Conductors and Insulators

The governing principle is electrostatic equilibrium: the electric field within the conducting material must be zero. If a charge +q is placed inside a cavity without touching the conductor, Gauss’s law requires an induced charge −q on the cavity’s inner surface so that the enclosed net charge for a Gaussian surface in the metal is zero. Hence A is correct; the induced charge is not positive, and charge is not destroyed.

Let the charge inside the cavity be −q. To keep the electric field zero in the conducting material, electrostatic induction places +q on the inner surface. The conductor as a whole was initially neutral, so its total surface charge must remain zero: (+q)inner + Qouter = 0. Therefore Qouter = −q, an equal negative charge, making option B correct. It is not zero because the inner induced charge must be balanced.

This is the principle of electrostatic shielding. An external charge exerts forces on the conductor’s free charges, causing them to redistribute over its surfaces. In electrostatic equilibrium, the induced surface-charge field cancels the external field throughout the empty closed cavity, so the net field there is zero. Therefore B is correct. Colour, absence of air, and destruction of the external charge have no role in the result.

The governing condition applies only to the interior of the conducting material in electrostatic equilibrium, not to all space around it. Excess charge can reside on the surface, and that surface charge produces an electric field outside the conductor. For example, a charged conducting sphere has zero field inside its material but a nonzero external field. Thus B is correct; zero internal field does not imply zero field everywhere.

In electrostatic equilibrium, free charges in a conductor must have no unbalanced force. A component of electric field parallel to the surface would exert tangential force F = qE_parallel on mobile surface charges, causing them to drift along the surface. Their movement would continue until that component became zero. Hence A is correct. The normal component may exist and is related to surface charge density; the other options do not describe electrostatic behavior.

In electrostatic equilibrium, free charges in a conductor cannot continue moving. If the electric field had a tangential component along the surface, it would exert a force on surface charges and produce motion. Therefore that tangential component must be zero, leaving only the normal component outside the surface. Thus option A is correct. The other options incorrectly deny surface charge or misunderstand the location of the field.

Excess charge on an isolated conductor resides on its outer surface and redistributes until electrostatic equilibrium is reached. The local surface charge density is not generally uniform when curvature changes: sharper regions require a greater concentration of charge and consequently have a stronger nearby electric field. Therefore option A is correct. The other options deny mobile charge or introduce irrelevant colour and centre-based ideas.

At electrostatic equilibrium, charge density tends to become larger at a sharply curved or pointed part of a conductor. Since the field just outside is related to surface charge density by E = σ/ε₀, a larger σ produces a stronger local electric field. If this field is sufficiently high, it can accelerate electrons and ionise air molecules. Hence A is correct; the other choices have no physical basis.

A charged isolated sphere has the same geometrical environment in every direction. Electrostatic equilibrium and spherical symmetry therefore require the surface charge density to be uniform at corresponding points. An irregular conductor has regions with different curvatures, so charge density can vary, usually becoming larger near sharper parts. Thus option A is correct; earthing is not required, and charge is not destroyed.

A conductor contains mobile free electrons. When an external charge approaches, its electric field exerts forces on these electrons, causing them to move over the conductor's surface until electrostatic equilibrium is restored. This separation is electrostatic induction; the conductor's net charge remains unchanged if it is isolated. Therefore A is correct, while the other options incorrectly claim insulation, destruction, or a colour change.

A positively charged rod attracts the conductor's mobile electrons. Electrons shift toward the near end, leaving the far end with an electron deficit and therefore an induced positive charge. The conductor as a whole remains neutral because no charge has entered or left; only separation has occurred. Thus option A is correct. Protons do not freely flow through the solid conductor, and earthing is not assumed.

The negatively charged rod repels the conductor's mobile electrons. These electrons move toward the far end, producing an excess of negative charge there, while the near end becomes relatively positive because it has lost electrons. Since the conductor is isolated, its total charge remains zero; only redistribution occurs. Therefore A is correct. Protons do not move freely, and no earthing is stated.

An external charge can exert forces on the conductor's free electrons and separate positive and negative regions. However, when there is no physical contact and no conducting path to Earth, electrons cannot enter or leave the conductor. Consequently, the algebraic sum of charge remains zero for an initially neutral conductor, although local surface charge densities change. Hence A is correct; induction changes distribution, not net charge.

In charging by induction, the external charged body first separates charges in the conductor. While earthing is connected, electrons can still flow between Earth and the conductor in response to the external field. Removing the earth connection first isolates the conductor and traps the acquired net charge. Only after that should the external body be removed, allowing the charge to spread over the conductor. Thus A is correct.

A positively charged rod creates an electric field that attracts electrons toward the nearby conductor. If the conductor is earthed, Earth acts as a vast reservoir of mobile charge and provides a conducting path. Electrons therefore flow from Earth into the conductor until the electrostatic condition is established. The rod does not create or destroy electrons, and the conductor remains a conductor. Hence A is correct.

The governing concept is electrostatic induction in a conductor. A negatively charged rod repels the conductor’s mobile electrons, pushing them toward the side connected to Earth. Because earthing provides a conducting path to the huge charge reservoir of Earth, these electrons can flow away until the potential condition is established. Thus A is correct; the rod repels electrons, does not pull protons, and conductors do contain mobile electrons.

The governing principle of earthing is potential equalisation, not equal mass or removal of matter. When a conductor is connected to Earth, mobile charges can move through the connecting path. They continue moving only while a potential difference drives them; the flow stops when the conductor reaches Earth’s potential. Therefore A is correct. Earthing does not destroy particles or turn the conductor into an insulator.

Polarisation separates positive and negative bound charges slightly inside an initially neutral insulator, so its net charge can remain zero. If an external charged body is nearby, the oppositely charged side is closer than the similarly charged side. Since electrostatic force varies as 1/r², the nearer attraction is stronger than the farther repulsion, producing a net attraction. Hence A is correct.

The microscopic distinction depends on the mobility of charge carriers. In a conductor, electrons are sufficiently free to redistribute over a macroscopic distance when an external electric field is applied; this is electrostatic induction. In an insulator, electrons remain bound to atoms or molecules and shift only slightly, creating polarisation. Therefore A is correct; neither process requires proton motion or a necessary change in net charge.

Rubbing transfers electrons between materials, so charge can be deposited on an insulator. However, an insulator has very few mobile charge carriers, and its electrons are strongly bound to atoms or molecules. The transferred charge therefore remains near the rubbed region instead of rapidly redistributing across the surface. Thus A is correct. The other choices incorrectly claim metallic content, universal equipotentiality, or absence of an electric field.