptions.

In electrostatic equilibrium, the electric field inside the conducting material is zero. Therefore, free charges experience zero net electric force, so there is no net drift or current caused by an electric field. Option B is incorrect because random microscopic thermal motion may still occur; only the net directed drift is zero.

An insulator has very few mobile free charge carriers, so it has high electrical resistance. A plastic covering confines the current to the metal wire and reduces the chance of leakage current reaching a person's body, thereby helping prevent electric shock. In contrast, a low-resistance conductor allows current to flow easily.

The governing idea is the contrast between conductors and insulators. Copper contains mobile electrons and has low resistivity, so it forms the conducting core and carries current efficiently. Rubber has very high resistivity, so it electrically insulates the user from the conductor and helps prevent shocks or short circuits. Hence A is correct; B reverses the roles, while C and D ignore the electrical properties of the materials.

This is the principle of electrostatic shielding. In a metal, free charges move under an applied electric field and redistribute themselves over the outer surface. Their induced field opposes the applied field in the enclosed region, making the net internal field very small or zero under ideal electrostatic conditions. Thus A is correct. The cage does not destroy charge, block gravity, or lack charge carriers, as the other options suggest.

Electrostatic equilibrium requires that free charges have no net force and therefore no continued drift. If excess charge is located within the bulk of a conductor, mutual repulsion and the resulting electric field drive the mobile charges toward the outer surface. Redistribution continues until the internal electric field is zero and excess charge resides on the surface. Therefore A is correct; the other choices do not describe the equilibrium condition.

In electrostatic equilibrium, the free charges in a conductor are at rest. If there were a tangential component of the electric field along the surface, it would exert a force on surface charges and make them move. Charges redistribute until the tangential component becomes zero. Hence, the electric field just outside the surface has only a normal component. Option B is incorrect because the normal component at the surface of a charged conductor is generally not zero.

A metal wire provides a low-resistance conducting path between the conductor and Earth. Charge then flows between Earth and the conductor until the conductor reaches approximately Earth’s potential. An insulating stand or a glass layer does not provide such a conducting path, so rapid earthing does not occur.

Rubbing can transfer electrons and create excess charge on a region of an insulator's surface. However, electrons in an insulator are generally bound to atoms or molecules, so there are not enough mobile charge carriers to carry the charge through the material. The charge therefore usually remains localized near the rubbed region. In contrast, free electrons in a conductor can redistribute the charge. Option C describes the behavior of a conductor, not an insulator.

In charging by contact, a charged conductor makes direct contact with a neutral conductor. In option A, electrons transfer from the negatively charged sphere to the neutral sphere, so the second sphere is negatively charged after they are separated. In option B, there is no contact and only temporary charge separation due to induction occurs; option C describes charging by induction with earthing, whereas option D describes charging by friction.

In charging by induction, the charged object does not touch the conductor; it causes separation of charges in the conductor. When the conductor is earthed while the charged object is nearby, electrons flow between the earth and the conductor. Disconnecting the earth connection first leaves the conductor with a net charge; removing the charged object afterwards does not remove this charge. In option D, there is only temporary charge separation, so the conductor does not retain a permanent net charge.

The governing concept is electrostatic induction. A neutral conductor has equal total positive and negative charge, so its net charge is zero, but an external charged body can attract mobile electrons toward one side and leave the opposite side relatively positive. This spatial separation creates local effects without changing the total charge. Hence A is correct; charge is not destroyed, mass need not change, and the conductor is not an insulator.

Rubber is an insulator with very high resistivity. It has very few freely mobile charge carriers, so it opposes the flow of current between the electrical source and a person's hand. Consequently, the current through the body is reduced, lowering the risk of electric shock. Option B is incorrect because rubber does not absorb current; it primarily resists the flow of current.

Copper has many free electrons and low resistance, so it allows electric current to flow easily. Plastic has very high resistance, so it prevents current leakage from the conductor and reduces the risk of electric shock on touching the wire. Option D is incorrect because the main role of plastic covering is electrical insulation and safety, not merely mechanical strength.

In electrostatic equilibrium, free charges in a conductor rearrange until the electric field inside the conducting material becomes zero. If an internal electric field were non-zero, it would exert a force on free charges and cause further motion. Therefore, the excess charge on an isolated solid conductor resides on its surface. Option B is incorrect because excess charge is not uniformly distributed throughout the conductor’s volume.

In an insulator, charge carriers cannot move freely throughout the material, so deposited excess charge may remain localized. An external electric field can cause a small relative displacement of bound positive and negative charges; this is polarization. Option C describes electrostatic equilibrium in a conductor, whereas the electric field inside an insulator need not be zero.

In a conductor, some charge carriers, usually electrons, are relatively free from atoms and can move in an electric field. In an insulator, electrons are tightly bound, so there are very few mobile charge carriers. This difference arises from the material's internal electronic structure, not merely from the number of atoms or the object's shape.

The broad governing idea is that material structure controls charge behaviour. Conductors have many mobile charge carriers, so charge moves and redistributes readily and can produce shielding. Insulators hold charges more tightly, so charge remains localized and polarisation occurs mainly through small displacements. Thus A correctly combines motion, distribution, and shielding; B, C, and D conflict with the defining electrical properties of materials.

A conductor contains free charges. If a nonzero electric field exists in its conducting material, the free charges experience an electric force and redistribute. Therefore, electrostatic equilibrium cannot be established until the electric field within the conducting material becomes zero. Option D is incorrect because
\(\mathbf{E}=-\nabla V\); a nonzero electric field implies that the potential changes with position.

The governing principle is electrostatic equilibrium in a conductor. A tangential electric field would exert a force qE along the surface on mobile charges, causing them to move. Since charges are at rest in electrostatic equilibrium, the tangential component must be zero; only the normal component may remain. Option B is correct. The other options falsely deny surface charge, claim zero net charge, or confuse conductors with insulators.