Conductors and Insulators

Dry plastic is an insulator. Most of its electrons are bound to atoms or molecules, so it has very few mobile charge carriers. Therefore, an added charge does not move easily throughout the plastic and can remain localized. In contrast, a metal has mobile free electrons, so charge spreads through it quickly.

Charges in a conductor are mobile, so they repel one another and move in response to any internal electric field. This redistribution continues until electrostatic equilibrium is reached, at which point the electric field inside the conductor is zero and the conductor is at one potential. It is not caused by colour, mass, or an always-decreasing temperature. Therefore, option A is correct.

In electrostatic equilibrium, the free charges in a conductor are at rest. If the electric field had a tangential component at the surface, it would exert a force along the surface and make the charges move. Therefore, the tangential component must be zero. In contrast, the normal component just outside the surface need not be zero; it is related to the surface charge density.

Rubber has very few free charge carriers, so it has high electrical resistance and does not allow current to flow easily. Dry, undamaged rubber gloves rated for the appropriate voltage form an insulating barrier between the hand and a live part, greatly reducing current through the body. In contrast, a conductor allows current to flow readily and would not provide this protection.

In a material with many free electrons, the electrons move easily when an electric field is applied and carry electric current. Therefore, the material is generally a good conductor. In contrast, an insulator or dielectric has very few free charge carriers.

When charge carriers are tightly bound to atoms, they cannot move easily in response to an external electric field. Therefore, very little electric current flows, and the material generally behaves as a good insulator. In contrast, a good conductor has many charge carriers that can move relatively freely.

Free charges in a conductor already have random thermal motion, but an applied electric field exerts an electric force on them. This produces motion with a definite average direction, called drift motion. A potential difference can establish an electric field in a conductor, but the direct cause of directed motion is the electric field.

A conductor contains free charges. If the electric field inside it were nonzero, the free charges would experience an electric force and move to redistribute themselves. This redistribution continues until the electric field inside the conductor becomes zero in electrostatic equilibrium. Therefore, option A is correct; an unchanged nonzero internal field would not satisfy electrostatic equilibrium.

In a conductor, free charges can move relatively easily and hence redistribute; in electrostatic equilibrium, excess charge resides on the outer surface of the conductor. In an insulator, charge motion is highly restricted, so charges generally remain localized near where they are placed or produced. Thus, conductors and insulators explain the motion and distribution of charge, not the quantisation of charge.

The governing principle is electrostatic equilibrium in a conductor. Mobile electrons redistribute until the electric field inside the conducting material becomes zero. Any excess charge therefore remains on the conductor’s surface; its density can be greater near sharp edges, but it is not restricted to the centre or a single edge. Hence option B is correct, while A, C, and D contradict conductor behavior.

Dry plastic is an insulator. It has very few mobile charge carriers, so the charge produced by rubbing cannot move easily through the material and remains localized near the rubbed region. In contrast, free electrons move easily in a metal, so charge can spread quickly.

A conductor contains free charges. If the electric field inside it were nonzero, it would exert a force on these charges and make them move and redistribute. This redistribution continues until the resultant electric field inside becomes zero. Option C is incorrect because a conductor may have a nonzero net charge; in electrostatic equilibrium, any excess charge resides on its surface.

Copper is a conductor because it contains mobile electrons that can move through the material. When charge is supplied, these carriers redistribute over the object until electrostatic equilibrium is reached. Glass, rubber, and dry wood are generally insulators under ordinary conditions, so their charges remain relatively localized. Therefore option C is correct; the other materials do not permit easy charge spreading.

Rubber is an insulator, so it has high electrical resistance. Dry, undamaged rubber shoes provide a high-resistance path between the body and the ground, reducing the chance of current flowing through the body. In contrast, conductors allow current to flow easily. Wet or damaged shoes, or high-voltage conditions, can reduce this protection; rubber shoes do not guarantee complete safety.

A metal contains free electrons. The electric field of the charged rod exerts a force on these free charges, so they redistribute within the sphere. This phenomenon is called electrostatic induction. Without contact, charges are separated within the sphere; unless the sphere is earthed, its net charge generally remains unchanged.

In electrostatic equilibrium, free charges redistribute until the electric field within the conducting material becomes zero. Since \(\mathbf{E}=-\nabla V\), \(\mathbf{E}=0\) means that the potential does not change from one point to another, so the conductor is an equipotential body. Option B is incorrect because a conductor need not have zero potential; its potential is only the same at all its points.

In an insulator, electrons are generally tightly bound to atoms, so there are very few free, mobile charge carriers. Therefore, electric current flows through an insulator only with great difficulty. In contrast, a semiconductor has some free charge carriers, and its conductivity can change with temperature or doping.

A free charge in an electric field experiences an electric force of magnitude \(F=qE\). Therefore, free charges begin to move and redistribute within the conductor. In contrast, in electrostatic equilibrium, this redistribution continues until the electric field inside the conductor becomes zero.

Plastic is an electrical insulator, so electric current does not pass through it easily. A plastic handle reduces the risk of current reaching the hand from a live metal part of the device. A conductor would not be safe for such a handle. Exam tip: Link insulating materials used in handles with protection from electric shock.

In electrostatic equilibrium, the surface of a conductor is an equipotential surface. If the closed cavity contains no charge, its boundary is at constant potential, so the electric field inside the cavity is zero. Therefore, the correct answer is zero. If a charge were placed inside the cavity, the field in the cavity would not generally be zero; hence the stated condition is essential.