Conductors and Insulators

Correct answer: A. The metal enclosure can provide electrostatic shielding

Explanation: The conducting metal body of a car provides a partially enclosed path for charge. During a lightning event, charge tends to travel mainly over the exterior surface, so the electric field inside the passenger space can be greatly reduced, illustrating electrostatic shielding. This protection is not because the car is a perfect insulator or because lightning is destroyed. Safety still requires avoiding contact with metal parts and following proper lightning precautions.

Correct answer: B. To provide a safe, low-resistance path for excess charge to Earth

Explanation: A lightning rod must conduct a very large and rapidly changing charge, so metal is selected because it has many mobile charge carriers and low resistance. Connecting the rod to Earth provides a comparatively safe, low-impedance path through which charge can flow into the vast Earth, reducing the chance that it will pass through the building. The rod does not make charge disappear or convert it into light; it controls the discharge path.

Correct answer: C. Because charge does not move freely through the whole insulator

Explanation: In an insulator, electrons are strongly bound to atoms or molecules, so the material has very few mobile charge carriers. A charge placed at one location therefore remains largely localized instead of spreading rapidly through the entire object. Earthing that point may remove or alter nearby charge, but the rest may not reach Earth quickly. This contrasts with a conductor, where free charges redistribute readily throughout the surface and discharge more effectively.

Correct answer: B. Bound positive and negative charges can shift slightly in opposite directions

Explanation: 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.

Correct answer: A. When an external charge causes electrostatic induction

Explanation: 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.

Correct answer: C. Surface charge can produce an electric field outside

Explanation: 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.

Correct answer: A. Keeping it inside a completely closed conducting enclosure

Explanation: 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.

Correct answer: B. Zero internal electric field, uniform potential, and excess charge on the surface

Explanation: 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.

Correct answer: C. It has few free charges; charge can remain localized and polarisation can occur

Explanation: 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.

Correct answer: A. Charge can remain localized and the material can become polarised

Explanation: 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.

Correct answer: B. Charge motion, distribution, induction, polarisation, and shielding depend on the nature of the material

Explanation: 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.

Correct answer: A. Free charges will feel force and rearrange

Explanation: 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.

Correct answer: B. There may be electric field inside the conductor

Explanation: 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.

Correct answer: B. Free charges spread to the outer surface due to repulsion

Explanation: 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.

Correct answer: C. Equal negative

Explanation: 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.

Correct answer: D. Equal negative

Explanation: 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.

Correct answer: A. Zero

Explanation: 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.

Correct answer: C. Surface charge density

Explanation: 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.

Correct answer: D. Surface charge density can be higher there

Explanation: 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.

Correct answer: A. Charge density and electric field can be higher

Explanation: The governing concept is field enhancement at a small-radius conductor tip. Free charge accumulates more densely near the sharp region, increasing the local surface charge density σ. The nearby field, approximately E = σ/ε₀, can therefore become large enough to ionise the surrounding air or another medium, initiating corona or discharge. Hence option A correctly gives both linked causes. The other options deny or misrepresent the electrical process and cannot explain discharge.

Correct answer: A. Complete symmetry

Explanation: The governing concept is symmetry combined with electrostatic equilibrium. An isolated spherical conductor has identical geometry in every direction, and no external object or earth connection selects one surface region over another. Mobile charges therefore redistribute until the conductor reaches one potential and the tangential electric field vanishes. Equal symmetry then gives equal surface charge density over the sphere. Thus option A is correct; earthing is absent, gravity is irrelevant, and charge is not destroyed.

Correct answer: B. The external charge affects free charges non-uniformly

Explanation: The governing concept is electrostatic induction. A nearby external charge produces an electric force that is stronger on the nearer portion of the conductor and weaker on the farther portion. Mobile charges consequently shift over the surface until electrostatic equilibrium is restored, creating a non-uniform induced distribution. The conductor’s total charge need not change if it remains isolated; only its locations change. Therefore option B is correct, while mass loss, insulation, or charge disappearance is not involved.

Correct answer: C. Positive effect

Explanation: The governing concept is electrostatic induction in a conductor. Electrons are mobile, so a negatively charged rod repels them toward the far end. The near end is left with an electron deficiency and therefore behaves as positively charged, although the conductor as a whole remains neutral. Hence option C is correct; option A reverses the charge, option B ignores redistribution, and option D is physically meaningless.

Correct answer: C. It remains zero if initially neutral

Explanation: Electrostatic induction initially causes only redistribution of free charges inside the conductor. If the charged object does not touch the conductor and no earth connection is made, no net charge can enter or leave it. Thus an initially neutral conductor remains neutral overall, though its two ends become oppositely charged. Option C is correct; A and B confuse separation with net charging, while D has no physical basis.

Correct answer: A. To provide a path for charge exchange

Explanation: In charging by induction, a nearby charged body first separates charges in the conductor. Earthing then connects the conductor to the Earth, which acts as a vast charge reservoir, allowing electrons to enter or leave. After the earth connection is removed, a net charge can remain on the conductor. Therefore option A is correct; the other choices describe no electrical function.