Conductors and Insulators

Correct answer: A. Because mobile charges redistribute due to mutual repulsion until the electric field within the conductor becomes zero.

Explanation: Free electrons can move in a metal. Excess like charges repel one another and redistribute until electrostatic equilibrium is established. At equilibrium, the electric field within the conducting material is zero, and the excess charge resides on the outer surface. Option B is incorrect because even when an external electric field is present, the electric field inside the conducting material is zero, not equal to the external field.

Correct answer: A. Charge carriers cannot move easily through rubber

Explanation: The governing concept is the difference between conductors and insulators. Dry rubber is an insulator, so its electrons are tightly bound and charge carriers cannot move freely through the material. Consequently, charge deposited at one point does not rapidly redistribute across the surface and remains localized for some time. Option A states this reason. Option B is false because rubber is not a metal; option C describes a conductor incorrectly, and option D is not generally true.

Correct answer: B. 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. Hence, the field is perpendicular to the surface. It may point outward for positive surface charge or inward for negative surface charge, but in both cases it remains perpendicular to the surface.

Correct answer: B. Conductor

Explanation: In electrostatic induction, bringing a charged object near a material causes charges within it to redistribute without direct contact. In a conductor, free electrons can move easily, so the effect is clearly observed. Glass, rubber, and dry wood are insulators; their charge carriers cannot move freely.

Correct answer: A. Because it has free electrons

Explanation: In metals, some electrons are loosely bound to atoms and can move as free electrons. When an electric field is applied, these electrons drift and produce electric current. Unlike option C, tightly bound electrons cannot move easily through the material.

Correct answer: C. Insulator

Explanation: In an insulator, electrons are tightly bound to their atoms, so there are very few free or mobile charge carriers. Therefore, electric current flows through it with difficulty. In contrast, a conductor has many free electrons, so current flows through it easily.

Correct answer: A. Same at every point

Explanation: In electrostatic equilibrium, \,\(\vec{E}=-\nabla V\). Since \,\(\vec{E}=0\)\, inside the conductor, there is no change in potential, so the potential is the same at every point of the connected conductor. Equal potential does not mean that the potential must be zero or negative; its value depends on the chosen reference potential.

Correct answer: A. Charge can flow between the sphere and the Earth

Explanation: The Earth is a very large conductor. When a charged metal sphere is connected to it, charge can flow between the sphere and the Earth until their potentials become equal. If the sphere is positively charged, electrons can flow from the Earth to the sphere; if it is negatively charged, electrons can flow from the sphere to the Earth. Charge does not change into mass, so option B is incorrect.

Correct answer: A. To keep current safely inside

Explanation: The governing concept is electrical insulation and safety. A wire’s metallic core has mobile electrons and allows current to flow, but direct contact with that conductor could cause electric shock or an unintended short circuit. A rubber or plastic covering has very high resistance and separates the core from people and nearby conductors, so option A is correct. The covering does not turn the wire into metal, increase current by itself, or destroy charge.

Correct answer: A. When a charged object is brought near the conductor without touching it

Explanation: When a charged object is brought near a neutral conductor without touching it, its electric field exerts forces on the conductor’s free charges. The positive and negative charges redistribute toward opposite sides of the conductor; this is charge separation by electrostatic induction. Cutting or cooling the conductor does not by itself produce this induced separation.

Correct answer: A. Bound positive and negative charges undergo a slight relative displacement, or permanent dipoles partly align with the electric field.

Explanation: In an insulator, charges do not become free to flow through the whole material. An external electric field causes a very small relative displacement of bound positive and negative charges; in polar materials, permanent dipoles may also partly align with the field. This produces or aligns electric dipole moments in the material, which is called polarisation. Option B describes the flow of free charges in a conductor, not polarisation of an insulator.

Correct answer: A. It has low electrical resistivity and is a good conductor

Explanation: The governing concept is resistivity. Copper contains mobile electrons and has low electrical resistivity, so for a wire of length L and area A, R = ρL/A is relatively small. Hence current can pass with less voltage drop and lower heating loss for a given current. Option A is correct. High resistivity would oppose current, glass is an insulator, and copper’s useful ductility means it can be drawn into wires rather than being highly brittle.

Correct answer: A. Free charges on the surface will move and redistribute.

Explanation: A tangential electric field exerts a force parallel to the surface on free charges in the conductor, so they move and redistribute. This redistribution continues until electrostatic equilibrium is reached; in equilibrium, the tangential electric field is zero. Unlike option C, the normal component of electric field at the surface need not be zero.

Correct answer: A. Electrostatic shielding

Explanation: The closed metal body of a car acts as a conducting enclosure. The lightning charge and current travel mainly over its outer surface, so the electric field inside is very small. This is electrostatic shielding, also called the Faraday-cage effect. Earthing is a different safety measure; the car does not need to be connected to the ground for this shielding effect. Exam tip: For protection inside a closed conducting enclosure, choose electrostatic shielding.

Correct answer: A. The sharp tip has a smaller radius of curvature, so the electric field there is stronger.

Explanation: In electrostatic equilibrium, the electric field inside a conductor is zero and the entire conductor surface is at the same potential. A sharp tip has a smaller radius of curvature, producing a stronger electric field just outside it. Since \(E=\sigma/\varepsilon_0\) immediately outside a conductor, a larger \(E\) means a larger surface charge density \(\sigma\). Option C is incorrect because the potential is the same over the entire conductor surface.

Correct answer: A. To provide a safe, low-resistance path for the lightning current to reach the earth

Explanation: A lightning conductor is made of metal and, when earthed, provides a low-resistance path for the lightning discharge. The charge and current therefore flow safely into the ground, protecting the building and the people inside. In contrast, making the conductor an insulator would obstruct the lightning current and would not provide protection.

Correct answer: A. Conductor

Explanation: A conductor has free electrons or other mobile charge carriers. Therefore, excess charge can redistribute easily through it and, at electrostatic equilibrium, resides on its surface. In an insulator, charge carriers are bound, so deposited charge generally remains near the location where it was placed.

Correct answer: C. Insulator

Explanation: The governing concept is charge mobility. In an insulator, electrons are bound to atoms or molecules and cannot travel freely through the bulk material. Therefore, excess charge deposited at one point generally remains localized rather than spreading over the entire solid. Option C is correct. A conductor would redistribute charge quickly over its surface, while a semiconductor has intermediate, condition-dependent conductivity; a superconductor is characterized by zero resistance under suitable conditions, not by ordinary charge localization.

Correct answer: A. Metal has mobile free electrons, whereas rubber has no appreciable mobile charge carriers.

Explanation: In a metal, metallic bonding provides electrons that can move through the lattice. When an electric field is applied, these electrons drift and produce electric current, so metal is a conductor. In rubber, electrons are strongly bound to atoms or molecules, so it has negligible mobile charge carriers and acts as an insulator. Option D is incorrect because positive ions are fixed in the solid metal lattice; current is mainly carried by electrons.

Correct answer: A. Because the Earth is a very large charge reservoir and its potential remains nearly unchanged.

Explanation: Because of its enormous size, the Earth has a very large capacitance. It can accept or supply charge with negligible change in its potential. When a conductor is connected to the Earth, a conducting path is provided, and charge flows until the conductor reaches the Earth's potential. Charge is not destroyed; it is transferred between the conductor and the Earth.

Correct answer: A. The electric field inside the conductor is zero, and its surface is equipotential.

Explanation: In electrostatic equilibrium, the free charges in a conductor must experience no net force; otherwise, they would move. Therefore, the electric field within the conducting material is zero. Consequently, the potential is constant throughout the conductor and its surface is equipotential. Option C is incorrect because excess charge resides on the outer surface of a conductor, not at its centre.

Correct answer: A. Polarisation

Explanation: The governing concept is electrostatic polarisation. In an insulator, charges are not free to travel across the whole body, but a nearby charged object can slightly displace bound positive and negative charges in opposite directions within atoms or molecules. This creates induced dipoles and is called polarisation, so option A is correct. Metallic current and complete charge flow require mobile charges, while earthing requires a conducting path to Earth; neither normally describes an isolated insulator.

Correct answer: A. Zero

Explanation: In electrostatic equilibrium, free charges in the conductor redistribute so that the resultant electric field inside the conducting material and in an empty closed cavity is zero. Thus, a closed conductor shields its interior from external electric fields. Option B is incorrect because even a uniform non-zero electric field would not represent electrostatic equilibrium in the conductor.

Correct answer: A. So that charge does not easily leak to the earth through the hand or support

Explanation: The governing concept is earthing and charge leakage. A metal sphere is a conductor, so any excess charge on it can move through a person or a conducting support to the Earth. An insulating rod has very few mobile charge carriers and interrupts that conducting path, allowing the sphere to retain its charge more effectively. Thus option A is correct. The rod neither melts the sphere, destroys charge, nor changes the metal itself into an insulator.

Correct answer: A. Rubber

Explanation: Rubber is an electrical insulator, so it offers high resistance to the flow of electric current and is suitable for safety gloves. In contrast, copper is a good conductor and allows current to flow easily. In actual use, only specially tested and properly rated rubber gloves should be used for electrical protection.