Conductors and Insulators

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.

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.

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.

The governing concept is electrostatic induction in a neutral conductor. A positively charged rod attracts the conductor’s mobile electrons toward the nearer end, so that end develops an induced negative charge. The electrons’ movement leaves an induced positive charge at the farther end. The conductor’s net charge remains zero because these are separated induced charges, not charge creation. Therefore option B is correct; option A reverses the polarity, while C and D incorrectly give the same sign at both ends.

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.

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.

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.

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.

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.

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.

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.

Earthing establishes an electrical connection between the conductor and the Earth. Because the Earth is an enormous charge reservoir, it can accept or supply charge until the conductor reaches the same electric potential as the Earth, conventionally taken as zero potential. Therefore option B is correct. The potential is not necessarily positive or negative, and it does not become infinite merely because the conductor is earthed.

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.

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.

Charge distribution depends on whether charge carriers can move freely. Friction transfers electrons to or from the insulator, but the material does not provide enough mobile carriers to conduct those electrons across its complete surface. The charge therefore stays localized near the contact or rubbing area, making option A correct. Charge is conserved rather than destroyed, and the other choices confuse insulation with equipotential behavior or incorrectly mention metal.

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.

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.

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.

A metal object allows excess charge to move freely, so it must be electrically isolated to retain that charge. Placing it on an insulating stand greatly reduces the conducting path to the body and earth; consequently, charge leakage is minimized. Option C is correct. A metal wire to earth deliberately discharges the object, holding it connects it through the body, and a wet cloth provides an additional conducting path.

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.