Conductors and Insulators

A negatively charged rod repels the mobile electrons in the nearby neutral conductor. When the conductor is connected to Earth, Earth acts as a very large charge reservoir, so the repelled electrons can flow from the conductor into the Earth through the grounding connection. The rod remains nearby during this process. The electrons do not move toward the negative rod, become fixed at the centre, or transform into mass; the effect is charging by induction with earthing.

The zero-field result applies to the conducting material in electrostatic equilibrium, because any internal field would drive free charges and cause motion. It does not imply that the conductor produces no field outside. Excess charge resides on the surface and its electric field extends into the surrounding space. In fact, just outside a charged conductor the normal field is E = σ/ε0. Thus internal equilibrium and an external field are perfectly consistent.

An equipotential conductor is established by the redistribution of mobile free charges: they move until the internal electric field vanishes. In an insulator, charge carriers are strongly bound to atoms or molecules, so charge cannot generally redistribute freely throughout the material. Treating it as an equipotential conductor therefore incorrectly assumes free charge mobility and zero internal field everywhere. Polarization may occur in an insulator, but that is not the same as conductor-like equipotential behavior.

The external charge changes the electric potential and redistributes mobile charges in the earthed conductor. A positive external charge attracts electrons from Earth into the conductor, whereas a negative external charge repels electrons from the conductor toward Earth. Therefore the direction of electron flow depends primarily on the sign of the external charge, with the exact amount also affected by geometry and distance. Colour, name, and mass do not determine the direction.

The governing concept is electrostatic induction in conductors. Free charges inside a conductor can move, and a nearby external charge attracts or repels them, producing a non-uniform surface distribution. Thus the presence, sign, and position of nearby charged objects can change the distribution even without contact. The conductor’s name, book colour, and sound intensity have no role in this electrostatic effect, so option A is correct.

The governing principle is conservation of charge for an isolated conductor. An external charge exerts forces and causes the conductor’s free charges to redistribute, so positions, local surface effects, and surface density can change. However, without contact or an earthing path, no charge enters or leaves the conductor. Therefore its algebraic net charge remains constant, making option A correct; induction changes distribution, not total charge.

The governing concept is the limited mobility of charge in an insulator. Since its charge carriers are bound rather than freely moving throughout the material, charge can remain concentrated in a particular region. The electric field produced by that localized distribution generally varies with position and may be especially strong nearby. It is not always zero or uniform, and it need not imitate a conductor’s field, so option A is correct.

In electrostatic equilibrium, the electric field inside a conductor must be zero; otherwise its free charges would continue accelerating and equilibrium would not exist. The mobile charges therefore move until their arrangement cancels the internal field. Mutual repulsion also drives excess charge toward the outer surface, where it remains for an isolated conductor. Charge is not destroyed, and colour or gravity is irrelevant, so option A gives both reasons.

Earthing removes charge efficiently only when charge carriers can move through a conducting path. In a conductor, mobile carriers redistribute and flow to Earth, bringing the potential toward ground. In an insulator, carriers are bound and the charge may remain trapped in localized regions, so contact with Earth does not immediately remove all of it. Insulators can hold charge, and Earth can accept charge; hence option A is correct.

For a conductor in electrostatic equilibrium, the electric field just outside its surface is normal to the surface and has magnitude E⊥ = σ/ε₀, where σ is the local surface charge density. Therefore, if σ is zero at a particular point, the normal field component there can also be zero. It is not necessarily infinite or maximum; option D is physically unrelated.

When a charged body is brought near an insulating object, its bound charges shift slightly, producing polarisation. The induced opposite charge appears closer to the external charge, while the like charge is farther away. Since electrostatic force varies as 1/r², the nearer attraction is stronger than the farther repulsion, giving a net attraction. Thus option A is correct; the other statements are false.

At a sharp portion of a conductor, the radius of curvature is small, so surface charge tends to accumulate there. The local surface charge density becomes large, and the nearby electric field, approximately E = σ/ε₀, becomes strong. This strong field helps ionise nearby air and provides a preferred path for charge leakage or discharge. Therefore option A is correct; the other choices contradict electrostatic principles.

In electrostatic equilibrium, the electric field inside the conducting material must be zero; otherwise free charges would continue to move. A charge placed inside the cavity induces an equal and opposite net charge on the cavity’s inner surface, producing a field that cancels the field within the metal. The conductor’s outer surface may carry the remaining charge required by its total charge. Hence option A is correct.

The governing concept is electrostatic equilibrium in a conductor. Since the electric field is related to the potential gradient, a nonzero potential difference across an interior region implies a nonzero electric field. That field exerts force qE on mobile charges, causing them to drift until the interior becomes equipotential. Therefore B is correct; sound, insulation, and conversion of charge into mass do not follow from a potential difference.

The governing concept is electrostatic equilibrium of free charges on a conductor. Suppose the external field had a tangential component along the surface. It would exert a tangential force qE_t on mobile surface charges, making them move and rearrange. In equilibrium that component must therefore be zero, leaving only the normal component; the field just outside is perpendicular. Hence A is correct, and the other options contradict basic conductor behaviour.

The governing condition for electrostatic equilibrium is that the tangential component of electric field at a conductor’s surface must be zero. If E_t is nonzero, a surface charge experiences a force qE_t parallel to the surface. Because conductor charges are mobile, they move and redistribute, producing a transient current and destroying the assumed static equilibrium. Therefore B is correct; charges do not vanish and the conductor does not lose its mass.

The governing relation at a conductor surface is E just outside = sigma divided by epsilon_0 in vacuum, where sigma is the local surface charge density. Thus a region with greater charge per unit area generally has a stronger nearby normal electric field, subject to the surrounding medium. Option A is correct. Colour, sound, and the conductor’s name do not determine the electrostatic field and are irrelevant distractors.

The governing concept is electrostatic induction in a conductor. Free electrons can move through the conducting material, so an external charge attracts or repels them more strongly on the nearer side than on the farther side. This redistribution makes surface charge density non-uniform, although the conductor’s total charge need not change. Therefore, option A is correct; the other options contradict the presence of mobile charges and charge conservation.

This is electrostatic induction. A negatively charged rod repels the conductor’s mobile electrons toward its farther end. The near end therefore has an electron deficiency and behaves positively, while the far end has an excess of electrons and behaves negatively. The conductor remains neutral overall because no charge has entered or left. Hence option A is correct; B reverses the electron movement, while C and D ignore charge separation.

Charging by induction requires more than merely separating charges. First, the nearby charged body polarises the conductor. Earthing then provides a conducting path through which electrons can enter or leave, depending on the sign of the external charge. After the earth connection is removed, the conductor can retain a net charge. Thus option B states the essential role; the remaining options have no physical connection with induction.

A positively charged rod attracts electrons. When the nearby conductor is connected to Earth, Earth acts as a very large reservoir of mobile charge, so electrons can flow from Earth into the conductor toward the positive rod. This gives the conductor an excess of electrons while the external rod remains separate. Therefore option A is correct; electrons do not turn into protons or disappear, and their direction is opposite to conventional current.

The governing idea is repulsion between like charges. A negatively charged rod pushes the conductor’s mobile electrons away from the rod. If the conductor is earthed, these repelled electrons can continue through the connection and flow from the conductor into Earth. Hence option A is correct. They do not flow into the isolated rod, and earthing means that movement is possible rather than impossible; conventional current would be opposite to electron flow.

The governing concept is the mobility of charge carriers. In electrostatic induction, free electrons in a conductor can travel through the material and collect on different regions, although the total charge is conserved. In polarisation, charges are bound to atoms or molecules, so their centres shift only slightly or dipoles orient. Therefore B is correct; A and C violate charge conservation, while D is not necessary.

Charging by rubbing transfers electrons from one material to another; it does not destroy charge. The governing distinction is that an insulator has no large population of mobile charge carriers. Consequently, the transferred electrons remain near the rubbed region instead of redistributing over the whole surface. Thus A is correct. B is contradictory, C violates conservation of charge, and D is a property associated with electrostatic conductors, not all insulators.

The electric field is determined by the spatial distribution of charge. Since charge on an insulator can remain concentrated in a limited region, the field strength and direction can vary from point to point around that region; it is therefore local and non-uniform. A is correct. A zero field is not implied by an insulator, uniformity requires special symmetry, and conductor boundary conditions cannot automatically be transferred to an insulator.