Conductors and Insulators

An electric field inside a conductor exerts a force on its free charges, causing them to move and redistribute. This redistribution continues until the field produced by the redistributed charges cancels the internal electric field. At electrostatic equilibrium, the electric field inside the conductor and hence the net force on free charges are zero. Option B is incorrect because the conductor is at constant potential in equilibrium, but that potential need not be zero.

In electrostatic equilibrium, the electric field within the conductor is zero, so free charges experience no net electric force due to the field. Hence, they have no net directed drift. This does not mean that electrons lose their random thermal motion, so option D is incorrect.

Free charges can move in a conductor. If an electric field existed within the conducting material, the charges would continue to move under its influence. They therefore rearrange until electrostatic equilibrium is reached, at which point the electric field inside the conductor is zero. Thus, excess charge resides on the outer surface. The surface distribution need not be uniform; it can depend on the conductor's shape and nearby charges. In option A, gravity is not responsible for this electrical effect.

In a metal, some electrons are weakly bound and gain a drift motion when an electric field is applied, producing a steady current. In rubber, electrons are generally tightly bound to atoms or molecules, so there are very few mobile charge carriers. The polarization in option C can occur in rubber, but it does not provide the continuous motion of free charges needed for steady conduction.

The electric field of the charged rod exerts a force on the conductor’s free electrons. The electrons therefore shift toward one part of the conductor, depending on the sign of the rod, leaving an electron-deficient region on the other part. The positive-ion lattice remains nearly fixed. Thus, electrostatic induction causes charge separation, but the isolated conductor still has zero net charge; no new charge is created.

In an insulator, charges cannot move freely through the entire material. However, the electric field of the external charge can slightly displace bound positive and negative charges in opposite directions within atoms or molecules. This relative displacement produces induced electric dipoles and hence polarisation. Option B describes the flow of free charges in a conductor, not polarisation in an insulator.

In electrostatic induction, an external electric field redistributes the free charges in a conductor until electrostatic equilibrium is reached. In an insulator, charges are not free to travel through the material; bound positive and negative charges undergo only a very small relative displacement, or permanent dipoles align with the field. Therefore, an insulator does not develop free-charge conduction like a conductor.

In electrostatic equilibrium, free charges redistribute on the surface of a conductor so that the electric field within the conducting material becomes zero. Therefore, a closed conducting enclosure prevents external electrostatic fields from reaching its interior cavity. Unlike option B, the field inside a conductor at equilibrium is not maximum; it is zero.

In electrostatic equilibrium, free charges in the metal redistribute in response to the external electric field. The field produced by these redistributed charges cancels the external field inside the empty cavity of the closed conductor, so the external electric field there is zero. This is called electrostatic shielding. Option B is incorrect because metals do contain free charges.

The governing concept is electrostatic equilibrium in a conductor. If the electric field had a component parallel to the surface, free charges on that surface would experience a tangential force and move. They would continue redistributing until the tangential component became zero. The remaining field is normal, or perpendicular, to the surface, so option A is correct. The other options deny the existence of a surface, confuse charge with mass, or incorrectly place the field only inside.

Free charges in a conductor can move. A tangential electric field at the surface exerts a force parallel to the surface on surface charges, causing them to move and redistribute. This redistribution continues until the field produced by the redistributed charges cancels the tangential component. Thus, in electrostatic equilibrium, the electric field at a conductor's surface can only be normal to the surface; a normal field does not drive charges along the surface.

Because Earth is extremely large, it has a very large capacitance. Hence, transfer of charge produces only a very small change in its potential, since
\(\Delta V=\frac{Q}{C}\). Therefore, Earth can receive or supply a large amount of charge while remaining at nearly constant potential. Option B is incorrect because Earth provides a path for charge flow through conducting connections; it is not a perfect insulator.

A metal sphere is a conductor. If it is supported by a conducting path connected to Earth, excess charge can flow to Earth. An insulating rod electrically isolates the sphere from Earth and prevents this loss of charge. Uniform distribution of charge is due to the conducting nature of the sphere, not due to the insulating rod.

A metal is a conductor, so excess charge on it can move easily. A dry insulating handle isolates the rod from the hand and Earth, preventing a path for charge leakage; therefore, the rod can remain charged longer. In contrast, in option B the hand and body can provide a conducting path to Earth, allowing charge to leak away.

When the conductor is connected to the Earth through a conducting wire, charge flows between them until their potentials become equal. Because the Earth has an enormously large size and capacitance, its potential remains practically unchanged; hence the conductor acquires the Earth’s potential. A glass enclosure or an insulating stand keeps the conductor electrically isolated from the Earth, so this equalisation does not occur.

The governing concept is the relation between surface charge density and the electric field near a conductor. A pointed region has a small radius of curvature, so charge tends to accumulate there. Greater surface charge density produces a greater field just outside the conductor; in simplified form, the normal field is proportional to surface charge density. Therefore option A is correct. Option B confuses the zero field inside with the external field, while C and D are not generally true.

A lightning conductor is a metal conductor. When it is earthed, it provides a low-resistance path for lightning current to reach the earth. Thus, the current passes through the conductor and earthing wire rather than through the building. Option B is incorrect because charge may still appear on the conductor; earthing safely carries the resulting current to the earth.

Metals have low electrical resistance because they contain mobile electrons, so the inner metal part carries current. Plastic has very high resistance; its outer covering greatly reduces current flow into a person or another object during accidental contact with the metal conductor, reducing the risk of electric shock. Option B reverses the properties of a conductor and an insulator.

A closed metal body acts as a conducting enclosure. During a lightning strike, current flows mainly along the vehicle's outer metal surface, so the electric field inside is very small. This is the Faraday-cage, or electrostatic-shielding, effect. For safety, a person should avoid touching metal parts of the vehicle. Option B is incorrect because the main protection is due to the conducting metal enclosure, not the rubber tyres.

A negatively charged object repels the free electrons in the conductor toward the farther end. The nearer end is therefore deficient in electrons and acquires an induced positive charge. The conductor still has zero net charge overall because an equal induced negative charge accumulates at the farther end.

A positively charged object attracts the free electrons in the conductor. Hence, electrons accumulate at the nearer end, making that end negative. This is redistribution of charge by electrostatic induction: electrons are neither created nor destroyed, and they do not leave the conductor. The conductor remains net neutral, while its farther end becomes relatively positive. Unlike option B, electrons move toward, not away from, the positive object.

Dry wood generally has very few mobile charge carriers and therefore behaves approximately as an insulator. In wet wood, dissolved salts and other impurities in the moisture can provide ions. These ions move in an electric field and increase the current. Option B is incorrect because wet wood does not acquire metallic free-electron conduction; the increase is mainly due to ionic conduction.

Free charges in a conductor move until the electric field within its material becomes zero. Since \(\vec{E}=-\nabla V\), a zero electric field means that the potential is the same at all points of the conductor. Therefore, option A is correct. Unlike option B, the electric field inside a conductor is zero in electrostatic equilibrium; excess charge generally resides on its surface rather than throughout its volume.

A conductor has many mobile charge carriers. Therefore, excess charge moves quickly and redistributes over the outer surface of the conductor. An insulator has very few mobile charge carriers, so deposited charge generally remains localized near the region where it was placed. Option B incorrectly reverses this difference in charge mobility.

In electrostatic equilibrium, free charge on a conductor arranges itself on its surfaces, and the surface charge density is generally not uniform. It depends on the shape and local curvature of the surface; charge density is higher near sharp regions or regions with a smaller radius of curvature. Although the electric field inside a conductor is zero, this does not imply that the surface charge density is uniform.