Conductors and Insulators

In electrostatic equilibrium, free charges in a conductor can move. If an electric field existed within the conducting material, these charges would experience a force and continue moving. They therefore redistribute, generally onto the surface, until the field due to the redistributed charges cancels the field of external and other charges inside the material. Hence the net internal electric field is zero. This happens because of charge redistribution, not because charge is destroyed.

The governing concept is electrical conductivity. Copper, silver, and aluminium are conductors, so free electrons can move through them and distribute excess charge over the surface. Glass is an insulator because its electrons are tightly bound; therefore charge cannot move easily away from the point of contact and remains localized there. Hence option C is correct, while the metallic options are plausible but unsuitable distractors.

In electrostatic equilibrium, free charges redistribute on a conductor’s surface so that the electric field within the conducting material becomes zero. Therefore, a closed conducting enclosure shields its interior from external electrostatic fields. Option B is incorrect because an electric field need not be zero inside an insulator.

A closed, continuous conducting enclosure works on the Faraday-cage principle. Ideally, the electric field inside a conductor-enclosed region is zero; therefore, the electric field inside the enclosure is very small. Lightning current flows mainly along the outer surface. The metal does not become an insulator, so option B is incorrect.

Free charges cannot move easily through an insulator. Hence, an insulating rod does not provide a conducting path for charge to leak from the metal sphere through the hand to the earth. In contrast, if the sphere is held directly by hand, charge may flow through the body to the earth.

The Earth acts as a very large charge reservoir. When a conductor is connected to the Earth, charge flows until the conductor reaches the same potential as the Earth. The Earth's potential is conventionally taken as zero. The conductor does not have to become negative; it becomes equal to the Earth's potential.

Copper is a conductor and contains mobile free electrons. When an electric field is applied, these electrons acquire a drift motion, producing a sustained electric current. Dry rubber, glass, and plastic are insulators; they may undergo polarization, but they do not normally allow significant sustained charge flow.

A conductor has free charge carriers, such as free electrons in metals. When an electric field is applied, these carriers can move and produce electric current. In electrostatics, they redistribute to establish electrostatic equilibrium. In contrast, charge carriers in an insulator cannot move freely.

During friction, electrons can be transferred between two materials, giving the insulator a net charge. Charge carriers cannot move freely in an insulator, so the transferred charge usually remains localized near the place where it was deposited. Unlike option D, immediate spreading of charge throughout an object is characteristic of a conductor, not an insulator.

In electrostatic equilibrium, free charges in a conductor move until the electric field inside the conductor becomes zero. Hence, excess charge given to a conductor spreads over and remains on its outer surface. In an insulator, charges cannot move easily, so they generally remain near the place where they are deposited.

Rubbing the glass rod with silk transfers electrons, so the rod can become charged. Glass is an insulator and dry skin has relatively high resistance; therefore, the charge does not quickly leak through the hand to Earth and can remain on the rod. In contrast, holding it through a conducting path or with a wet hand would allow charge to leak away more easily.

Free charges in a metal move and redistribute under the electric force. This process stops when electrostatic equilibrium is established. In this state, the electric field inside the conductor is zero and the potential is the same throughout the conductor. Unlike option B, the internal electric field at equilibrium is not maximum; it is zero.

In electrostatic equilibrium, excess charge resides on the outer surface of a conductor, but the surface charge density

trac{dq}{dA}

need not be uniform. It is generally greater at sharper or more highly curved parts of the conductor. Option B can be true in special cases, such as an isolated spherical conductor, but it is not true for every conductor.

In electrostatic equilibrium, charge is not distributed uniformly over a conductor’s surface. Where the radius of curvature is small, such as at a pointed tip, the surface charge density is greater and the external electric field is stronger. Contrary to option D, the electric field just outside a pointed tip is not zero.

Metals are good conductors because they contain free electrons. A lightning conductor is installed on top of a building and connected to the Earth, so during lightning it provides a safe, low-resistance path for electric charge to reach the Earth, reducing the chance of damage to the building. Options B and D are incorrect because metals do not stop charge flow; they are conductors.

A metal mesh is a conductor, so its free charges redistribute in response to an external electric field. The induced charges produce an electric field that opposes the external field inside the enclosure. In a closed mesh enclosure with sufficiently small openings, the resultant electric field inside can be nearly zero. Earthing may be useful, but the fundamental cause of shielding is charge redistribution in the conductor, not earthing alone.

In electrostatic equilibrium, free charges in a conductor redistribute until the electric field inside the conducting material becomes zero. Therefore, the excess charge of an isolated conductor resides on its outer surface. Option D is incorrect because excess charge does not spread uniformly through the conductor’s volume.

The nearby charged object causes separation of charges in the metal conductor. Because the conductor is earthed, the induced potential difference can make electrons flow between the Earth and the conductor. The direction of electron flow depends on the sign of the external charge. Unlike option B, earthing does not make a metal lose its conductivity.

In an insulator, electrons are generally tightly bound to atoms, so there are very few freely mobile charge carriers. Therefore, excess charge deposited at one place does not spread readily through the material. In contrast, a conductor has free electrons, so charge redistributes quickly.

In electrostatic equilibrium, \(\vec{E}=0\) throughout the conductor. The potential difference is given by \(\Delta V=-\int \vec{E}\cdot d\vec{l}\). Therefore, the integral between any two points inside the conductor is zero, so \(\Delta V=0\). Hence, the potential is the same throughout a connected conductor. Option B is incorrect because a constant potential gives zero potential difference between two points.

Rubber is an electrical insulator. Covering a metal wire with rubber prevents unintended current from reaching the body and reduces the risk of electric shock. In contrast, silver and copper are conductors, so gloves or shoes made from them would not provide electrical insulation.

In electrostatic equilibrium, free charges rearrange until the electric field inside the conductor becomes zero. There is then no net electric force on the charges, so their net motion stops. Option B is incorrect because a non-zero internal electric field would continue to move free charges. Exam tip: the electric field inside a conductor in electrostatic equilibrium is always zero.

In a conductor, free charge carriers, such as free electrons in metals, can move easily. Therefore, an applied electric field can produce electric current and cause charge redistribution. In an insulator, by contrast, most charges remain bound to their atoms or molecules.

In an insulator, electrons and other charge carriers are tightly bound to atoms, so they cannot move freely through the material. Therefore, an insulator does not allow electric current to flow easily. Options B and C describe conductors, while an insulator can hold electric charge on its surface, so D is incorrect.

In a charged conductor, free charges redistribute as long as an electric field exists inside the conductor. At electrostatic equilibrium, the electric field inside the conductor is zero, so the net redistribution of free charges stops. The conductor need not have zero net charge; it may still retain a net charge.