Conductors and Insulators

An external electric field exerts opposite forces on positive and negative charges within each atom or molecule. In an insulator, these charges cannot travel freely through the material, but their bound distributions can shift slightly relative to one another. This produces induced dipoles and is called polarisation. The material does not normally become a perfect conductor, and no charge is destroyed; the displacement is small and largely microscopic.

Paper is an insulator, but its molecules can become polarised in the electric field of a charged comb. The side nearer the comb acquires an induced charge effect opposite to the comb’s charge, while the like effect is displaced farther away. Because the attractive force is stronger at the smaller separation, the net force is attraction, even though the paper may have zero net charge. The comb neither increases gravity nor destroys charge.

A neutral conductor has equal total positive and negative charge, so its net charge is zero. If an external charged body is brought near it, the conductor’s mobile electrons redistribute: one region becomes relatively negative and another relatively positive. This separation is electrostatic induction. The total charge remains conserved and zero unless charge is transferred by contact or grounding. Thus local charge regions can exist without the conductor acquiring a net charge.

For a conductor in electrostatic equilibrium, free charges rearrange until the electric field within the conducting material becomes zero; otherwise they would continue moving. This condition does not require the surface charge density to be zero. Surface charges can produce a nonzero field outside the conductor, and just outside a surface with charge density σ the normal field is related to σ by E = σ/ε₀. Therefore, zero internal field and nonzero external field are fully consistent.

The governing concept is electrostatic shielding. In a closed conducting enclosure, free charges in the conductor redistribute themselves so that the electric field inside the enclosure is greatly reduced, and in ideal electrostatic conditions it is zero. Therefore option A provides the best protection. Rubber and coloured glass do not produce reliable electrostatic shielding, while an ungrounded bare wire may conduct disturbances toward the device rather than block them.

In electrostatic equilibrium, free charges in a conductor have no net force and therefore stop moving. This requires the electric field inside the conducting material to be zero. Since the potential difference inside is zero, the conductor is at the same potential throughout. Any excess charge resides on the surface, with its distribution depending on the shape. Hence option B is correct; the other choices contradict these conditions.

An insulator contains very few mobile charge carriers because most of its charges are bound to atoms or molecules. Consequently, an added charge can remain localized instead of spreading freely over the surface. In an external electric field, the bound positive and negative charges can shift slightly, producing polarisation. Thus option C correctly describes an insulator. Options A and D are conductor-like claims, while B wrongly denies polarisation.

The key physical distinction is the mobility of charge carriers. In a conductor, electrons or other charge carriers can move comparatively freely through the material, allowing current and charge redistribution. In an insulator, charge carriers are tightly bound and cannot move over macroscopic distances easily. Therefore option D is the fundamental basis. Colour, name, and sound do not determine electrical conduction and are irrelevant distractors.

A material with very few free carriers behaves in an insulator-like manner. An added charge cannot travel easily through the material, so it may remain localized. However, bound charges can undergo a small relative displacement in an external electric field, producing polarisation. Thus option A is correct. Uniform surface spreading, zero internal field, and equipotential behaviour are characteristic idealisations of conductors, not general properties of insulators.

The broad conclusion is that material properties control how charges respond to electric fields. Conductors have mobile charges, so they allow redistribution, induction, and electrostatic shielding. Insulators have mainly bound charges, so they support localized charge and polarisation instead. Therefore option B gives the complete conclusion. Option A ignores material differences, C reverses the nature of insulators, and D confuses charge with mass.

The governing principle is electrostatic equilibrium in a conductor. A charge q placed in a non-zero electric field E experiences force F = qE. Since conduction electrons are mobile, they would begin to drift and redistribute until their rearrangement cancels the internal field. Thus option A is correct; the other choices violate charge behaviour and electrostatic principles.

Potential difference is related to the electric field by E = −∇V; along a finite separation, a non-zero voltage difference generally indicates a component of electric field. In a conductor at electrostatic equilibrium, free charges move until the interior becomes equipotential. Therefore option B is the best answer. The other options incorrectly equate equilibrium with voltage difference or neutrality.

In electrostatic equilibrium, excess charge on an isolated conductor resides on its surface. The reason is that like charges repel, while conduction charges are free to move; they redistribute until the electric field inside the conducting material becomes zero. Hence option B is correct. The alternatives wrongly claim that space is absent, charge is destroyed, or all charges are immobile.

Gauss’s law gives the result. Choose a Gaussian surface lying within the conducting material, where the electric field is zero in electrostatic equilibrium. The net enclosed charge must therefore be zero, so an inner-surface charge of −q is induced to cancel the cavity charge +q. Thus option C is correct; the outer surface receives compensating charge if the conductor is initially neutral.

The governing concept is electrostatic induction and conservation of charge in a conductor. If a charge −q is placed inside a cavity, the electric field inside the conducting material must be zero, so +q is induced on the inner surface. The conductor was initially neutral, hence its total induced charge must remain zero: (+q) + outer charge = 0. Therefore the outer surface carries −q, making option D correct; zero or half negative would violate charge conservation.

The governing idea is electrostatic induction in an earthed conductor. A charge +q inside the cavity induces −q on the inner surface so that the field within the conducting material is zero. Because the conductor is connected to Earth, charge can flow between the conductor and the Earth; the Earth acts as a reservoir and fixes the conductor’s potential. With no external charge and suitable grounding, the outer surface can have zero net charge. Thus option A is the intended answer; option C describes the inner surface, not necessarily the outer one.

The governing concept is electrostatic equilibrium in a conductor. A component of electric field parallel to the surface would exert a tangential force F = qE_parallel on mobile charges. These charges would move along the surface, producing a current and redistributing themselves until that tangential component became zero. Therefore a persistent parallel component is impossible in electrostatic equilibrium, so option A is correct. The other choices do not describe any electric-force effect.

In electrostatic equilibrium, the electric field immediately outside a conductor is normal, or perpendicular, to its surface. If a tangential component existed, it would exert a force on mobile surface charges and make them move, contradicting equilibrium. The normal component may be nonzero and is related to surface charge density by E = σ/ε₀ just outside the surface. Therefore option B is correct; A and D describe tangential directions, while C is not generally true for an arbitrary conductor.

The governing relation at a conductor’s surface is E_out = σ/ε₀, where σ is the local surface charge density and ε₀ is the permittivity of free space. Thus, at locations with greater charge per unit area, the nearby electric field is stronger. The field depends on the local distribution of charge, not directly on colour or mass; temperature alone is also not the determining quantity in this ideal electrostatic question. Hence option C is correct.

The governing concept is non-uniform surface charge distribution on a conductor. At a sharp tip, the radius of curvature is small, so charges crowd more densely there than on a broad, gently curved region. Since the field just outside is approximately E = σ/ε₀, a larger local surface charge density produces a stronger local electric field. Therefore option D is correct. Gravity, a change into an insulator, or charge destruction does not explain the enhancement.