Unit 15: Electrostatics — Multiple Choice Questions

Electrical breakdown occurs when the electric field is so strong that it forces electrons away from gas atoms, creating ions. This ionization allows the gas to conduct electricity. The other options describe different electrical processes but not this specific phenomenon.

Corona discharge is the visible bluish glow around a conductor at very high potential, caused by ionization of the surrounding air. This is the most common visible example of electrical breakdown in everyday life.

Coulomb's law states that opposite charges attract each other. Unlike charges have opposite signs, so they experience an attractive force.

Coulomb's law is F = k q₁ q₂ / r². As distance r increases, the denominator increases, so the force F decreases. The attraction weakens with greater separation.

A photocopier uses electrostatic attraction between charged toner powder and an electrostatically charged drum to transfer images. An electric bell uses electromagnets, a heater uses thermal energy, and a transformer uses electromagnetic induction.

The Coulomb (C) is the SI unit of electric charge. Ampere is the unit of current, Volt is the unit of potential, and Ohm is the unit of resistance.

When a plastic comb is rubbed through dry hair, it pulls electrons away from the hair. Since plastic gains extra electrons, it becomes negatively charged.

Charge can be transferred by rubbing, direct contact, and induction. Radiation (electromagnetic waves) does not transfer charge between objects.

This is the law of conservation of charge. In a closed, isolated system, the total amount of electric charge cannot be created, destroyed, or lost; it remains constant.

When a charged rod approaches a neutral conductor without touching, the charges in the conductor rearrange due to the electric field of the rod. This process is called induction. Conduction would require direct contact.

During induction, charges opposite to the inducing charge gather on the near side of the conductor. When earthed, like charges escape to ground, leaving the conductor with the opposite charge to the inducing object.

Earthing (grounding) connects the conductor to the earth, allowing excess charge to flow away and unwanted charge to be removed. This leaves the conductor with only the desired induced charge.

When a charged object is brought near the electroscope, it induces charge on the leaves, causing them to have like charges that repel each other and diverge. A neutral object produces no effect.

An electric field is a region where an electric charge experiences an electric force due to another charge. It is the mechanism by which charges exert forces on each other.

By convention, the electric field direction is defined by the force that would act on a positive test charge. A negative charge would experience a force in the opposite direction.

Inside a conductor in electrostatic equilibrium, charges have redistributed to cancel any internal electric field. The field inside is always zero because charges stop moving when E = 0.

Between parallel oppositely charged plates, the electric field is uniform (constant magnitude and direction) and parallel everywhere inside. This creates a region of unidirectional field lines.

Electric field E is defined as force per unit charge: E = F / q. So its SI unit is Newton per Coulomb (N/C). Newton measures force and Coulomb measures charge.

Copper is a metal and contains free electrons that move easily. Rubber, plastic, and wood are insulators with very few free charge carriers and do not conduct electricity.

The free electron model describes electrons that are responsible for electrical conduction in conductors. By definition, these electrons are called 'free electrons' (c) because they are loosely bound and can move freely through the material to carry electric current.

Insulators are materials where electrons remain tightly bound to atoms and cannot move freely. This is why insulators do not conduct electricity — the electrons cannot move through the material even when a potential difference is applied (b).

When the bulb lights up, it means current flows through the circuit. For current to flow, the material must allow electrons to move freely — making it a conductor (a). An insulator or semiconductor would block the current and the bulb would not light.

A photocopier works by charging the drum positively and coating it with negatively charged toner powder. The toner sticks to the charged areas of the drum, then transfers to paper (b). This electrostatic principle is the core of photocopier technology.

An electrostatic precipitator removes dust and particles from industrial gases by first charging them, then attracting them to oppositely charged plates where they stick (b). The particles collect on the plates and fall to the bottom, leaving the gas clean.

The visible glow around high-voltage power lines occurs when the electric field is strong enough to ionize the air molecules. This phenomenon is called corona discharge (b) because it creates a corona or halo of light around the conductors.

A lightning rod protects a building by providing a low-resistance path (b) for lightning discharge to reach the ground safely. If lightning strikes the building, the current follows the rod instead of passing through the structure, which would cause damage.

When static electricity accumulates on flammable dust or gas vapors, even a small spark can ignite them, causing dust explosions (b). This is a major industrial hazard in grain silos, coal mines, and chemical plants.

Coulomb's law states that the electrostatic force between two point charges is F = k(q₁q₂)/r². The force is directly proportional to the product of the charges (b), not their sum. Doubling one charge doubles the force; doubling both charges increases the force four times.

The constant k in Coulomb's law equals 1/(4πε₀ε_r), where ε_r is the relative permittivity of the medium between the charges (b). Different materials have different permittivity values, so k depends on what medium separates the charges. It does not depend on the charge amounts, shape, or temperature.

From Coulomb's law, force is inversely proportional to the square of distance: F ∝ 1/r². If the distance is doubled, r becomes 2r, so F becomes 1/(2r)² = 1/4 of the original force (b).