Conductors and Insulators

Conductors brought into contact exchange charge until their potentials become equal, not necessarily until their charges become equal. For an isolated conductor, the relation is approximately Q = CV. Unequal conductors generally have different capacitances, so at the common potential their charges satisfy Q1/Q2 = C1/C2 rather than Q1 = Q2. Option B is correct. Charge can reach either conductor, electrons exist in both, and total charge is conserved.

In electrostatic equilibrium, free charges in a conductor rearrange until the electric field within the conducting material becomes zero. This zero field indicates equilibrium, not zero total charge. Any excess charge can reside on the outer surface, where it produces fields outside the conductor. Option B is correct. Charge is not destroyed, and it is not required to gather at the centre; option A confuses field cancellation inside with absence of charge.

A conductor contains mobile charge carriers. If excess charge remained in its bulk, it would create an internal electric field and exert forces that would continue moving the carriers. In electrostatic equilibrium, this redistribution ends with excess charge on the outer surface, so the excess volume charge density inside the conducting material is zero. Option A is correct. Charge is not converted into mass, electrons are present, and the result is not due to lack of space.

Free charges in a conductor respond to the electric force and rearrange themselves. They continue moving until the internal electric field is zero, so no net electric force drives further macroscopic charge motion. The conductor is then at electrostatic equilibrium and is at one potential throughout its connected body. Option A states this condition. Charges are not locked at the centre or destroyed, and the material does not become an insulator.

The electric-field energy density in vacuum or a linear dielectric is u = 1/2 εE². Therefore, in the conducting material where the electrostatic field is zero, the local energy density associated with that field is also zero. Option B is the intended answer: no electric-field energy is stored in the conductor’s bulk under this ideal condition. This does not rule out energy in fields outside the conductor or in a cavity containing a field.

The governing concept is electrostatic shielding. A metal contains mobile electrons, so an external electric field makes them redistribute over the cage’s surface. Their induced field opposes the applied field inside the closed conductor, giving zero electric field in electrostatic equilibrium, provided there is no charge inside the cage. Option C is correct; A, B, and D contradict basic properties of matter and electricity.

The relevant principle is electrostatic shielding by a conducting enclosure. During a lightning strike, current tends to travel along the outside metal body, while charges redistribute on its outer surface. This greatly reduces the electric field inside the passenger compartment, so option A is correct. The protection is not because the car is a perfect insulator, destroys lightning, or lacks gravity. Occupants must still avoid touching metal parts and remain inside.

The governing ideas are electrical conductivity and earthing. Metal has many mobile charge carriers and therefore offers a low-resistance path for the large current associated with a lightning discharge. Connecting the rod to the Earth allows charge to flow safely into a vast charge reservoir instead of through the building. Thus B is correct; the rod is not intended to insulate, convert charge into light, or change colour.

The governing concept is the concentration of charge on a conductor. In electrostatic equilibrium, surface charge density is greater where the radius of curvature is smaller. A sharp tip therefore can have a stronger nearby electric field than a blunt region. This strong field helps ionise surrounding air and encourages controlled leakage or discharge. Hence C is correct; a tip does not become an insulator, destroy charge, or reduce mass.

The governing distinction is between mobile charge in a conductor and bound charge in an insulator. In an insulator, electrons are tightly bound to atoms or molecules, so a localized excess charge cannot quickly spread through the entire object and reach the earthed contact. Option C is correct. Earth can accept or supply charge, but poor internal mobility prevents rapid discharge; option B states the opposite of the material’s behavior.

The governing concept is polarisation of a dielectric. An external electric field exerts opposite forces on bound positive and negative charges within atoms or molecules. They may separate slightly, producing tiny induced dipoles, even though the charges do not travel freely through the material. Therefore B is correct. The insulator does not become a perfect conductor, all electrons do not flow freely, and electric charge is not destroyed.

The governing concept is electrostatic polarisation and the nonuniform force on an induced dipole. The electric field of the charged comb slightly separates bound charges in each neutral paper piece. The side nearer the comb acquires an induced effect of opposite sign and is closer, so its attraction is stronger than the repulsion of the farther side. Thus C is correct; the paper need not become metal or lose its net charge.

The governing concept is electrostatic induction. A neutral conductor has equal total positive and negative charge, so its net charge is zero. When an external charged body is brought nearby, mobile electrons shift within the conductor: one surface region becomes relatively negative and another relatively positive. No charge is created or destroyed; only its distribution changes. Therefore A is correct, while B, C, and D do not describe induction.

The governing condition is electrostatic equilibrium. Free charges in a conductor move until the internal electric field becomes zero; otherwise they would continue drifting. The excess charge then resides on the surface, and that surface charge produces an electric field in the surrounding space. Hence C is correct. A is false because a conductor may carry surface charge, while B and D contradict equilibrium and the definition of a conductor.

The governing principle is electrostatic shielding. In electrostatic equilibrium, excess charge on a closed conductor resides on its outer surface, and the electric field inside the conducting material is zero. A properly closed conducting enclosure therefore protects the device from external static electric fields, so option A is correct. Rubber is an insulator, a bare wire alone is not a complete shield, and coloured glass does not provide reliable electrostatic shielding.

Electrostatic equilibrium means that free charges in a conductor have no net motion. Consequently, the electric field throughout the conducting material is zero, the potential is constant at every point of the conductor and its surface, and any excess charge resides on the surface. Option B includes all three conditions. The other choices incorrectly place charge at the centre, claim charge is destroyed, or describe impossible field and material properties.

The governing concept is the availability of mobile charge carriers. In an insulator, most charges are bound to atoms or molecules, so an added charge generally remains near its point of deposition instead of spreading freely. An external electric field can slightly separate bound positive and negative charges, producing polarisation. Therefore C is correct; A and D describe conductor-like behaviour, while B incorrectly denies polarisation.

The governing concept is electrical conductivity, which depends primarily on how easily charge carriers can move. In a conductor, electrons or other carriers can respond and drift through the material under an electric field. In an insulator, carriers are tightly bound and cannot move freely over macroscopic distances. Thus D gives the physical basis; colour, name, and sound do not determine electrostatic conduction.

The governing idea is the distinction between mobile and bound charges. With very few free carriers, an excess charge cannot travel easily through the material, so it may remain localized. Bound charges can nevertheless shift slightly in an applied electric field, giving polarisation. Hence A is correct. Uniform surface spreading, zero internal field, and equipotential behaviour are characteristic idealizations of conductors, not insulators.

The governing conclusion is that material structure controls the response of charge. In conductors, mobile carriers permit redistribution, electrostatic shielding, and induction. In insulators, charges are largely bound, so localization and polarisation are more important. Therefore B is the comprehensive statement. A ignores material differences, C reverses the nature of insulators, and D confuses charge with mass.