Conductors and Insulators

Charges can move easily through a metal. When the rod is held in a bare hand and the person is in contact with the Earth, the rod, body, and Earth form a conducting path. Any excess charge on the rod therefore leaks to the Earth, so static charge does not remain on it. In contrast, holding the rod with an insulating handle prevents this leakage path to Earth.

Glass is an electrical insulator. Its electrons are not free to move throughout the rod, so charge transferred during rubbing remains concentrated near the rubbed region instead of quickly spreading away. A conductor would redistribute charge readily, while a semiconductor or electrolyte is not the standard classification intended here. Thus option B is correct because limited charge mobility allows the glass rod to retain charge.

In electrostatic equilibrium, the free charges in a conductor are at rest. If the electric field had a component parallel to the surface, it would exert a force on the free charges and make them move. Therefore, the parallel (tangential) component must be zero, so the electric field is perpendicular to the surface. Option B is incorrect because the electric field inside a conductor is zero in electrostatic equilibrium.

In an insulator, electrons or other charge carriers are tightly bound to atoms or molecules, so they cannot move easily under an electric field. In contrast, a conductor has mobile electrons, allowing charge to flow more readily. Therefore, option B is correct.

Copper is a good electrical conductor because its free electrons move easily and it has low resistivity. Therefore, it is used as the conducting core of electric wires. Rubber and plastic are generally insulators and are used as the outer covering of wires for electrical insulation.

In electrostatic induction, bringing a charged object near a conductor causes the conductor’s free charges to redistribute, producing charge separation in different parts of the conductor. No contact with the charged object is required. Burning wood is a chemical change, not an example of induction.

In electrostatic equilibrium, free charges in a conductor are at rest. If a tangential electric field existed at the surface, it would exert a force parallel to the surface and move the charges. The charges would redistribute until they produced a field that cancels the tangential component. Hence, just outside a conductor's surface, the electric field is normal to the surface. Option B is incorrect because the electric field inside a conductor is zero in electrostatic equilibrium.

A metal is a conductor, and the Earth acts as a very large charge reservoir. When the sphere is grounded, electrons can flow between the sphere and the Earth, removing the excess charge and bringing the sphere to earth potential. Unlike option B, excess charge does not remain permanently on a sphere that is connected to the Earth.

In electrostatic equilibrium, free charges in a conductor move and redistribute until the electric field inside it becomes zero. Since \(\mathbf{E}=-\nabla V\), a zero electric field means there is no change in potential within the conductor; hence the conductor is equipotential. Unlike option D, a non-zero internal electric field would keep charges moving, so equilibrium could not exist.

Insulators have very few free charge carriers, so they have high electrical resistance and strongly oppose the flow of electric charge. Therefore, insulating materials such as plastic or rubber coverings on wires help prevent electric current from reaching the body. In contrast, conductors have many free electrons and allow charge to flow easily.

In a metal, some outer electrons are not bound to individual atoms and can move through the lattice. When an electric field is applied, these free, or conduction, electrons drift and produce electric current. The positive metal ions remain essentially fixed in the lattice, so they are not the main mobile charge carriers.

In an insulator, charges are generally bound to atoms or molecules, so they cannot move freely through the material to produce a current. An external electric field causes a small relative displacement of the centres of positive and negative bound charges, producing induced dipoles. This is called polarisation. In electrical conduction, free charges flow through the material, which does not occur here.

In a conductor, free electrons or other mobile charges can redistribute within the material when an external electric field is applied; this is electrostatic induction. In an insulator, charges cannot move freely through the material, but the centres of bound positive and negative charges may shift slightly relative to each other, or dipoles may align; this is polarisation. Option B reverses these behaviours.

A metal contains free charges that move and redistribute when an electric field is present. In electrostatic equilibrium, this redistribution continues until the resultant electric field inside the metal becomes zero. Thermal expansion is related to a change in temperature, not to a zero electric field inside the metal.

Copper is a conductor because it has free electrons that allow electric charge to flow easily. Rubber is an insulator because the motion of charge carriers in it is highly restricted. Therefore, option A gives the materials in the required order: conductor first and insulator second. Option B contains the same materials, but in the reverse order.

In electrostatic equilibrium, free charges in a conductor can move. If an electric field existed within the conducting material, these charges would experience a force and continue moving. They therefore redistribute, generally onto the surface, until the field due to the redistributed charges cancels the field of external and other charges inside the material. Hence the net internal electric field is zero. This happens because of charge redistribution, not because charge is destroyed.

The governing concept is electrical conductivity. Copper, silver, and aluminium are conductors, so free electrons can move through them and distribute excess charge over the surface. Glass is an insulator because its electrons are tightly bound; therefore charge cannot move easily away from the point of contact and remains localized there. Hence option C is correct, while the metallic options are plausible but unsuitable distractors.

In electrostatic equilibrium, free charges redistribute on a conductor’s surface so that the electric field within the conducting material becomes zero. Therefore, a closed conducting enclosure shields its interior from external electrostatic fields. Option B is incorrect because an electric field need not be zero inside an insulator.

A closed, continuous conducting enclosure works on the Faraday-cage principle. Ideally, the electric field inside a conductor-enclosed region is zero; therefore, the electric field inside the enclosure is very small. Lightning current flows mainly along the outer surface. The metal does not become an insulator, so option B is incorrect.

Free charges cannot move easily through an insulator. Hence, an insulating rod does not provide a conducting path for charge to leak from the metal sphere through the hand to the earth. In contrast, if the sphere is held directly by hand, charge may flow through the body to the earth.