Conductors and Insulators

Correct answer: B. Because its free charges do not move far enough to redistribute easily

Explanation: An electrostatic conductor becomes equipotential because its mobile charges redistribute until no tangential electric field remains. In an insulator, charges are largely bound and cannot move freely over the body, so an uneven potential can persist. Hence B gives the governing reason. An insulator can contain charge, it certainly contains atoms or molecules, and it is not necessarily metallic; therefore A, C and D are false.

Correct answer: A. Charge can flow through the body and then to Earth

Explanation: A metal has mobile electrons, so excess charge can move readily through it. When the object is held, the human body provides a conducting path with some resistance to the surrounding Earth; leakage therefore removes the charge, especially in humid conditions. A is correct. Metals do contain electrons, charge does not turn into colour, and metals are conductors rather than insulators, so B, C and D are invalid.

Correct answer: A. When it is isolated on an insulating stand

Explanation: A charged metal object retains charge only when an easy leakage path is prevented. An insulating stand separates it from Earth and from other conductors, so its mobile electrons cannot readily escape. Therefore A is correct. A metal wire deliberately provides an earthing path, holding it connects it through the body, and a wet cloth generally increases surface conduction; B, C and D therefore favour discharge rather than retention.

Correct answer: A. They move toward an arrangement in which no further electric force drives their motion

Explanation: The governing concept is electrostatic equilibrium in a conductor. Mobile charges rearrange under electric forces until the conductor reaches a minimum-energy stable arrangement. At equilibrium, the electric field inside the conductor is zero and the potential is uniform, so there is no net force to drive continued charge motion. Therefore, option A is correct; charges do not necessarily collect at the centre, and charge is neither destroyed nor converted into mass.

Correct answer: A. No energy associated with the electric field is stored in the field-free interior

Explanation: Electric-field energy density is given by u = 1/2 εE² in a linear medium. If the electric field E inside the closed conductor is zero, this local field-energy density is also zero. Hence no electric-field energy is stored in that field-free interior region, making option A correct. The other choices contradict the relation, introduce an unrelated form of energy, or incorrectly claim that charge becomes mass.

Correct answer: A. Rearrangement of surface charges can make the internal electric field zero

Explanation: This is the principle of electrostatic shielding. A metal has mobile electrons, so an external electric field causes charges to redistribute over its outer and inner surfaces as required. Their induced field opposes the applied field inside the closed conductor; in electrostatic equilibrium the net interior field becomes zero, provided no internal charge is present. Thus option A is correct, while the other choices describe false or irrelevant ideas.

Correct answer: A. Electrostatic shielding

Explanation: The relevant principle is electrostatic shielding. The conducting metal body of a closed car allows excess charge from an external electrical discharge to redistribute mainly over its outer surface. In the ideal electrostatic picture, the electric field inside the enclosure is greatly reduced or zero, so the occupant is relatively protected. Option A is therefore correct; sound reflection, light refraction, and thermal expansion do not explain this electrical protection.

Correct answer: A. To provide a safe, low-resistance path to earth for large charge or current

Explanation: A lightning rod works through conductivity and earthing. Metal has low resistance, so it can carry a large transient current without behaving like an insulating barrier. Its connection to the Earth provides a broad charge reservoir and a comparatively safe path for the discharge to spread away from the protected structure. Therefore option A is correct. The rod does not destroy charge; charge is transferred, and colour has no physical role.

Correct answer: A. Charge does not move freely through the whole insulator

Explanation: 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.

Correct answer: A. Bound positive and negative charges can shift slightly in opposite directions

Explanation: 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.

Correct answer: A. Charge separation by electrostatic induction

Explanation: 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.

Correct answer: A. Surface charge can produce an outside field

Explanation: 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.

Correct answer: A. When it is inside a completely closed conducting enclosure

Explanation: 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.

Correct answer: A. Few free charges, localized charge possible, and polarisation possible

Explanation: 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.

Correct answer: A. Charge can remain localized and the material can polarise

Explanation: 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.

Correct answer: A. Free charges rearrange and cancel the internal field

Explanation: 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.

Correct answer: C. Because free charges spread outward due to repulsion

Explanation: 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.

Correct answer: D. Positive and equal

Explanation: 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.

Correct answer: A. Negative and equal

Explanation: 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.

Correct answer: C. Surface charges will move and equilibrium will break

Explanation: 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.

Correct answer: D. Perpendicular to the surface

Explanation: 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.

Correct answer: A. Surface charge density

Explanation: For a conductor in electrostatic equilibrium, the field just outside its surface has magnitude E = σ/ε₀ in vacuum, where σ is the local surface charge density. Thus regions with larger local σ have a stronger nearby field. The result depends on charge distribution, not on the object’s name or colour, and classroom temperature is irrelevant to this basic relation. Therefore option A is correct.

Correct answer: B. Charge density can be higher at the pointed part

Explanation: The governing idea is nonuniform surface charge distribution on a conductor. At a sharp tip, the small radius of curvature allows free charge to become more concentrated, so the local surface charge density σ increases. Since the surface field is approximately E = σ/ε₀, the nearby electric field becomes stronger. Thus option B is correct; a tip does not become an insulator, lose charge, or gain a relevant mass effect.

Correct answer: D. Because there is complete symmetry in all directions

Explanation: An isolated spherical conductor with no external charge has complete rotational symmetry: no direction or surface point is physically preferred. Mobile charges therefore redistribute until the conductor is an equipotential, producing the same surface charge density at equivalent points. The charge is on the surface, not absent, and isolation does not mean the sphere is earthed. Thus option D is correct; options A, B, and C state false conditions.

Correct answer: A. The external charge rearranges free charges

Explanation: A nearby external charge produces an electric field that exerts forces on the conductor’s mobile electrons. They shift until the conductor again becomes an equipotential, creating induced regions of higher and lower surface charge density; the near side and far side need not carry equal local density. The material remains conducting and charge is not converted into mass. Therefore option A correctly describes electrostatic induction.