Conductors and Insulators

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.

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.

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.

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.

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.

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.