Conductors and Insulators

Correct answer: A. Plastic has very few mobile charge carriers.

Explanation: Dry plastic is an insulator. It has very few mobile charge carriers, so the charge produced by rubbing cannot move easily through the material and remains localized near the rubbed region. In contrast, free electrons move easily in a metal, so charge can spread quickly.

Correct answer: B. Because a nonzero electric field would cause free charges to move

Explanation: A conductor contains free charges. If the electric field inside it were nonzero, it would exert a force on these charges and make them move and redistribute. This redistribution continues until the resultant electric field inside becomes zero. Option C is incorrect because a conductor may have a nonzero net charge; in electrostatic equilibrium, any excess charge resides on its surface.

Correct answer: C. Copper

Explanation: Copper is a conductor because it contains mobile electrons that can move through the material. When charge is supplied, these carriers redistribute over the object until electrostatic equilibrium is reached. Glass, rubber, and dry wood are generally insulators under ordinary conditions, so their charges remain relatively localized. Therefore option C is correct; the other materials do not permit easy charge spreading.

Correct answer: D. Rubber is an insulator; its high resistance reduces current between the body and the ground.

Explanation: Rubber is an insulator, so it has high electrical resistance. Dry, undamaged rubber shoes provide a high-resistance path between the body and the ground, reducing the chance of current flowing through the body. In contrast, conductors allow current to flow easily. Wet or damaged shoes, or high-voltage conditions, can reduce this protection; rubber shoes do not guarantee complete safety.

Correct answer: A. The electric field of the rod causes free charges in the metal to move.

Explanation: A metal contains free electrons. The electric field of the charged rod exerts a force on these free charges, so they redistribute within the sphere. This phenomenon is called electrostatic induction. Without contact, charges are separated within the sphere; unless the sphere is earthed, its net charge generally remains unchanged.

Correct answer: A. Because the electric field inside a conductor is zero in electrostatic equilibrium.

Explanation: In electrostatic equilibrium, free charges redistribute until the electric field within the conducting material becomes zero. Since \(\mathbf{E}=-\nabla V\), \(\mathbf{E}=0\) means that the potential does not change from one point to another, so the conductor is an equipotential body. Option B is incorrect because a conductor need not have zero potential; its potential is only the same at all its points.

Correct answer: B. Insulator

Explanation: In an insulator, electrons are generally tightly bound to atoms, so there are very few free, mobile charge carriers. Therefore, electric current flows through an insulator only with great difficulty. In contrast, a semiconductor has some free charge carriers, and its conductivity can change with temperature or doping.

Correct answer: C. They will experience a force due to the electric field and begin to move

Explanation: A free charge in an electric field experiences an electric force of magnitude \(F=qE\). Therefore, free charges begin to move and redistribute within the conductor. In contrast, in electrostatic equilibrium, this redistribution continues until the electric field inside the conductor becomes zero.

Correct answer: C. Because plastic is an electrical insulator

Explanation: Plastic is an electrical insulator, so electric current does not pass through it easily. A plastic handle reduces the risk of current reaching the hand from a live metal part of the device. A conductor would not be safe for such a handle. Exam tip: Link insulating materials used in handles with protection from electric shock.

Correct answer: A. Zero

Explanation: In electrostatic equilibrium, the surface of a conductor is an equipotential surface. If the closed cavity contains no charge, its boundary is at constant potential, so the electric field inside the cavity is zero. Therefore, the correct answer is zero. If a charge were placed inside the cavity, the field in the cavity would not generally be zero; hence the stated condition is essential.

Correct answer: A. Charge leaks to the Earth through the rod and the person's body.

Explanation: Charges can move easily through a metal. When the rod is held in a bare hand and the person is in contact with the Earth, the rod, body, and Earth form a conducting path. Any excess charge on the rod therefore leaks to the Earth, so static charge does not remain on it. In contrast, holding the rod with an insulating handle prevents this leakage path to Earth.

Correct answer: B. Insulator

Explanation: Glass is an electrical insulator. Its electrons are not free to move throughout the rod, so charge transferred during rubbing remains concentrated near the rubbed region instead of quickly spreading away. A conductor would redistribute charge readily, while a semiconductor or electrolyte is not the standard classification intended here. Thus option B is correct because limited charge mobility allows the glass rod to retain charge.

Correct answer: A. Because an electric field parallel to the surface would cause free charges to move

Explanation: In electrostatic equilibrium, the free charges in a conductor are at rest. If the electric field had a component parallel to the surface, it would exert a force on the free charges and make them move. Therefore, the parallel (tangential) component must be zero, so the electric field is perpendicular to the surface. Option B is incorrect because the electric field inside a conductor is zero in electrostatic equilibrium.

Correct answer: B. Charges are bound to atoms or molecules

Explanation: In an insulator, electrons or other charge carriers are tightly bound to atoms or molecules, so they cannot move easily under an electric field. In contrast, a conductor has mobile electrons, allowing charge to flow more readily. Therefore, option B is correct.

Correct answer: C. Copper

Explanation: Copper is a good electrical conductor because its free electrons move easily and it has low resistivity. Therefore, it is used as the conducting core of electric wires. Rubber and plastic are generally insulators and are used as the outer covering of wires for electrical insulation.

Correct answer: A. Separation of charges in a conductor when a charged object is brought near it without contact

Explanation: In electrostatic induction, bringing a charged object near a conductor causes the conductor’s free charges to redistribute, producing charge separation in different parts of the conductor. No contact with the charged object is required. Burning wood is a chemical change, not an example of induction.

Correct answer: A. Because a tangential electric field would move free charges along the surface until this component becomes zero.

Explanation: In electrostatic equilibrium, free charges in a conductor are at rest. If a tangential electric field existed at the surface, it would exert a force parallel to the surface and move the charges. The charges would redistribute until they produced a field that cancels the tangential component. Hence, just outside a conductor's surface, the electric field is normal to the surface. Option B is incorrect because the electric field inside a conductor is zero in electrostatic equilibrium.

Correct answer: A. It can flow between the sphere and the Earth, bringing the sphere to earth potential.

Explanation: A metal is a conductor, and the Earth acts as a very large charge reservoir. When the sphere is grounded, electrons can flow between the sphere and the Earth, removing the excess charge and bringing the sphere to earth potential. Unlike option B, excess charge does not remain permanently on a sphere that is connected to the Earth.

Correct answer: A. Free charges redistribute until the electric field inside the conductor becomes zero.

Explanation: In electrostatic equilibrium, free charges in a conductor move and redistribute until the electric field inside it becomes zero. Since \(\mathbf{E}=-\nabla V\), a zero electric field means there is no change in potential within the conductor; hence the conductor is equipotential. Unlike option D, a non-zero internal electric field would keep charges moving, so equilibrium could not exist.

Correct answer: B. Strongly resisting the flow of electric charge

Explanation: Insulators have very few free charge carriers, so they have high electrical resistance and strongly oppose the flow of electric charge. Therefore, insulating materials such as plastic or rubber coverings on wires help prevent electric current from reaching the body. In contrast, conductors have many free electrons and allow charge to flow easily.

Correct answer: A. Free (conduction) electrons

Explanation: In a metal, some outer electrons are not bound to individual atoms and can move through the lattice. When an electric field is applied, these free, or conduction, electrons drift and produce electric current. The positive metal ions remain essentially fixed in the lattice, so they are not the main mobile charge carriers.

Correct answer: A. Polarisation

Explanation: In an insulator, charges are generally bound to atoms or molecules, so they cannot move freely through the material to produce a current. An external electric field causes a small relative displacement of the centres of positive and negative bound charges, producing induced dipoles. This is called polarisation. In electrical conduction, free charges flow through the material, which does not occur here.

Correct answer: A. Free charges redistribute in a conductor, whereas bound charges undergo a small relative displacement or alignment in an insulator.

Explanation: In a conductor, free electrons or other mobile charges can redistribute within the material when an external electric field is applied; this is electrostatic induction. In an insulator, charges cannot move freely through the material, but the centres of bound positive and negative charges may shift slightly relative to each other, or dipoles may align; this is polarisation. Option B reverses these behaviours.

Correct answer: A. Electrostatic equilibrium

Explanation: A metal contains free charges that move and redistribute when an electric field is present. In electrostatic equilibrium, this redistribution continues until the resultant electric field inside the metal becomes zero. Thermal expansion is related to a change in temperature, not to a zero electric field inside the metal.

Correct answer: A. Copper and rubber

Explanation: Copper is a conductor because it has free electrons that allow electric charge to flow easily. Rubber is an insulator because the motion of charge carriers in it is highly restricted. Therefore, option A gives the materials in the required order: conductor first and insulator second. Option B contains the same materials, but in the reverse order.