Conductors and Insulators

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

Explanation: Charging by induction requires the conductor to gain or lose electrons without touching the charged body. The nearby charged object first separates charges in the conductor, and earthing then provides a conducting path through which electrons can flow to or from the Earth. After the earth connection is removed in the proper sequence, a net charge can remain. Therefore option A states the essential role; B, C, and D are unrelated to electrostatics.

Correct answer: B. From Earth to conductor

Explanation: Electrons are attracted by a positively charged rod. In an isolated conductor, this attraction merely shifts electrons toward the near side. When the conductor is connected to Earth, however, Earth acts as a huge reservoir of electrons, so electrons can flow from Earth into the conductor under the electric influence of the rod. Hence option B is correct. Electrons do not become protons or disappear, and option A gives the opposite direction for this setup.

Correct answer: C. From conductor to Earth

Explanation: A negatively charged rod repels electrons in the nearby conductor. If the conductor is earthed, the repelled electrons have an available conducting route into the Earth, which can accept a very large amount of charge. Therefore electrons may flow from the conductor to Earth, making option C correct. They do not enter the negatively charged rod, remain permanently fixed at the centre, or move from Earth toward the conductor as suggested by the other choices.

Correct answer: D. Because wrong sequence may not retain the final charge

Explanation: In induction charging, the external charged object maintains separation of charges while the conductor is earthed. The earth connection must be removed first, so the transferred electrons cannot flow back and the conductor retains a net charge. Only after that should the external object be removed. If the order is reversed, earthing remains available and the charge can neutralise. Thus option D is correct; the other choices are unrelated physical claims.

Correct answer: A. The desired charge may not remain

Explanation: The external charged object must remain nearby while the earth connection is removed because it maintains the separation of charges and determines the direction of electron transfer. If the external object is removed first, its electric influence disappears while the conductor is still connected to Earth. The induced charge can then flow to or from Earth until the conductor becomes neutral again. Therefore option A is correct; the fixed signs in B and C are unjustified, and D violates the nature of electric charge.

Correct answer: C. Because the opposite effect on the near side is closer

Explanation: Polarisation separates the centres of positive and negative charge within an otherwise neutral insulator; it does not create net charge. When a charged object approaches, the induced opposite sign appears slightly closer to it than the like sign. Since electrostatic force varies inversely with the square of distance, the attraction from the nearer opposite charges is stronger than the repulsion from the farther like charges. Hence option C is correct, while A, B, and D misdescribe polarisation.

Correct answer: D. Free charges rearrange in conductor and bound charges shift slightly in insulator

Explanation: The governing concept is the different mobility of charge carriers. In electrostatic induction, free electrons in a conductor move through the material and redistribute, usually without changing the conductor’s total charge. In polarisation, charges remain bound to atoms or molecules; their positive and negative centres shift slightly in opposite directions. Therefore D is correct. A incorrectly assumes total charge must change, while B and C wrongly describe charge destruction or compulsory earthing.

Correct answer: A. Because there are very few free charge carriers

Explanation: The governing idea is limited charge mobility in an insulator. Rubbing transfers electrons between the contacting materials, but an insulating substance has very few mobile charge carriers. After the rubbing stops, the transferred charge therefore remains concentrated near the rubbed region instead of quickly redistributing over the entire surface. A is correct. B is false because an insulator need not contain metal, and C and D contradict charge conservation and the non-equipotential nature of insulators.

Correct answer: B. Local and non-uniform

Explanation: The governing concept is the electric field produced by a non-uniform charge distribution. Because charge on an insulator can remain concentrated in a limited region, the field is generally stronger near that region and changes in magnitude and direction with position. Thus it can be local and non-uniform, so B is correct. A would apply only in special shielding or equilibrium situations, C assumes uniform charge distribution, and D incorrectly treats an insulator as a conductor.

Correct answer: C. Because free charges do not move far easily

Explanation: An equipotential conductor in electrostatic equilibrium results from the redistribution of free charges until no tangential electric field remains. In an insulator, most charges are bound to atoms or molecules, so they cannot move freely over the body to equalise potential. Therefore C gives the correct reason. An insulator can certainly carry charge, it contains molecules, and it is not necessarily metallic; those claims make A, B and D incorrect.

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

Explanation: The governing concept is earthing through a conducting path. A metal object has mobile electrons, and when it is held, the person’s body provides a conducting route to the surroundings and often to Earth. Excess charge can therefore flow away until the object approaches electrical equilibrium, making retention difficult. D is correct. Metals do have electrons, charge does not become colour, and metals are conductors, so A, B and C are false.

Correct answer: B. Because they are identical conductors and become equipotential

Explanation: The governing conditions are charge conservation and equal potential. When identical conducting spheres touch, free charge flows between them until both reach the same potential. Since their radii, and hence their capacitances, are equal, equal potential implies equal final charges. If the initial total charge is Q, each sphere finally carries Q/2 in the ideal isolated case. Therefore B is correct; charge is not destroyed, and gravity or insulation is not responsible.

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

Explanation: The governing concept is electrostatic equilibrium, not absence of charge. In a conductor at equilibrium, free charges rearrange so that the electric field within the conducting material is zero. This does not prevent excess charge from residing on the outer surface, where it produces fields outside the conductor. Hence D is correct. A and B confuse zero internal field with zero charge, while C incorrectly places excess charge necessarily at the centre.

Correct answer: A. On the outer surface

Explanation: The governing principle is electrostatic equilibrium in a conductor. Because free charges can move, any excess charge in the bulk would create an electric field and continue driving charge motion. The equilibrium condition therefore leaves zero volume charge density, with excess charge residing on the conductor’s surface; for an isolated solid conductor this is the outer surface. Thus A is correct, while B, C, and D conflict with charge mobility and conservation.

Correct answer: B. Where no further motion is caused by electric force

Explanation: In a conductor, free charges initially move because an internal electric field exerts force on them. Their redistribution continues until electrostatic equilibrium is reached. At that stage, the electric field inside the conducting material is zero, the potential is uniform, and no net charge motion is driven by an internal electric force. Therefore, option B correctly describes the equilibrium state; charge is not destroyed, and the conductor does not become an insulator.

Correct answer: C. The energy associated with the electric field inside is taken as zero

Explanation: The energy density of an electrostatic field in vacuum is u = ½ε₀E²; in a linear material it is similarly proportional to E². Inside a conductor in electrostatic equilibrium, E = 0, so the local energy density associated with that electric field is zero. Hence option C is correct. Zero field does not imply infinite energy, sound energy, or conversion of charge into mass.

Correct answer: D. Redistribution of surface charges can make the internal field zero

Explanation: A metal contains mobile electrons. When an external electrostatic field is applied, these charges redistribute over the outer surface of the cage. Their induced electric field opposes the applied field within the enclosed region. In electrostatic equilibrium, the resultant field inside the conducting enclosure is zero, provided there is no charge placed inside the cavity. Thus option D gives the principle of electrostatic shielding.

Correct answer: A. Electrostatic shielding

Explanation: The conducting metal body of a car behaves approximately as an enclosing conductor. During a lightning event, charge and current preferentially remain on or travel along the exterior conducting surface, so the electric field inside the passenger space can be much smaller. This is the principle of electrostatic shielding, making option A correct. The effect is not caused by sound reflection, light refraction, or thermal expansion; occupants must still avoid touching exposed metal.

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

Explanation: A lightning rod is made of metal because metal has high electrical conductivity and can carry a large transient current. Connecting it to earth provides a low-resistance path to the ground, which acts as a huge charge reservoir and helps keep the protected structure near earth potential. Thus option B is correct. The rod does not destroy charge, make itself an insulator, or change colour; it safely conducts and disperses charge.

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

Explanation: In an insulator, electrons are strongly bound to atoms or molecules, so charge cannot move freely through the entire material. If charge is localized, touching one point to earth may remove or alter charge only near that contact; the rest may remain charged because there is no continuous low-resistance path. Hence option D is correct. Earth can accept charge, but the insulating material prevents rapid redistribution throughout the object.

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

Explanation: The governing principle is electrostatic equilibrium in a conductor. Free charges move until the electric field inside the conducting material becomes zero. This does not mean that the conductor has no charge or that the external field vanishes. Excess charge resides on the surface and its electric field can exist outside the conductor. Therefore option D is correct; A, B and C contradict the properties of a conductor.

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

Explanation: The governing concept is electrostatic shielding. In a closed conducting enclosure, external charges cause free charges on the enclosure to rearrange so that the electric field within the conducting material is zero; under suitable shielding conditions, the enclosed region is protected from external electrostatic influence. Thus option A is correct. Rubber and coloured glass do not provide conducting shielding, while a bare wire alone is not a complete enclosure.

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. If a nonzero electric field existed inside, these charges would continue to move, so the internal field must be zero. Since the field inside is zero, the potential is constant throughout the conductor and on its surface. Any excess charge settles on the surface. Hence option B combines all three correct conditions.

Correct answer: C. Few free charges, localized charge possible, and polarisation possible

Explanation: An insulator contains very few mobile charge carriers; most of its charges are bound to atoms or molecules. Consequently, an added charge can remain localized instead of spreading freely over the surface. An external electric field can slightly displace bound positive and negative charges, producing polarisation. Therefore option C gives the correct deeper description. Options A and D describe conductor-like behaviour, while B incorrectly denies polarisation.

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

Explanation: When a material has very few free charge carriers, charges cannot travel easily across it. An introduced charge may therefore remain localized. However, bound positive and negative charges can undergo a small relative displacement in an external electric field, producing polarisation. This is typical insulating behaviour, so option A is correct. Options B, C and D are characteristic of ideal conductor behaviour and are not generally valid for an insulator.