Electric Charge and Electric Field

16-1 Static Electricity; Electric Charge and Its Conservation

Static electricity arises when objects are charged by friction, resulting in the transfer of electrons. Electric charge exists in two types: positive and negative. Like charges repel, while opposite charges attract. The total electric charge in any interaction is conserved; the arithmetic sum of all charges remains constant.

• Definition: Electric charge is a fundamental property of matter responsible for electric forces.

• Conservation of Charge: The total charge before and after any process remains unchanged.

• Example: Rubbing a plastic ruler with a cloth transfers electrons, charging the ruler.

16-2 Electric Charge in the Atom

An atom consists of a small, positively charged nucleus and a large, negatively charged electron cloud. Atoms are electrically neutral overall. Rubbing objects moves electrons, causing them to become charged. Polar molecules are neutral but have uneven charge distribution.

• Nucleus: Contains protons (positive charge) and neutrons (neutral).

• Electron Cloud: Contains electrons (negative charge).

• Polar Molecule Example: Water (H2O) has a partial positive and negative side.

16-3 Insulators and Conductors

Materials are classified based on their ability to conduct electric charge. Conductors (e.g., metals) allow charge to flow freely, while insulators (e.g., wood) do not. Semiconductors have intermediate properties.

• Conductor: Electrons move easily (e.g., copper, aluminum).

• Insulator: Electrons are tightly bound (e.g., rubber, glass).

• Semiconductor: Conductivity can be controlled (e.g., silicon).

16-4 Induced Charge; the Electroscope

Metal objects can be charged by conduction (direct contact) or induction (without contact). Nonconductors experience charge separation but do not become charged. The electroscope detects electric charge and can be charged by either method.

• Conduction: Transfer of charge by direct contact.

• Induction: Redistribution of charge without direct contact.

• Electroscope: Device to detect and determine the sign of electric charge.

• Example: Gold-leaf electroscope separates leaves when charged.

16-5 Coulomb’s Law

Coulomb’s law quantifies the electric force between two point charges. The force is proportional to the product of the charges and inversely proportional to the square of the distance between them.

• Formula: where

• Unit of Charge: Coulomb (C)

• Charge on Electron:

• Permittivity of Free Space:

• Superposition Principle: Forces from multiple charges are added as vectors.

16-6 Solving Problems Involving Coulomb’s Law and Vectors

The net force on a charge is the vector sum of all individual forces acting on it. Vector addition methods include the parallelogram and tip-to-tail methods.

• Net Force Formula:

• Vector Addition: Forces are combined using geometric methods.

16-7 The Electric Field

The electric field is defined as the force per unit charge experienced by a small test charge. For a point charge, the electric field decreases with the square of the distance.

• Electric Field Formula:

• Point Charge Field:

• Superposition Principle:

16-8 Electric Field Lines

Electric field lines visually represent the direction and strength of the electric field. They start on positive charges and end on negative charges. The density of lines indicates field strength.

• Field Line Properties:

  • Direction: Tangent to the field line.

  • Strength: Proportional to line density.

  • Origin/End: Start on positive, end on negative charges.

• Electric Dipole: Two equal and opposite charges create a characteristic field pattern.

• Parallel Plates: Field between closely spaced plates is uniform.

16-9 Electric Fields and Conductors

In electrostatic equilibrium, the electric field inside a conductor is zero. Any excess charge resides on the surface, and the field at the surface is perpendicular.

• Key Points:

  • Field inside conductor: Zero

  • Surface charge: Resides on the surface

  • Field direction: Perpendicular to surface

16-10 Electric Forces in Molecular Biology: DNA Structure and Replication

Electrostatic forces play a crucial role in molecular biology, particularly in the structure and replication of DNA. The double helix is stabilized by attractions between nucleotide bases (A-T and G-C) via electrostatic forces.

• DNA Structure: Double helix with complementary base pairing.

• Replication: Electrostatic attraction ensures correct base pairing during DNA replication.

16-11 Photocopy Machines and Computer Printers Use Electrostatics

Electrostatics are used in photocopy machines and laser printers. A charged drum attracts toner particles to form images, which are then transferred to paper and fixed by heat.

• Photocopy Process:

  • Drum charged positively

  • Image focused on drum

  • Charged areas attract toner

  • Image transferred and sealed

• Laser Printer: Computer controls laser to form image on drum.

16-12 Gauss’s Law

Gauss’s law relates the electric flux through a closed surface to the charge enclosed by that surface. It is especially useful for calculating electric fields in symmetric situations.

• Electric Flux:

• Gauss’s Law:

• Application: Used for spheres, cylinders, and planes with symmetry.

Summary Table: Key Concepts of Chapter 16

Additional info: These notes expand on the original slides by providing definitions, formulas, and context for each topic, ensuring a comprehensive and self-contained study guide for college-level physics students.