Conductors and Insulators

Correct answer: A. Because total positive and negative charges can be equal

Explanation: The governing concept is net charge, not the freedom of individual charge carriers. Metals contain mobile electrons, but they also contain an equal amount of positive nuclear charge when the object is neutral. The net charge is calculated as total positive charge plus total negative charge; equal and opposite amounts give zero. Thus option A is correct. Electrons are charged, protons are present in atomic nuclei, and metals are made of particles, so B, C and D are false.

Correct answer: A. Using copper inside a wire to carry current

Explanation: Copper contains many mobile charge carriers and has low electrical resistivity, so it allows current to pass efficiently and is commonly used as the conducting core of a wire. Therefore option A is the correct application. Rubber, glass and plastic are generally insulators; they are useful for protection and insulation rather than for carrying the main current or increasing it. The option refers to copper as the conducting part, not its outer covering.

Correct answer: A. Copper carries current and rubber provides safety

Explanation: The governing idea is the contrast between conductors and insulators. Copper contains mobile electrons and has low resistivity, so it forms the conducting core and carries current efficiently. Rubber has very high resistivity, so it electrically insulates the user from the conductor and helps prevent shocks or short circuits. Hence A is correct; B reverses the roles, while C and D ignore the electrical properties of the materials.

Correct answer: A. Freedom of charge carriers

Explanation: The fundamental distinction is the mobility of charge carriers. In a conductor, electrons or other carriers can move relatively freely through the material, allowing current and rapid charge redistribution. In an insulator, charge carriers are strongly bound, so charge does not move freely over the whole body and may remain localized. Length, shine, and the object’s name do not determine this electrical classification; therefore option A is correct.

Correct answer: A. Because an insulator has very few free carriers

Explanation: Rubbing transfers electrons between two materials, leaving one surface charged. In a conductor, many mobile electrons move readily, so excess charge redistributes over the surface until electrostatic equilibrium is reached. An insulator has very few mobile charge carriers, so the transferred charge remains near the region where rubbing occurred. Therefore option A is correct; the other options misstate the nature of insulation.

Correct answer: B. It remains zero if it was neutral before

Explanation: The governing principle is conservation of charge for an isolated conductor. A nearby charged rod can produce induction, so positive and negative charges separate locally on different regions. However, because there is no contact and no earth connection, no charge is supplied or removed from the conductor. Its net charge therefore remains zero if it was initially neutral. Option B is correct; the other choices confuse redistribution with net charging.

Correct answer: A. Freedom of charge carriers

Explanation: 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.

Correct answer: B. Near negative and far positive

Explanation: 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.

Correct answer: C. Near positive and far negative

Explanation: 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.

Correct answer: D. If it was neutral before, it remains zero

Explanation: 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.

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

Explanation: 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.

Correct answer: D. Freedom of charge carriers

Explanation: The fundamental basis is the availability and mobility of charge carriers. In a conductor, electrons or other carriers can move comparatively freely through the material, allowing charge redistribution and current. In an insulator, carriers are tightly bound and cannot move appreciably over macroscopic distances, although slight displacement and polarisation may occur. Thus option D is correct; colour, name and sound do not determine electrical conduction.

Correct answer: B. The near end becomes negative and the far end positive

Explanation: The governing concept is electrostatic induction in a conductor. Free electrons inside the neutral conductor are attracted toward the nearby positive rod, so the near end has an excess of electrons and becomes negatively charged by induction. The far end is left electron-deficient and becomes positive. No net charge is created because the conductor remains isolated; only charge separation occurs. Therefore, option B is correct, while A reverses the induced signs and C and D ignore separation.

Correct answer: A. Positive effect

Explanation: The governing principle is electrostatic induction. A negatively charged rod repels the mobile electrons in a nearby neutral conductor. Electrons move toward the far side, leaving an electron deficit at the near side. That deficit is described as a positive induced charge or positive effect. The conductor need not gain a net charge, and the amount is not infinite. Hence option A is correct; B reverses the electron movement, while C and D contradict the physical process.

Correct answer: C. If initially neutral, the net charge remains zero

Explanation: The governing concept is conservation of charge during electrostatic induction. Bringing a charged body nearby produces redistribution of free charges within the conductor, but there is no physical contact and no conducting path to earth. Thus electrons cannot enter or leave the isolated conductor. If it was initially neutral, the positive and negative induced regions have equal total magnitude, so its net charge remains zero. Option C is correct; A and B confuse local charge separation with net charging, and D is impossible.

Correct answer: A. Because there are very few free charge carriers

Explanation: The governing concept is charge mobility in materials. Rubbing transfers electrons between surfaces, but an insulator contains very few mobile charge carriers. Consequently, the transferred charge remains near the rubbed region instead of redistributing over the entire surface. Option A is correct. Rubbing does not destroy charge, an insulator is not necessarily equipotential, and the presence of metal is not the reason; options B, C, and D are therefore incorrect.

Correct answer: C. Isolating it on an insulating stand

Explanation: A metal allows excess charge to move easily, so charge is retained only when leakage paths are minimised. Placing the object on an insulating stand separates it from the earth and reduces conduction through supports or the body. Thus option C is correct. Earthing, holding the object, or using a wet cloth provides conducting paths and causes faster discharge rather than charge retention.

Correct answer: D. The freedom of charge carriers to move

Explanation: The key physical distinction is the mobility of charge carriers. In a conductor, electrons or other charge carriers can move comparatively freely through the material, allowing current and charge redistribution. In an insulator, charge carriers are tightly bound and cannot move over macroscopic distances easily. Therefore option D is the fundamental basis. Colour, name, and sound do not determine electrical conduction and are irrelevant distractors.

Correct answer: A. It will move surface charges along the surface

Explanation: The governing concept is electrostatic equilibrium in a conductor. A component of electric field parallel to the surface would exert a tangential force F = qE_parallel on mobile charges. These charges would move along the surface, producing a current and redistributing themselves until that tangential component became zero. Therefore a persistent parallel component is impossible in electrostatic equilibrium, so option A is correct. The other choices do not describe any electric-force effect.

Correct answer: B. Perpendicular to the surface

Explanation: In electrostatic equilibrium, the electric field immediately outside a conductor is normal, or perpendicular, to its surface. If a tangential component existed, it would exert a force on mobile surface charges and make them move, contradicting equilibrium. The normal component may be nonzero and is related to surface charge density by E = σ/ε₀ just outside the surface. Therefore option B is correct; A and D describe tangential directions, while C is not generally true for an arbitrary conductor.

Correct answer: B. Near negative and far positive

Explanation: The governing concept is electrostatic induction in a neutral conductor. A positively charged rod attracts the conductor’s mobile electrons toward the nearer end, so that end develops an induced negative charge. The electrons’ movement leaves an induced positive charge at the farther end. The conductor’s net charge remains zero because these are separated induced charges, not charge creation. Therefore option B is correct; option A reverses the polarity, while C and D incorrectly give the same sign at both ends.

Correct answer: B. Earth's potential

Explanation: Earthing establishes an electrical connection between the conductor and the Earth. Because the Earth is an enormous charge reservoir, it can accept or supply charge until the conductor reaches the same electric potential as the Earth, conventionally taken as zero potential. Therefore option B is correct. The potential is not necessarily positive or negative, and it does not become infinite merely because the conductor is earthed.

Correct answer: A. There are very few free charge carriers

Explanation: Charge distribution depends on whether charge carriers can move freely. Friction transfers electrons to or from the insulator, but the material does not provide enough mobile carriers to conduct those electrons across its complete surface. The charge therefore stays localized near the contact or rubbing area, making option A correct. Charge is conserved rather than destroyed, and the other choices confuse insulation with equipotential behavior or incorrectly mention metal.

Correct answer: C. Isolating it on an insulating stand

Explanation: A metal object allows excess charge to move freely, so it must be electrically isolated to retain that charge. Placing it on an insulating stand greatly reduces the conducting path to the body and earth; consequently, charge leakage is minimized. Option C is correct. A metal wire to earth deliberately discharges the object, holding it connects it through the body, and a wet cloth provides an additional conducting path.

Correct answer: D. The freedom of charge carriers to move

Explanation: The governing concept is electrical conductivity, which depends primarily on how easily charge carriers can move. In a conductor, electrons or other carriers can respond and drift through the material under an electric field. In an insulator, carriers are tightly bound and cannot move freely over macroscopic distances. Thus D gives the physical basis; colour, name, and sound do not determine electrostatic conduction.