Conductors and Insulators

Correct answer: A. Conductor

Explanation: Aluminium is a metal. Metals have free electrons that can move easily and carry electric charge, so aluminium is an electrical conductor. Unlike semiconductors, whose conductivity is controlled by conditions, aluminium shows metallic conduction under ordinary conditions.

Correct answer: D. Insulator

Explanation: The governing concept is the distinction between bound and mobile charge carriers. In an insulator, electrons are strongly attached to atoms or molecules and cannot drift freely through the bulk material, so charges remain largely bound. Therefore, option D is correct. Copper, silver, and aluminium are metals containing mobile conduction electrons, which is why they generally allow electric current to flow much more easily.

Correct answer: A. Because they contain free electrons that allow charge to flow easily.

Explanation: The governing concept is metallic conduction. Metals contain mobile conduction electrons; when a potential difference creates an electric field inside the wire, these electrons acquire a small net drift and produce current. Thus option A gives the correct reason. High resistance would oppose current, and an insulator would not carry current readily. Cost is not the physical explanation, although particular metals may be chosen for economic or engineering reasons.

Correct answer: B. Insulators

Explanation: Plastic, rubber, and glass are insulators because their electrons are tightly bound to atoms and cannot move freely through the material. Consequently, they offer high resistance and allow only a very small current under ordinary conditions. Metals such as copper are conductors because they contain mobile electrons. Semiconductors have intermediate conductivity, while magnetic behaviour is unrelated to the defining property asked here.

Correct answer: C. Conductors

Explanation: Copper, silver, and aluminium are metals whose loosely bound outer electrons can move through the solid. These mobile charge carriers respond to an applied electric field, allowing current to flow readily; therefore, option C is correct. Insulators strongly bind their electrons, semiconductors have intermediate conductivity, and superconductors require special conditions such as very low temperature, so those alternatives do not describe these materials in ordinary use.

Correct answer: B. Zero

Explanation: The electric force on a test charge q is F = qE. Here, E = 0 at the point inside the conductor, so F = q(0) = 0. Therefore, the force is zero. Option A describes the usual dependence of force on charge, but when the electric field is zero, any finite test charge experiences no electric force.

Correct answer: A. Because mobile charge carriers in a conductor move due to mutual repulsion until equilibrium is established.

Explanation: In a metallic conductor, free electrons can move, whereas positive ions are normally fixed near their lattice positions. Mutual repulsion among excess charges causes the mobile charges to redistribute until electrostatic equilibrium is reached and the electric field inside the conductor becomes zero. Hence, excess charge resides on the outer surface. Its distribution is not necessarily uniform; charge density can be greater near sharp points. Unlike option B, positive ions do not flow freely in an ordinary metal conductor.

Correct answer: A. Because charge carriers in an insulator cannot move freely.

Explanation: In an insulator, most electrons are bound to atoms, so there are very few mobile charge carriers. Therefore, deposited charge cannot easily move from one place to another and often remains localized near where it was placed. In contrast, a conductor such as a metal has free electrons, allowing charge to redistribute over its surface; thus option B describes a conductor, not an insulator.

Correct answer: A. In electrostatic equilibrium, the electric field inside a conductor is zero.

Explanation: In electrostatic equilibrium, free charges redistribute on a conductor so that the electric field within the conducting material becomes zero. Hence, an enclosed hollow cavity is protected from an external electrostatic field; this is electrostatic shielding. Properties such as low mass or transparency do not cause shielding.

Correct answer: A. Because the enclosed metal body carries the lightning current mainly over its outer surface, keeping the electric field inside very small.

Explanation: An enclosed metal car body acts as a conductor. When lightning strikes, the current travels mainly over the outside of the body, so the electric field inside is very small; this is related to electrostatic shielding or the Faraday-cage effect. Rubber tyres are not the main reason for this protection. For relative safety, the occupant should remain inside and avoid touching metal parts.

Correct answer: C. Metal

Explanation: Metals are conductors and contain mobile electrons, so excess charge redistributes rapidly. In electrostatic equilibrium, the excess charge resides on the outer surface of a metal. Rubber, dry glass, and plastic are insulators, so charge does not spread through them readily.

Correct answer: D. Rubber

Explanation: A deposited charge can remain localized on rubber because rubber is an insulator. Its charge carriers are not free to travel throughout the material, so the charge does not rapidly redistribute away from the placement region. Silver, copper, and aluminium are conductors; their mobile electrons allow excess charge to move and redistribute over the surface when electrostatic equilibrium is reached. Therefore, option D is correct.

Correct answer: A. Conductor

Explanation: The defining electrical property of a conductor is the presence of mobile charge carriers, usually free electrons in metals. When an electric field is applied, these carriers acquire a directed drift and produce current, so option A is correct. An insulator binds its charges strongly. A perfect vacuum contains no material charge carriers, and dry soil is generally a poor and variable conductor rather than the standard answer to this definition.

Correct answer: B. Insulator

Explanation: An insulator is a material in which electrons or other charge carriers are tightly bound and cannot move freely under a small applied electric field. This produces high electrical resistance and very little current, so option B is correct. Conductors have many mobile carriers. Semiconductors have limited but controllable conductivity, while electrolytes conduct through ions in solution; neither is the general category described by the question.

Correct answer: A. Because charge can flow to Earth through the body.

Explanation: The human body has some electrical conductivity because it contains water and ions. If the person is connected to Earth, touching the metal can provide a path for excess charge to flow to Earth. Thus, the charge on the metal object can decrease; this is called earthing. Option B is incorrect because touching does not make the metal's resistance infinite.

Correct answer: A. Because plastic has very few mobile charge carriers.

Explanation: Dry plastic is an insulator. Most of its electrons are bound to atoms or molecules, so it has very few mobile charge carriers. Therefore, an added charge does not move easily throughout the plastic and can remain localized. In contrast, a metal has mobile free electrons, so charge spreads through it quickly.

Correct answer: A. To attain electrostatic equilibrium

Explanation: Charges in a conductor are mobile, so they repel one another and move in response to any internal electric field. This redistribution continues until electrostatic equilibrium is reached, at which point the electric field inside the conductor is zero and the conductor is at one potential. It is not caused by colour, mass, or an always-decreasing temperature. Therefore, option A is correct.

Correct answer: A. Zero

Explanation: In electrostatic equilibrium, the free charges in a conductor are at rest. If the electric field had a tangential component at the surface, it would exert a force along the surface and make the charges move. Therefore, the tangential component must be zero. In contrast, the normal component just outside the surface need not be zero; it is related to the surface charge density.

Correct answer: A. Rubber is an insulator, so it greatly reduces the flow of current to the body.

Explanation: Rubber has very few free charge carriers, so it has high electrical resistance and does not allow current to flow easily. Dry, undamaged rubber gloves rated for the appropriate voltage form an insulating barrier between the hand and a live part, greatly reducing current through the body. In contrast, a conductor allows current to flow readily and would not provide this protection.

Correct answer: A. Good conductor

Explanation: In a material with many free electrons, the electrons move easily when an electric field is applied and carry electric current. Therefore, the material is generally a good conductor. In contrast, an insulator or dielectric has very few free charge carriers.

Correct answer: A. As a good insulator

Explanation: When charge carriers are tightly bound to atoms, they cannot move easily in response to an external electric field. Therefore, very little electric current flows, and the material generally behaves as a good insulator. In contrast, a good conductor has many charge carriers that can move relatively freely.

Correct answer: A. Electric field

Explanation: Free charges in a conductor already have random thermal motion, but an applied electric field exerts an electric force on them. This produces motion with a definite average direction, called drift motion. A potential difference can establish an electric field in a conductor, but the direct cause of directed motion is the electric field.

Correct answer: A. Free charges would experience a force and move to redistribute themselves.

Explanation: A conductor contains free charges. If the electric field inside it were nonzero, the free charges would experience an electric force and move to redistribute themselves. This redistribution continues until the electric field inside the conductor becomes zero in electrostatic equilibrium. Therefore, option A is correct; an unchanged nonzero internal field would not satisfy electrostatic equilibrium.

Correct answer: A. Motion and distribution of charges

Explanation: In a conductor, free charges can move relatively easily and hence redistribute; in electrostatic equilibrium, excess charge resides on the outer surface of the conductor. In an insulator, charge motion is highly restricted, so charges generally remain localized near where they are placed or produced. Thus, conductors and insulators explain the motion and distribution of charge, not the quantisation of charge.

Correct answer: B. On the surface of the metal

Explanation: The governing principle is electrostatic equilibrium in a conductor. Mobile electrons redistribute until the electric field inside the conducting material becomes zero. Any excess charge therefore remains on the conductor’s surface; its density can be greater near sharp edges, but it is not restricted to the centre or a single edge. Hence option B is correct, while A, C, and D contradict conductor behavior.