Conductors and Insulators

Correct answer: B. Negative charge

Explanation: This is charging by induction, involving a conductor and earthing. The positive rod attracts electrons within the conductor. While the rod is nearby, electrons flow from Earth into the conductor. Disconnecting Earth first traps these extra electrons; after the rod is removed, the electrons spread over the conductor. Therefore the conductor retains a net negative charge. Option B is correct; removing the rod before disconnecting Earth would instead leave no net charge.

Correct answer: A. Negative

Explanation: This is charging by induction. The nearby positive rod attracts electrons toward the conductor’s near side. While the rod remains present, earthing provides a path for additional electrons to enter from Earth. Removing the earth connection first traps this excess negative charge; removing the rod afterward only redistributes it. Thus the conductor remains negatively charged, so option A is correct.

Correct answer: A. Positive

Explanation: In induction, the negative rod repels the conductor’s free electrons. When the conductor is earthed while the rod remains nearby, some electrons flow from the conductor into Earth. The earth connection must be removed before taking away the rod; this leaves an electron deficiency on the conductor. After the rod is removed, that deficiency spreads over the conductor, so its final charge is positive. Option A is correct.

Correct answer: A. Because the earthed conductor can become neutral again

Explanation: A charged rod maintains the separation of charges in the nearby conductor. If the rod is removed while the conductor is still connected to Earth, the external influence disappears and electrons can flow between the conductor and Earth until the conductor becomes neutral. The charge is not destroyed, and neither protons nor the elementary charge changes. Therefore option A correctly explains the failure.

Correct answer: A. Because free charges move to and reside on the surface

Explanation: A conductor contains mobile electrons. If an electric field existed inside it during electrostatic equilibrium, these free charges would continue to move, so equilibrium would not exist. They redistribute until the internal field is zero and excess charge lies on the outer surface. Thus A is correct; charge is not destroyed, protons do not normally leave, and conductors can certainly be charged.

Correct answer: B. There is a deficiency of free electrons

Explanation: In a metal, the positive ions and their protons are bound within atomic nuclei, while conduction electrons can move through the material. A conductor becomes positively charged when some of its mobile electrons are removed, leaving an electron deficiency. Thus option B is correct. Protons do not flow out, neutrons do not become charged, and charge is transferred rather than destroyed.

Correct answer: A. Because free electrons can move while positive ions remain bound in the lattice

Explanation: The relevant conductor principle is that metals contain mobile conduction electrons, whereas the positively charged atomic ions occupy nearly fixed lattice sites. When an electric field or contact disturbs charge balance, electrons drift and redistribute rapidly through the material; the heavy ions do not move appreciably. Therefore option A is correct. Protons are bound inside nuclei, neutrons are neutral, and lattice charge does not simply disappear.

Correct answer: A. Perpendicular to the surface

Explanation: In electrostatic equilibrium, the electric field has no tangential component at a conductor’s surface. If a tangential component existed, free charges would move along the surface until that component vanished. Consequently, the field just outside the conductor is normal, or perpendicular, to the surface; its magnitude may be σ/ε₀ for a suitable surface charge density. Thus A is correct.

Correct answer: A. Free charges arrange themselves so that the internal field cancels

Explanation: The governing electrostatic principle is that the electric field inside a conductor in electrostatic equilibrium is zero. Conduction electrons are free to move, so any internal electric field would exert force on them and produce a current. They redistribute themselves, usually on the surface, until their induced field cancels the interior field. Thus A is correct; charge can exist on the surface, while B, C, and D are physically false.

Correct answer: A. The diagram is not correct for electrostatic condition

Explanation: In electrostatic equilibrium, the electric field at a conductor’s surface must be normal to the surface. If field lines meet it obliquely, they have a tangential component. That component would exert a force on mobile surface charges and make them move, contradicting electrostatic equilibrium. Thus the shown diagram cannot represent the stated condition, so A is correct. The other choices incorrectly claim universal validity, a large interior field, or absence of surface charge.

Correct answer: A. Because charges would move if a parallel field existed

Explanation: In electrostatic equilibrium, free charges inside a conductor have stopped moving. If the electric field had a tangential component along the conductor’s surface, it would exert a force on these mobile charges and make them move. Charges redistribute until the tangential component becomes zero, leaving only the normal component. Therefore field lines meet the surface normally. Option A gives the governing reason; the other statements are false or irrelevant.

Correct answer: A. Free charges arrange themselves so that the net field inside becomes zero

Explanation: The governing concept is electrostatic equilibrium in a conductor. Conductors contain mobile charges, and any internal electric field would exert force on them, causing motion. The charges therefore redistribute themselves, mainly on the surface, until their induced field cancels the internal field. Hence the net electric field inside is zero. Options B, C, and D are false because conductors can contain charge, obey force laws, and are not defined as magnetic materials.

Correct answer: A. Because charge density can be higher at the pointed part

Explanation: The governing concept is electrostatic charge distribution on a conductor. In equilibrium, excess charge resides on the outer surface and concentrates more strongly where the surface is sharply curved. A pointed region can therefore have greater surface charge density, producing a stronger nearby electric field; field-line density represents field strength. The point is not necessarily negative, the material remains a conductor, and field lines never intersect.

Correct answer: A. Field is zero inside and lines are radially outward outside

Explanation: The governing electrostatic-conductor rule is that the electric field inside a conductor in equilibrium is zero; otherwise free charges would move. For a positively charged spherical conductor, symmetry makes the external field radial and directed outward, equivalent to the field of a point charge at the centre outside the sphere. Thus A is correct. The other choices incorrectly assign a field inside, deny the external field, or violate the properties of electrostatic field lines.

Correct answer: A. Otherwise free charges would move along the surface

Explanation: In electrostatic equilibrium, free charges in a conductor must have zero net force. A tangential electric-field component would exert a force qE_parallel along the surface, causing mobile charges to drift. Their redistribution would continue until that component disappeared. Hence the field at the surface can have only a normal component, making option A correct. Conductors do contain free charges, and their surfaces need not be spherical.

Correct answer: A. Free charges arrange themselves so that the net internal field becomes zero

Explanation: A conductor contains mobile free charges. If a nonzero electric field persisted inside it, these charges would experience force and continue moving, contradicting electrostatic equilibrium. They redistribute on the conductor’s surface until their field cancels the internal field, giving zero net field inside. Therefore option A is correct. The conductor still has atoms, does not become an insulator, and distance remains physically meaningful.

Correct answer: A. Because excess charges can arrange on the surface and create an external field

Explanation: The governing electrostatic-conductor principle is that the electric field inside a conductor is zero in equilibrium because free charges redistribute until the internal force vanishes. This does not mean the surface charge disappears. Excess charge remains on the outer surface and produces an electric field in the exterior region. Thus option A is correct. Option B contradicts equilibrium, while C and D incorrectly deny the conductor’s influence and surface charge.

Correct answer: A. A tangential component would move free charges along the surface.

Explanation: In electrostatic equilibrium, free charges inside a conductor must have no continuing motion. If the electric field had a tangential component along the surface, a free charge would experience a force qE_t and would accelerate or drift along that surface. Charges redistribute until the tangential component becomes zero; only the normal component may remain just outside. Therefore A is correct. B is wrong because conductors do contain mobile charges, while C and D misunderstand electric-field properties.

Correct answer: A. Zero, provided there is no charge inside the cavity.

Explanation: 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.

Correct answer: A. In electrostatic equilibrium, the field line must be normal to the conductor surface.

Explanation: 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.

Correct answer: A. Zero

Explanation: 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.

Correct answer: A. Free charges arrange on the surface and cancel the internal field

Explanation: 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.