Conductors and Insulators

A localized charge produces an electric field whose magnitude and direction vary from point to point, especially near the charged region. The field is stronger closer to the charge and generally weaker farther away, so it is non-uniform. Therefore B is correct. It is not necessarily zero, and a localized charge on an insulator cannot automatically be treated as a uniformly charged or conducting surface.

A conductor becomes equipotential in electrostatic equilibrium because its free charges redistribute until no tangential electric field remains. The wrong assumption is that an insulator has the same freely mobile charge carriers. In an insulator, charges are mainly bound and cannot move over large distances, so its surface need not become equipotential. Hence A is correct; mass, shape, and molecular structure are not the mistaken assumptions described.

Rubbing can transfer charge to either material, but the subsequent charge retention differs. In a metal, mobile electrons move readily through the rod, the hand, and the body; this creates a conducting path to Earth, allowing charge to leak away. Plastic has strongly bound charges and normally blocks this path, so it retains charge locally. Therefore A is correct; the other statements contradict these material properties.

A metal contains mobile charge carriers, so any charge placed on it can flow away if a conducting path to Earth exists. An insulating stand has very high resistance and interrupts that path, reducing leakage through the support and surroundings. Consequently, the metal object retains its static charge for longer. Therefore A is correct; insulation neither creates charge nor changes the object’s mass or melting point.

When two conducting spheres touch, mobile charges flow between them until both reach the same electric potential. If the spheres are identical in size and material, their capacitances are equal; equal potential then requires equal final charges, because q = CV. Thus A is correct. Charge is conserved during the redistribution, and neither insulation nor zero gravity is needed for equal sharing.

The governing concept is electrostatic equilibrium: when two conductors touch, charge flows until their potentials become equal, not until their charges become equal. Since the relation is Q = CV, different-sized conductors generally have different capacitances C. At the same potential V, their charges Q can therefore be unequal. Options B and C incorrectly claim that charge stays on only one conductor, while D is not generally true.

The zero-field condition inside a conductor in electrostatic equilibrium means that mobile charges have rearranged until the net force is zero. It does not mean that the conductor has no charge: any excess charge resides on its outer surface, and the potential is constant throughout the conductor because E = −dV/dr = 0. Thus A is the incorrect conclusion; B, C, and D are valid consequences or compatible statements.

In a conductor, free charges can move. If excess charge remained in the bulk at electrostatic equilibrium, it would produce an internal electric field and continue pushing charges, contradicting equilibrium. The charges therefore redistribute until the field inside is zero; excess charge is then located on the surface. This is a rearrangement, not creation or destruction of charge. Hence A correctly explains the phenomenon, while B, C, and D are physically incorrect.

Free charges in a conductor move whenever a net electric force acts on them. Their rearrangement stops in electrostatic equilibrium, when the internal electric field and hence the net force on each mobile charge are zero. The conductor is then an equipotential body. Charges do not necessarily collect at the centre, nor do they change into mass; a colour change has no relation to electrostatic equilibrium. Therefore A is correct.

The electromagnetic energy density of an electrostatic field in vacuum is u = ½ε₀E², and in a material it is similarly proportional to E². Therefore, if the electric field inside the conducting enclosure is exactly zero, the energy density associated with that field is zero, so the total field energy in that region is taken as zero. This does not claim that every form of energy disappears. Hence A is correct; B, C, and D do not follow.

This is electrostatic shielding. When an external electric field is applied to a conducting cage, its mobile electrons redistribute over the outer surface. The induced surface-charge distribution produces a field that opposes the applied field within the enclosed region; in an ideal closed conductor, the net internal field is zero. The metal does not block gravity or destroy charge, and light-to-charge conversion is irrelevant. Thus A is correct.

A car with a substantially conducting metal body behaves approximately like a Faraday enclosure. During a lightning event, charge and current preferentially travel along the exterior conducting surface, while the induced electric field inside the closed body is greatly reduced. The protection depends on the conducting enclosure, not on paint colour, vehicle mass, or the air alone. Occupants should still avoid touching metal parts and follow safety guidance. Therefore A is the scientific basis.

A lightning rod is a good conductor that provides a preferred, low-resistance path between a structure and the ground. Connecting it to Earth allows charge and, when applicable, lightning current to be carried safely into the ground rather than through the building. Earth acts as a very large charge reservoir, so its potential changes only slightly when charge is transferred. Earthing does not make the rod an insulator or transform charge into light. Hence A is correct.

For a conductor, charge density is greater where the radius of curvature is smaller. A pointed tip therefore develops a large local surface-charge density and a strong electric field. This can encourage ionisation of nearby air and provide a preferred discharge path, helping the rod protect the structure when properly earthed. The tip is not uncharged, and gravity or sound is unrelated. Thus A correctly identifies the relevant conductor effect.

In an insulator, electrons are strongly bound to atoms or molecules, so charge cannot flow freely through the entire material as it can in a metal. A local charge may remain concentrated near its original region, and earthing one point does not instantly provide a conducting route for all of it to reach Earth. Earth can accept charge, but the material’s poor conductivity limits the discharge rate. Therefore A is correct; C states the opposite property.

The governing concept is polarisation of an insulator. Its positive and negative charges are normally bound, so they cannot travel through the material as free electrons do in a metal. However, an external electric field exerts opposite forces on the two charge centres, causing a small relative displacement and an induced dipole. Therefore option A is correct; B confuses an insulator with a conductor, while C and D violate ordinary charge and matter behaviour.

The governing concept is electrostatic induction or polarisation. The electric field of the charged comb slightly separates bound positive and negative charges in each paper piece. The nearer side acquires an induced charge of opposite sign and experiences a stronger attraction because it is closer; the farther-side repulsion is weaker. Thus A is correct. Paper need not become metallic, and neither mass increase nor charge destruction explains the force.

The governing concept is electrostatic induction in a neutral conductor. A nearby external charge exerts forces on the conductor’s mobile electrons, so positive and negative charges redistribute over its surface. This creates local charge regions, but the algebraic sum of all charge remains zero because no charge has been added or destroyed. Option A is therefore correct. B violates charge conservation, C confuses zero net charge with absence of charge, and D identifies the wrong force.

The governing concept is electrostatic shielding. In electrostatic equilibrium, free charges in a closed conductor redistribute themselves on its surfaces so that the electric field inside the conducting material is zero; a suitable closed enclosure also protects its interior from external electrostatic influence. Therefore option A gives the best protection. An open insulator, a bare wire, and an exposed glass strip do not provide a continuous conducting shield around the device.

Electrostatic equilibrium means that free charges in a conductor have stopped experiencing a net driving force. Consequently, the electric field throughout the conducting material is zero; since the field is zero, the potential is constant throughout the conductor and its surface. Any excess charge resides on the surface, with its distribution depending on shape. Thus A is the complete statement; the other choices contradict equilibrium and charge behaviour.