Conductors and Insulators

This is electrostatic shielding. In a conductor at equilibrium, free charges rearrange on its outer surface so that the electric field inside the conducting material is zero. For a completely enclosed, empty cavity with no charge inside, the boundary condition on the inner conductor surface then gives zero field throughout the cavity, regardless of how strong the external field is. Hence A is correct. B and C ignore shielding, and D incorrectly limits the result to one point.

A positive charge inside the cavity produces an electric field in the cavity. To keep the field inside the conducting material zero, an equal amount of negative charge is induced on the inner surface of the conductor. Field lines may therefore originate at the positive charge and terminate on this induced negative charge. They do not pass through the conductor, and electrostatic field lines are not closed loops. Thus A is correct.

For a conductor in electrostatic equilibrium, charge is distributed over its surface, and the surface charge density is generally greater where the radius of curvature is smaller. A pointed region therefore can have a large local surface charge density. Since the field just outside a conductor is related to surface charge density by E = sigma divided by epsilon_0, the field is stronger there. Denser field lines represent this larger magnitude, so A is correct; B, C, and D contradict conductor-field rules.

At the surface of a conductor in electrostatic equilibrium, the tangential component of electric field is zero. Electric field lines indicate the direction of the field, so with no tangential component they must meet the surface along the normal, not at an oblique angle. A is therefore the main correction. B is too strong because the field just outside the surface can be nonzero; C is false for electrostatic lines, and D has no physical significance.

For an empty cavity completely enclosed by a conductor in electrostatic equilibrium, the electric field inside the conducting material is zero, and the cavity contains no charge or independent source of field. Consequently, electrostatic shielding makes the field throughout the empty cavity zero, even if charges or an external field exist outside. Option B ignores shielding, while C and D do not follow from the equilibrium condition. This conclusion assumes the cavity is empty and the conductor is in electrostatic equilibrium.

In electrostatic equilibrium, mobile free charges inside a conductor redistribute themselves, usually appearing on its surface, until their induced field cancels the net field within the conducting material. This cancellation is required to prevent continued charge motion. Outside the conductor, however, the surface-charge distribution and external charges can produce a nonzero field. Thus A gives both the mechanism and the limitation. B is not the reason, and C and D are unsupported generalizations.

In electrostatic equilibrium, free charges cannot remain distributed through the conductor’s interior because an internal electric field would make them move. They rearrange until the field inside the conducting material is zero, leaving excess charge on the surface. For a conductor with no internal cavity charge, it resides on the outer surface. Thus A is correct; volume, centre, and cavity-only distributions are not generally valid.

In electrostatic equilibrium, the electric field at a conductor’s surface cannot have a tangential component. If it did, free charges would experience a force along the surface and continue moving, contradicting equilibrium. Therefore, just outside the conductor, the field is normal to the surface; its direction is outward for a positively charged conductor and inward for a negatively charged one. Thus option A is correct, while B describes an impossible equilibrium condition and D applies inside the conductor, not just outside.

The governing condition is electrostatic equilibrium. In a conductor at equilibrium, free charges are able to move and rearrange until the electric field inside the conducting material becomes zero; otherwise they would continue to experience force and move. This result is not restricted to vacuum, temperature, or a flowing-current state. Therefore option A is correct. A conductor carrying steady current generally has a nonzero internal electric field.

For a conductor in electrostatic equilibrium, the electric field everywhere within the conducting material is zero, including a point just inside its surface. If a nonzero field existed there, mobile charges would feel a force and move, contradicting equilibrium. The field just outside may be nonzero and is normal to the surface, but that is a different location. Hence option A is correct; the other choices confuse inside and outside behavior.

The relevant conductor result is the boundary condition just outside a charged conductor: E_out = σ/ε₀ in the normal direction, where σ is the local surface charge density. Thus the field becomes larger where charge is more densely concentrated, especially near sharp regions. It is not determined by colour, the name of the material, or mass alone. Therefore option A is correct, with the understanding that the statement refers to electrostatic equilibrium.

In electrostatic equilibrium, the free charges inside a conductor have no net motion. If a nonzero electric field existed within the conducting material, it would exert force on these charges and produce a current. The charges therefore redistribute themselves until the internal field becomes zero. This is a fundamental property of conductors in electrostatics, so option A is correct; the other choices are not general results.

A conductor contains mobile charges. In electrostatic equilibrium, they move until the electric field inside the conducting material is zero. Gauss’s law then shows that a Gaussian surface wholly within the conductor encloses no net excess charge. Consequently, excess charge is located on the conductor’s surface; for an isolated conductor without an internal cavity charge, it resides on the outer surface. Therefore option A is correct.

At the surface of a conductor in electrostatic equilibrium, the electric field cannot have a tangential component. Such a component would exert a force along the surface and make free charges move, contradicting equilibrium. Hence the field is normal, or perpendicular, to the surface just outside the conductor. Its external magnitude may be nonzero, so option D is incorrect. Therefore option A is correct.

The governing concept is electrostatic equilibrium in a conductor. Free charges move until the electric field within the conducting material becomes zero; excess charge remains on the outer surface. Gauss’s law also gives zero enclosed net charge for a Gaussian surface completely inside the conductor, so the electric flux and field are zero. Therefore, option A is correct; the other choices incorrectly predict a nonzero or infinite interior field.

The governing boundary condition for a conductor is that 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. Thus, greater charge concentration produces a stronger nearby field, especially near sharp regions. Option A is correct; colour, name, and sound have no role in this electrostatic relation.