Conductors and Insulators

Free charges in a conductor rearrange under the electric field until the resultant electric field inside the conductor becomes zero. This condition is called electrostatic equilibrium. Rearrangement of charge does not increase the conductor’s mass. Exam tip: In electrostatic equilibrium, the electric field inside a conductor is zero.

In an insulator, electrons are generally tightly bound to atoms or molecules. Therefore, it has very few free charge carriers and does not conduct electricity easily. A semiconductor can provide some charge carriers on heating or doping, so it differs from an insulator. Exam tip: bound charge carriers are a key feature of insulators.

At a sharp region of a charged conductor, the surface charge density is higher. The magnitude of the electric field just outside the conductor is proportional to the surface charge density, so the field is stronger near the sharp point. In contrast, the electric field inside a conductor in electrostatic equilibrium is zero.

A grounded person's body can provide a conducting path between the charged conductor and Earth. Excess charge can therefore flow through the body to Earth, reducing the conductor's net charge. In option B, charge is only redistributed on the conductor's surface, so the conductor's total charge does not decrease.

The governing concept is frictional charging and charge mobility. Plastic is an insulator, so electrons transferred during rubbing cannot move freely across its surface; the charge therefore remains localized for a relatively long time in dry conditions. Option A is correct. Copper, silver, and aluminium are metals with mobile electrons, so any excess charge can redistribute rapidly, especially when the object is touched or connected to Earth. Environmental moisture can shorten retention, but it does not change the material classification.

The governing concept is the complementary use of a conductor and an insulator. Copper has low resistivity and mobile electrons, so it forms the core and carries current efficiently. Rubber has high resistance and prevents contact with the live conductor, so it is suitable for the protective outer covering. Therefore, option A is correct. Options B, C, and D place insulating materials in the core or conducting metals on the outside, reversing or defeating the required functions.

In electrostatic induction, a nearby charged object attracts or repels the free charges in a conductor. Thus, charges redistribute and separate into different regions of the conductor without touching it. In charging by friction, contact and rubbing between objects are required.

The governing concept is the response of bound charges to an external electric field. In an insulator, positive and negative charges remain attached to their atoms or molecules; they shift slightly in opposite directions rather than travelling through the material. This separation produces induced dipoles and is accurately called polarisation, so option A is correct. In ordinary electrostatic induction of a conductor, free charges redistribute over macroscopic distances. Charges are neither completely free in an insulator nor destroyed.

The governing concept is electrostatic shielding. In a conductor at electrostatic equilibrium, excess charge resides on its outer surface and the electric field inside a closed hollow conducting enclosure is ideally zero when no charge is placed inside. Therefore, a sensitive instrument inside can be protected from external static electric fields. The other options concern mass, light production, or material type, none of which follows from enclosing the device in a conductor.

A metal has free electrons, so charge can move through it easily. If the metal has a conducting path to earth, charge may leak away to the earth. An insulating stand offers very high resistance and prevents this leakage path, so the charge remains on the metal for a longer time. The metal itself does not become an insulator. Exam tip: A charged conductor is placed on an insulating stand to avoid earthing and charge leakage.

In electrostatic equilibrium, free charges redistribute until the electric field within the conducting material becomes zero. Therefore, the conductor is at the same potential throughout, and when there is no charge inside its cavities, the excess charge resides on its outer surface. Hence, option A is correct. Option C is incorrect because the electric field inside the conductor is zero, not maximum.

An insulator contains very few mobile charge carriers under ordinary conditions. Its electrons are generally tightly bound to atoms or molecules, so an applied electric field produces only a very small current. Hence option A is correct. A large population of free electrons, very low resistance, and easy current flow are characteristic of conductors, so options B, C, and D describe the opposite behaviour.

Mobile charges can move in a conductor. Because like charges repel, they redistribute until electrostatic equilibrium is reached. In this state, the electric field inside the conductor is zero and the excess charge resides on the outer surface. The distribution need not be uniform; it depends on the conductor’s shape. Unlike option B, the electric field inside a conductor at equilibrium is not large; it is zero.

Dry glass is an insulator, so the movement of electric charges through it is highly restricted and charge does not leak away easily. In contrast, copper, silver, and aluminium are conductors, in which charge can move readily. The word “dry” is important because moisture can increase charge leakage over the glass surface.

Metal is a conductor. When a metal object is held by a person connected to Earth, charge can flow through the hand and body to Earth, making it difficult for the object to retain static charge. Unlike metal, rubber is generally an insulator and can hold charge for longer because charge does not move through it easily.

This is electrostatic induction in a conductor. The positive external object attracts the conductor’s mobile electrons, so electrons shift toward the nearer end and that end becomes negatively charged by induction. The farther end is left relatively positive because it loses some electrons. No charge is created overall: the conductor remains neutral, with equal and opposite induced charges separated across its surface. Thus A is correct, while B reverses the attraction and C-D ignore charge separation.

Both the rod and the electrons are negatively charged, so the rod repels the electrons. Since free electrons in a metal can move, they redistribute towards the surface of the sphere away from the rod. This leaves an electron-deficient, induced positive region on the side near the rod; electrons do not move towards the rod because like charges repel.

A positively charged rod attracts free electrons, so the electrons in the metal sphere shift toward the side nearer the rod. This is electrostatic induction. The sphere remains neutral overall; only its charges are redistributed. Thus, the electrons do not leave the sphere or remain uniformly distributed.

Rubber is an insulator because its charge carriers, such as electrons, cannot move freely. Therefore, it reduces leakage of charge from a wire and helps protect a person touching the covering. In contrast, metals have free electrons that move easily, so they are conductors.

A conductor contains free charges that redistribute until electrostatic equilibrium is reached. In this equilibrium, the electric field inside the conductor is zero, so all points of the conductor are at the same potential; hence, it is equipotential. Unlike option D, the electric field inside a conductor is not maximum but zero.

An insulator can be charged by rubbing or by contact. It has very few free charge carriers, so deposited charge generally remains near the region where it is placed rather than spreading easily through the material. An external electric field can polarize an insulator, but polarization does not necessarily involve a transfer of net charge. Therefore, option A is correct, whereas option B is false because an insulator can be charged.

In electrostatic equilibrium, the electric field inside a conductor is \(E=0\). Hence, the net electric force on a free charge \(q\) is \(F=qE=0\). Therefore, option A is correct. Option B is incorrect because the potential is the same at all points inside a conductor, but its value is not necessarily zero.

The central idea in studying conductors and insulators is how electric charges behave inside different materials. In conductors, mobile charges can move comparatively freely and redistribute over the surface or through the material. In insulators, charges are largely bound and remain localized, although polarization can occur. Therefore option A captures both charge motion and charge distribution. Options B, C, and D concern unrelated properties of light, sound, and gravitation.

In conductors such as copper, free electrons can move easily, so they are used inside wires to allow electric current to flow. Insulators such as plastic or rubber strongly oppose the flow of charge, so they are used as the outer covering of a wire to help prevent electric shock. Option B reverses the roles of conductors and insulators.

The governing concept is electrical conduction. In an insulator, electrons are strongly bound to atoms or molecules, so charge carriers cannot move freely through the material. When charge is supplied, it therefore tends to remain near the region where it was placed, although polarization may occur. Option A is correct. Option B describes a conductor, option C contradicts the definition of an insulator, and option D is false because electric fields can exist inside insulators.