Conductors and Insulators

Correct answer: A. Conductor

Explanation: The governing concept is electrical conductivity: a conductor contains mobile charge carriers, such as free electrons in metals, that can respond and move when an electric field is applied. Therefore, option A, conductor, is correct. An insulator restricts charge movement; a neutral object describes net charge, not conductivity; and a magnet is defined by magnetic properties, not by easy electric-charge motion.

Correct answer: B. Insulator

Explanation: The governing idea is resistance to movement of charge. An insulator has tightly bound electrons or otherwise lacks mobile charge carriers, so electric charge cannot move through it easily; rubber, glass, and dry plastic are familiar examples. Thus option B is correct. A conductor permits charge movement, metal is a broad material category, and earthing is a safety connection rather than a material type.

Correct answer: C. Copper

Explanation: Electrical conduction depends on the availability of mobile charge carriers. Copper is a metal whose electrons can move through the material when an electric field is applied, giving it high conductivity and making it widely useful in wires. Option C is therefore correct. Rubber, glass, and dry wood generally have strongly bound electrons and offer much greater resistance, so they are treated as insulators in this context.

Correct answer: D. Rubber

Explanation: An electrical insulator has very few mobile charge carriers under ordinary conditions, so it strongly resists the flow of current. Rubber fits this description and is commonly used to cover wires for protection. Therefore, option D is correct. Silver, copper, and iron are metals containing mobile electrons and are generally conductors, although their conductivities differ. The word generally appropriately refers to normal conditions.

Correct answer: A. Because it has delocalized (mobile) valence electrons

Explanation: In metals, valence electrons are not tightly bound to individual atoms; they are delocalized and can move through the lattice. When an electric field is applied, these electrons acquire a directed drift and produce electric current. In contrast, the positive ions of a solid metal remain at their lattice positions and are not the main carriers of current.

Correct answer: B. Because they have very few free, mobile charge carriers

Explanation: In insulators, electrons are tightly bound to their atoms. Therefore, there are very few free, mobile charge carriers, so charge cannot move easily through the material when an electric field is applied. In contrast, conductors have many free electrons, allowing charge to flow readily.

Correct answer: A. On the outer surface of the conductor

Explanation: In electrostatic equilibrium, the electric field within the conducting material is zero. If excess charge remained in the volume of the conductor, it would produce an internal electric field and free charges would continue to move. Therefore, when no charge is present inside a cavity, the excess charge resides on the outer surface of the conductor. Charge may be induced on an inner surface only when a charge is placed inside the cavity.

Correct answer: D. On the outer surface of the conductor

Explanation: In electrostatic equilibrium, the electric field inside a conductor is zero. If excess charge remained in the interior, it would produce an electric field and free charges would continue moving until equilibrium was reached. Therefore, the excess charge on an isolated conductor resides on its outer surface, not uniformly throughout its volume.

Correct answer: A. Availability and mobility of free charge carriers

Explanation: Conductors have free electrons or other free charge carriers that can move easily when an electric field is applied. In insulators, such charge carriers are very few or tightly bound, so they allow very little electric current to flow. Mass, melting point, and colour may be properties of materials, but they do not define the main difference between conductors and insulators.

Correct answer: B. Insulator

Explanation: Rubbing can transfer electrons between the hair and the plastic. Plastic is generally an insulator, so the acquired charge cannot move freely through it and can remain on the comb. In contrast, charge moves more easily in a conductor and may spread away or flow to Earth.

Correct answer: C. Metal

Explanation: In metals, some electrons are loosely bound and delocalised, so they can move easily through the metallic lattice. Hence, they carry electric charge through the material. In contrast, electrons in glass, rubber, and plastic are tightly bound and generally cannot move freely.

Correct answer: D. Insulator

Explanation: The governing concept is the availability of mobile charge carriers. Dry wood contains very few freely moving electrons or ions, so an applied electric field produces only a negligible current. Therefore, it generally behaves as an insulator, making option D correct. Copper is a conductor, a superconductor requires special low-temperature conditions, and dry wood is not normally classified as a semiconductor. Moisture may increase wood’s conductivity.

Correct answer: A. Copper

Explanation: The governing concept is electrical conductivity, which depends on the presence of mobile charge carriers. Copper contains loosely bound electrons that can drift through the metal when an electric field is applied, so it conducts current well and option A is correct. Rubber, glass, and plastic hold their charges much more tightly and generally act as insulators. Thus the distractors do not permit easy charge flow under ordinary conditions.

Correct answer: B. As a large charge reservoir

Explanation: The governing idea is that the Earth has an enormous size and capacitance compared with ordinary objects. It can accept or supply a large amount of charge while its electric potential changes negligibly, so it is treated as a large charge reservoir. Hence option B is correct. It is not a small conductor, a perfect insulator, or a magnet; those descriptions do not explain the function of earthing.

Correct answer: C. Because the electric field inside the conductor is zero

Explanation: In electrostatic equilibrium, the electric field inside a conductor is zero. Therefore, free charges experience no electric force and have no net directed drift. They may still have random thermal motion, but this is not a net motion caused by an electric field. Option A is incorrect because the potential inside a conductor is constant, but it is not necessarily zero.

Correct answer: A. Because it has few free charge carriers

Explanation: The governing concept is charge mobility in a material. In an insulator, electrons are strongly bound to atoms or molecules, and there are very few mobile carriers available to transport excess charge over macroscopic distances. Consequently, deposited charge usually remains localized, so option A is correct. Being a metal would describe a conductor, while colour and gravity have no relevant role in the spreading of electric charge.

Correct answer: B. On the surface

Explanation: The governing concept is electrostatic equilibrium in a conductor. Free electrons in a metal move until the electric field inside the conducting material becomes zero. As a result, any excess charge resides on the outer surface of the metal sphere, so option B is correct. Charge does not remain at one arbitrary point, disappear, or stay only inside; for a symmetrical isolated sphere it distributes uniformly over the surface.

Correct answer: C. For safety, because plastic is an electrical insulator

Explanation: The wire usually contains a metal such as copper or aluminium, which conducts electricity. The outer plastic layer is an electrical insulator, so it prevents accidental contact with the conducting metal and reduces the risk of electric shock. Plastic is used to insulate the wire, not to increase the current.

Correct answer: D. Copper wire

Explanation: The governing concept is the use of a low-resistance conducting path in an electrical device. A copper wire contains mobile electrons, allowing current to pass efficiently, so option D is correct. Rubber, plastic, and glass are generally insulators and are selected because they restrict charge flow rather than carry it. A copper wire may also be covered by an insulator for safety, but the metal core is the conductor.

Correct answer: A. Plastic covering on an electric wire

Explanation: The governing safety principle is electrical insulation: a material with very few mobile charge carriers prevents current from reaching a person. Plastic surrounding a live wire provides a high-resistance barrier, so option A is correct. Copper, iron, and silver are metals with mobile electrons and are conductors; they are useful for carrying current but are not suitable as exposed insulating coverings. The plastic must remain intact and properly rated.

Correct answer: B. Electrostatic induction

Explanation: The electric field of the charged object causes the free electrons in the conductor to redistribute, producing separation of opposite charges on the nearer and farther sides. This non-contact process is called electrostatic induction. In friction, charge is transferred by rubbing, whereas here the charges are redistributed within the conductor.

Correct answer: B. Because free charges in a conductor can move under the influence of an external electric field

Explanation: In electrostatic induction, charges redistribute in a conductor when a charged object is brought near it. Free electrons in a conductor can move easily under an external electric field, so induction occurs readily. Option C is incorrect because an ordinary conductor need not have zero electrical resistance.

Correct answer: D. Perpendicular to the surface

Explanation: In electrostatic equilibrium, the electric field cannot have a tangential component at a conductor’s surface, because such a component would make free charges move along the surface. Therefore, just outside the surface, the electric field is perpendicular to the surface. Whether it points outward or inward depends on the sign of the surface charge, but it is always normal to the surface.

Correct answer: A. Because the electric field inside the conductor must be zero; otherwise free charges would move.

Explanation: In electrostatic equilibrium, the electric field inside a conductor is zero. If excess charge remained within the conducting material, it would produce an electric field and exert a force on free charges, causing them to move. Charges redistribute until they reside on the outer surface and the field inside becomes zero. Option B is incorrect because the potential inside a conductor is constant, but it need not be zero.

Correct answer: B. Insulator

Explanation: The governing concept is the movement of charge carriers. In ordinary glass, electrons are tightly bound within atoms or molecular structures, so very little current flows under normal electric fields. Therefore, a glass rod is generally an insulator and option B is correct. It is not a metal, and classifying it as a conductor or ordinary semiconductor would not describe its usual electrical behavior. Special conditions can alter conductivity, but not the standard classification.