Conductors and Insulators

Correct answer: B. From the Earth to the conductor

Explanation: A positive rod attracts electrons. When the nearby conductor is earthed, electrons can be supplied from the Earth, which is a large reservoir of charge, and they move toward the conductor and especially its near side. Thus option B is correct. Option A describes the opposite tendency associated with a negative external rod; option C incorrectly assumes contact with the rod, and D violates charge conservation.

Correct answer: B. From the conductor to the Earth

Explanation: The governing principle is repulsion between like charges. A negatively charged rod pushes the conductor's mobile electrons away from the near side. If the conductor is earthed, these electrons can continue through the connection into the Earth, leaving the conductor deficient in electrons. Hence option B is correct. Option A is appropriate for a positive rod, while C assumes contact and D ignores the conducting earth path.

Correct answer: A. So that the final charge can be retained

Explanation: The correct induction sequence is to bring the charged body near, earth the conductor, remove the earth connection, and only then remove the external body. Removing the earth first isolates the conductor and prevents the accumulated charge from flowing away when the external field is removed. Therefore option A is correct. The other choices have no connection with charge conservation or the induction process.

Correct answer: A. The desired charge may not remain

Explanation: During induction, the external charged object maintains the separation of charges while earthing permits charge exchange. If the external object is removed before the earth connection, the separating electric influence disappears while the conductor is still connected to the Earth. Its charge can then flow back or readjust, so the intended net charge may be lost. Hence option A is correct; B and C are unjustified certainties, and D is impossible.

Correct answer: A. The opposite charge effect on the near side is closer

Explanation: Polarisation shifts bound positive and negative charges slightly in opposite directions without changing the insulator's total charge. When an external charge is nearby, the induced opposite charge is closer than the like charge on the far side. Since electrostatic force varies inversely with the square of distance, the nearer attraction is stronger than the farther repulsion, producing a net attraction. Thus option A is correct.

Correct answer: C. Free charges rearrange in a conductor, while bound charges shift slightly in an insulator

Explanation: The key distinction is the mobility of charge carriers. In a conductor, free electrons can move over macroscopic distances and redistribute on its surface when an external electric field is applied. In an insulator, electrons and nuclei remain bound within atoms or molecules; the field causes only a small displacement or orientation of bound charges. Therefore option C is correct, while A, B, and D contradict charge behaviour.

Correct answer: B. Local and non-uniform

Explanation: The governing idea is that charges in an insulators are bound to atoms or molecules and cannot freely redistribute over the material. Therefore, a localized excess charge produces a stronger field near its location and a field that varies from point to point. Option B is correct. Option A is false because the field need not vanish; option C ignores spatial variation, and option D incorrectly applies conductor behaviour to an insulator.

Correct answer: A. Assuming that free charges can move far in it

Explanation: An equipotential conductor is established because its mobile charges redistribute until the tangential electric field and the potential difference within the conductor vanish in electrostatic equilibrium. An insulator generally lacks freely mobile charge carriers, so it cannot automatically equalize its potential in the same way. Option A identifies the basic error. Mass and molecules are ordinary properties, while colour has no role in electrostatic equilibration.

Correct answer: C. Charge can flow through the body and earth

Explanation: Metals contain mobile electrons, so an excess charge placed on a metal object can move readily across its surface. When the object is held, the person provides a conducting path through the body toward the ground. Charge then leaks away, especially when humidity and contact conditions improve conduction. Option C is correct. Metals do contain electrons, charge does not become colour, and metals are conductors rather than insulators.

Correct answer: A. They are identical conductors and become equipotential

Explanation: When two conducting spheres touch, mobile charges flow between them until both spheres reach the same electric potential. For identical spheres, the same potential corresponds to the same capacitance and therefore equal final charges. If the initial total charge is Q, each sphere finally carries Q/2, provided there is no external charge or leakage. Option A is correct; charge is conserved, and neither insulation nor gravity explains the sharing.

Correct answer: B. No because excess charge can be on the surface

Explanation: 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.

Correct answer: A. A state where no further electric force drives them

Explanation: 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.

Correct answer: C. Rearrangement of surface charges can make the internal field zero

Explanation: 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.

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

Explanation: 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.

Correct answer: B. To provide a safe path to earth for large charge

Explanation: 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.

Correct answer: C. Charge density and field can be higher at the sharp tip

Explanation: 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.

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

Explanation: 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.

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

Explanation: 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.

Correct answer: C. Polarisation in paper creates opposite effect on the nearer side

Explanation: 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.

Correct answer: A. When an external charge causes induction

Explanation: 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.

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

Explanation: 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.

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

Explanation: 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.

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

Explanation: 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.

Correct answer: C. Few free charges exist; charge may remain localized and polarisation is possible

Explanation: 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.

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

Explanation: 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.