Conductors and Insulators

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.

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.

At a sharp metallic tip, charge tends to concentrate because the local radius of curvature is small. The increased surface charge density produces a large electric field near the tip, according to E = σ/ε₀ for the nearby surface region. If the field becomes sufficiently strong, it can ionize the surrounding air and initiate corona or discharge. Hence option C is correct; the other choices do not explain ionization or field enhancement.

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.

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.

This is electrostatic induction in a conductor. Free electrons inside the neutral conductor are attracted toward the nearby positive rod, so the near end develops an induced negative charge and the far end is left with an induced positive charge. The conductor as a whole remains neutral because no charge enters or leaves. Therefore, option B is correct; options A, C, and D either reverse the separation or incorrectly make both ends have the same sign.

A negatively charged rod repels the mobile electrons in a nearby neutral conductor. Electrons move toward the far side, leaving a deficiency of electrons, or induced positive charge, at the near side. The far side consequently becomes induced negative, while the total charge of the isolated conductor remains zero. Hence option C is correct. Option A reverses the polarities, while B and D ignore charge separation.

The governing principle is conservation of charge. A nearby charged rod produces induction, which only redistributes the conductor’s existing free electrons: one region may become relatively positive and another relatively negative. Because there is no physical contact and no conducting path to Earth, no net charge is transferred. Thus a conductor that was initially neutral remains net neutral, making option D correct. Options A and B confuse local induced charges with total charge, and C has no physical basis.

Charging by induction requires the conductor to gain or lose electrons without touching the charged body. The nearby charged object first separates charges in the conductor, and earthing then provides a conducting path through which electrons can flow to or from the Earth. After the earth connection is removed in the proper sequence, a net charge can remain. Therefore option A states the essential role; B, C, and D are unrelated to electrostatics.

Electrons are attracted by a positively charged rod. In an isolated conductor, this attraction merely shifts electrons toward the near side. When the conductor is connected to Earth, however, Earth acts as a huge reservoir of electrons, so electrons can flow from Earth into the conductor under the electric influence of the rod. Hence option B is correct. Electrons do not become protons or disappear, and option A gives the opposite direction for this setup.

A negatively charged rod repels electrons in the nearby conductor. If the conductor is earthed, the repelled electrons have an available conducting route into the Earth, which can accept a very large amount of charge. Therefore electrons may flow from the conductor to Earth, making option C correct. They do not enter the negatively charged rod, remain permanently fixed at the centre, or move from Earth toward the conductor as suggested by the other choices.

In induction charging, the external charged object maintains separation of charges while the conductor is earthed. The earth connection must be removed first, so the transferred electrons cannot flow back and the conductor retains a net charge. Only after that should the external object be removed. If the order is reversed, earthing remains available and the charge can neutralise. Thus option D is correct; the other choices are unrelated physical claims.

The external charged object must remain nearby while the earth connection is removed because it maintains the separation of charges and determines the direction of electron transfer. If the external object is removed first, its electric influence disappears while the conductor is still connected to Earth. The induced charge can then flow to or from Earth until the conductor becomes neutral again. Therefore option A is correct; the fixed signs in B and C are unjustified, and D violates the nature of electric charge.

Polarisation separates the centres of positive and negative charge within an otherwise neutral insulator; it does not create net charge. When a charged object approaches, the induced opposite sign appears slightly closer to it than the like sign. Since electrostatic force varies inversely with the square of distance, the attraction from the nearer opposite charges is stronger than the repulsion from the farther like charges. Hence option C is correct, while A, B, and D misdescribe polarisation.

The governing concept is the different mobility of charge carriers. In electrostatic induction, free electrons in a conductor move through the material and redistribute, usually without changing the conductor’s total charge. In polarisation, charges remain bound to atoms or molecules; their positive and negative centres shift slightly in opposite directions. Therefore D is correct. A incorrectly assumes total charge must change, while B and C wrongly describe charge destruction or compulsory earthing.

The governing idea is limited charge mobility in an insulator. Rubbing transfers electrons between the contacting materials, but an insulating substance has very few mobile charge carriers. After the rubbing stops, the transferred charge therefore remains concentrated near the rubbed region instead of quickly redistributing over the entire surface. A is correct. B is false because an insulator need not contain metal, and C and D contradict charge conservation and the non-equipotential nature of insulators.

The governing concept is the electric field produced by a non-uniform charge distribution. Because charge on an insulator can remain concentrated in a limited region, the field is generally stronger near that region and changes in magnitude and direction with position. Thus it can be local and non-uniform, so B is correct. A would apply only in special shielding or equilibrium situations, C assumes uniform charge distribution, and D incorrectly treats an insulator as a conductor.

An equipotential conductor in electrostatic equilibrium results from the redistribution of free charges until no tangential electric field remains. In an insulator, most charges are bound to atoms or molecules, so they cannot move freely over the body to equalise potential. Therefore C gives the correct reason. An insulator can certainly carry charge, it contains molecules, and it is not necessarily metallic; those claims make A, B and D incorrect.

The governing concept is earthing through a conducting path. A metal object has mobile electrons, and when it is held, the person’s body provides a conducting route to the surroundings and often to Earth. Excess charge can therefore flow away until the object approaches electrical equilibrium, making retention difficult. D is correct. Metals do have electrons, charge does not become colour, and metals are conductors, so A, B and C are false.

The governing principle is electrical isolation. A charged metal object can retain excess charge only when mobile charges do not have an easy conducting path to Earth. An insulating stand interrupts that path, so option A is correct. A metal earth wire, a person’s body, and a wet cloth all provide comparatively effective leakage paths; consequently B, C, and D would make the charge escape faster rather than preserve it.