Conductors and Insulators

For a conductor in electrostatic equilibrium, surface charge density is greater where the radius of curvature is smaller. A sharp tip therefore develops a strong local electric field, approximately related to surface charge density by E = σ/ε₀ just outside the surface. This helps initiate ionisation and discharge near the rod rather than the building. Option A is correct; sharpness does not make an insulator or destroy charge.

The governing distinction is between mobile charge in a conductor and bound or poorly mobile charge in an insulator. A localized charge on an insulator cannot readily travel through the entire material to a single earthing contact, so discharge may be slow or incomplete. Option A is correct. Earth can accept or supply charge, while an abundance of free electrons would describe a conductor, not an insulator; charge is not identical to mass.

The governing concept is polarization. In an insulator, electrons and nuclei are bound rather than freely conducting through the material. An applied electric field can produce a small relative displacement of the positive and negative charge centres, creating induced dipoles and possibly a net polarization. Thus option A is correct. The material does not automatically become a perfect conductor, and no charge is destroyed.

A charged comb polarizes the molecules in initially neutral paper. The side nearer the comb acquires an induced charge of opposite sign, while like-sign induced charge is displaced farther away. Because Coulomb attraction is stronger at the smaller separation, the nearer opposite charges produce a net attractive force, even though the paper’s total charge remains zero. Therefore option A is correct; the paper does not become metal or lose charge.

Net charge is the algebraic sum of all charge on the conductor, whereas local surface charge density can vary from place to place. An external charge or field can attract mobile carriers toward one region and repel them from another. Equal positive and negative induced amounts may then give zero net charge while producing local regions and a nonuniform field. Hence option A is correct; charge is separated, not destroyed, and the conductor remains a conductor.

The governing principle is electrostatic equilibrium in a conductor. Mobile charges rearrange themselves until the net electric field inside the conducting material becomes zero. Any excess charge resides on the surface, and this surface charge can produce a non-zero electric field outside. Therefore option A is correct. Option B is not necessary because a charged conductor may have surface charge; C contradicts equilibrium, and D misidentifies the material.

The governing concept is electrostatic shielding. In a closed conducting enclosure, free charges move over the conductor’s surfaces and arrange themselves so that the electric field within the enclosed conducting region is cancelled in electrostatic equilibrium. Thus a sensitive device placed inside the closed enclosure is best protected, making option A correct. Rubber and glass do not provide the same conducting shield, while a bare wire alone does not form a complete enclosure.

Electrostatic equilibrium means that the conductor’s free charges have no net tendency to move. If an electric field existed inside, charges would continue moving, so the internal field must be zero. Since E = -dV/dr, zero field throughout the conductor means its potential is constant, or the conductor is equipotential. Any excess charge remains on the surface. Hence option A gives the complete identity.

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 rather than spreading throughout the object. In an external electric field, the bound positive and negative charges may shift slightly, producing polarisation. Thus option A is correct. Options B and D describe conductor-like behaviour, while C falsely claims that charge and polarisation disappear.

The fundamental distinction is the mobility of charge carriers. In a conductor, electrons or other carriers can move relatively freely through the material, allowing charge redistribution and current. In an insulator, the charge carriers are strongly bound and cannot move freely over macroscopic distances, although small polarization can occur. Therefore option A is correct; colour, name, and sound are not the physical basis of electrical conduction.

A material with very few free charge carriers generally behaves as an insulator in electrostatics. Because charges cannot travel freely through the material, an introduced charge may remain localized. However, bound positive and negative charges can shift slightly under an applied field, creating polarization. Therefore option A is correct. Uniform surface spreading, zero internal field, and equipotential behaviour are characteristic of conductors, not insulating materials in general.

The broad governing idea is that material structure controls how charges respond to electric fields. In conductors, mobile carriers redistribute readily, support electrostatic induction, and enable shielding. In insulators, charges are mainly bound, so charge remains more localized and polarization is important. Hence option A correctly combines the major behaviours. B ignores material differences, C reverses the carrier property of insulators, and D confuses charge with mass.

The governing electrostatic principle is equilibrium of mobile charges. A conductor contains free charges that respond to any internal electric field. They redistribute on its surfaces until their field cancels the field within the conducting material, giving E = 0 in electrostatic equilibrium. A conductor may carry net charge, so option B is false; charge is not necessarily negative, and fields are not always absent inside matter. Option A is correct.

The governing relation is E = −∇V: a spatial potential difference indicates a non-zero electric field. That field exerts force F = qE on free charges in the conductor, producing motion or current. Electrostatic equilibrium requires no sustained motion and therefore a constant potential throughout a connected conductor. A potential difference does not reduce mass, make a conductor transparent, or destroy charge. Hence option B is correct.

The governing concept is electrostatic equilibrium in a conductor. Excess charges of the same sign repel one another, and the mobile charges can move through the conducting material. They continue redistributing until the electric field inside the conductor is zero; the stable arrangement places excess charge on the outer surface, with density depending on surface shape. The conductor does contain electrons, so option D is false, and gravity is irrelevant. Option C is correct.

The governing principle is electrostatic equilibrium: the electric field inside the conducting material must be zero. If the charge in the cavity is −q, a Gaussian surface drawn within the conductor encloses zero net charge, so the inner surface must carry +q. Therefore option D is correct. Option A ignores induction, option B has the wrong sign, and option C has the wrong magnitude.

Let the charge inside the cavity be −q. Electrostatic equilibrium requires +q on the inner surface, because the field within the conducting material is zero. The conductor itself was initially neutral, so its total induced surface charge must sum to zero: (+q) + Q_outer = 0. Hence Q_outer = −q, making option A correct. Options B and C violate charge conservation, while infinity is physically meaningless here.

For a +q charge inside the cavity, electrostatic equilibrium induces −q on the inner surface. Because the conductor is earthed, charge can flow between the conductor and Earth until the conductor reaches Earth potential. In the ideal symmetric arrangement, the outer surface need not retain any charge and can have Q_outer = 0. Thus option B is correct; the other choices incorrectly claim an unavoidable sign or an infinite charge.

A charge q in an electric field experiences force F = qE. Therefore, a tangential or parallel component E_parallel would exert a force along the conductor’s surface on its mobile charges. They would move and redistribute until that component vanished, so electrostatic equilibrium would be disturbed. Hence option C is correct. Charges are not destroyed, and the material does not become an insulator merely because a tangential field exists.

In electrostatic equilibrium, the electric field cannot have a tangential component at a conductor’s surface; otherwise free charges would experience a force and move. The remaining field direction is normal to the surface. Its inward or outward sense depends on the sign of the local surface charge, but its direction is perpendicular. Therefore option D is correct, while option A contradicts equilibrium.