Chapter 22: Electric Charges and Forces – Fundamental Concepts and Experimental Foundations

Electric Charge: Fundamental Concepts

What is Electric Charge?

Electric charge is a fundamental property of matter that gives rise to electric phenomena. The behavior of electric charges underlies many physical processes and is essential to understanding electricity and magnetism.

• Types of Charge: There are two kinds of electric charge: positive and negative.

• Constituents of Atoms: Electrons (negative charge) and protons (positive charge) are the basic charged particles in ordinary matter. Neutrons are neutral.

• Charging: The process of transferring electrons from one object to another is called charging.

Example: Rubbing a plastic rod with wool transfers electrons, charging the rod.

Behavior of Electric Charges

How Do Charges Interact?

• Like charges repel each other; opposite charges attract.

• Attraction of Neutral Objects: Small neutral objects are attracted to a charged object of either sign due to induced polarization.

• Charge Transfer: Charge can be transferred between objects, often by contact or friction.

• Conservation of Charge: The total electric charge in an isolated system is conserved.

Example: A charged rod attracts small pieces of paper, regardless of the sign of the charge.

Conductors and Insulators

Classification of Materials

• Conductors: Materials through or along which charge moves easily (e.g., metals).

• Insulators: Materials on or in which charge is immobile (e.g., glass, plastic).

Example: Metal spheres can be charged by contact and the charge spreads; plastic rods retain charge in localized regions.

Coulomb's Law

Fundamental Law of Electric Force

Coulomb's law describes the force between two point charges. It is an inverse-square law, similar to Newton's law of gravity.

• The electric force is inversely proportional to the square of the distance between charges.

• The force is attractive for opposite charges and repulsive for like charges.

Equation:

$F = k \frac{|q_1 q_2|}{r^2}$ where $k = 8.99 \times 10^9\ \mathrm{N\,m^2/C^2}$

SI Unit of Charge: The coulomb (C) is the SI unit of electric charge.

Experimental Foundations of Charge

Key Experiments

• Neutrality: Uncharged (neutral) objects do not exert forces on each other.

• Charging by Friction: Rubbing two objects (e.g., plastic rod with wool) can transfer charge, resulting in repulsion or attraction depending on the materials.

• Attraction of Neutral Objects: Charged objects attract neutral objects due to induced polarization.

• Charge Transfer by Contact: Charge can be transferred from a charged object to a conductor by touching.

• Discharging: Touching a charged object to a large conductor (e.g., the human body) can remove excess charge.

Example: A charged plastic rod can transfer charge to a metal sphere by contact, making the sphere capable of attracting small pieces of paper.

Charge Model: Postulates and Properties

Charge Model, Part I

• Frictional forces (e.g., rubbing) add or remove charge from objects.

• There are only two types of charge: "plastic charge" (now called negative) and "glass charge" (now called positive).

• Like charges repel; opposite charges attract.

• The force between charges is long-range and decreases with distance.

• Neutral objects have an equal mixture of both types of charge.

Charge Model, Part II

• Conductors allow charge to move easily; insulators do not.

• Charge can be transferred by contact.

Atomic Structure and Charge Quantization

Atoms and Electricity

• An atom consists of a dense nucleus (protons and neutrons) surrounded by electrons.

• Electrons and protons have equal but opposite charges.

• The fundamental unit of charge is $e$.

Particle	Mass (kg)	Charge
Proton	1.67 × 10-27	+e
Electron	9.11 × 10-31	-e

Charge Quantization

• The net charge $q$ of an object is given by $q = e(N_p - N_e)$, where $N_p$ and $N_e$ are the numbers of protons and electrons, respectively.

• Most objects are neutral ($q = 0$), but a charged object has an unequal number of protons and electrons.

• Charge is always an integer multiple of $e$ (quantization of charge).

Ionization

• Removing an electron from an atom creates a positive ion.

• Adding an electron creates a negative ion.

Properties of Insulators and Conductors

Insulators

• Electrons are tightly bound to nuclei and cannot move freely.

• Charging by friction leaves immobile patches of charge on the surface.

Conductors

• Outer electrons are weakly bound and can move freely throughout the material.

• The material remains electrically neutral overall, but electrons form a "sea of electrons" around positive ion cores.

Summary Table: Conductors vs. Insulators

Property	Conductors	Insulators
Charge Mobility	High (free electrons)	Low (bound electrons)
Example Materials	Metals	Glass, plastic
Charging by Contact	Charge spreads	Charge remains localized

Additional info: These notes summarize the foundational concepts of electric charge, experimental observations, and the atomic basis for charge, as well as the distinction between conductors and insulators, as covered in the first part of a college-level physics chapter on electricity.