Introduction to Electric Charges and Forces (Ch. 22): Study Notes for College Physics

Introduction to Phys 122 and Course Structure

Course Overview

• This course covers fundamental concepts in physics, including electric charges and forces, and is structured with lectures, labs, and tutorials.

• Active participation, group work, and a variety of learning methods are emphasized to support student success.

Course Components

• Lectures: Flipped classroom model with pre-lecture reading and in-class problem solving.

• Labs and Tutorials: Designed to reinforce concepts through hands-on and collaborative activities.

• Exams: Assess understanding through midterms and a final, with flexible grading methods to support improvement.

Grading Table

The following table shows the correspondence between grade points and final course scores:

Grade Point	Final Course Score	Grade Point	Final Course Score	Grade Point	Final Course Score
4.0	92.0	2.9	76.0	1.8	58.4
3.9	90.4	2.8	74.4	1.7	56.8
3.8	89.2	2.7	72.8	1.6	55.2
3.7	87.6	2.6	71.2	1.5	53.6
3.6	86.0	2.5	69.6	1.4	52.0
3.5	84.0	2.4	68.0	1.3	50.4
3.4	82.0	2.3	66.4	1.2	47.2
3.3	80.0	2.2	64.8	1.1	45.6
3.2	78.4	2.1	63.2	1.0	44.0
3.1	77.6	2.0	61.6	0.9	42.4
				0.8	40.8
				0.7	39.2

Chapter 22: Electric Charges and Forces

22.1 The Charge Model – Two Types of Charge

Electric charge is a fundamental property of matter that gives rise to electric forces and interactions.

• Two Types of Charge: Like charges repel each other, while opposite charges attract.

• Long-Range Force: The force between charges acts at a distance and does not require physical contact.

• Magnitude of Force:

  • Increases as the quantity of charge increases.

  • Decreases as the distance between charges increases.

• Neutral Objects: Contain equal mixtures of both types of charge; charged objects have an excess of one type.

• Charge Transfer: Charge can be transferred between objects by contact; removing excess charge is called discharging.

Glass Rods Demonstration

• When two rods of the same type (e.g., both red or both clear) are rubbed with wool, they repel each other.

• When a red rod and a clear rod are rubbed with wool, they attract each other.

• This demonstrates that there are two types of charge, and rubbing transfers charge between objects.

22.1 The Charge Model – Two Types of Material

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

• Insulators: Materials in or on which charges are immobile (e.g., glass, rubber).

Conductor and Insulator: Electroscope Example

• An electroscope is a device used to detect electric charge.

• When a charged rod touches the metal part of an electroscope, charge can flow and cause the vane to move due to repulsion of like charges.

• If the rod is an insulator, no charge is transferred, and the vane does not move.

22.2 Charge

Atomic Structure and Charge

• An atom consists of a nucleus (protons and neutrons) and an electron cloud (electrons).

• The fundamental charge is the magnitude of the charge of a single proton or electron: $e = 1.60 \times 10^{-19} \ \text{C}$.

• An object is charged if it has an unequal number of protons and electrons.

• Ionization: Removing or adding electrons forms ions:

  • More electrons than protons: Negative ion

  • Fewer electrons than protons: Positive ion

• Molecular Ions: Created by breaking molecular bonds (e.g., by rubbing a plastic rod with wool).

• Conservation of Charge: Charge is neither created nor destroyed; total charge remains constant.

For Fun: Fractional Charged Particles

• Quarks have charges smaller than the electron's charge (e.g., up quark: $+\frac{2}{3}e$, down quark: $-\frac{1}{3}e$).

• Quarks combine in groups so that observable particles (protons, neutrons) have whole-number multiples of $e$.

For Fun: Detecting Neutrinos

• Conservation of charge applies in particle reactions, such as neutrino detection:

$\bar{\nu}_e + p \rightarrow n + e^+$

• Proton and positron are positively charged; neutron and neutrino are neutral, so total charge is conserved.

22.3 Insulators and Conductors

Properties of Insulators

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

• Charging occurs by leaving patches of molecular ions on the surface.

Properties of Metals (Conductors)

• Outer electrons are detached and free to move throughout the solid.

• Metals remain electrically neutral overall, but electrons form a negatively charged 'liquid' over a positive ion core.

• Other conductors include ionic solutions (e.g., salt water).

Touching Electroscope

• When a charged electroscope is touched, charge can flow between the electroscope and the person, decreasing the excess charge (grounding).

22.3 Polarization

• Charge polarization: Slight separation of positive and negative charges in a neutral object.

• Two opposite charges with a slight separation form an electric dipole.

Charged Rod and Electroscope

• Bringing a negatively charged rod near an electroscope causes electrons to move, creating a separation of charge (polarization) without net charge transfer.

Attraction by Polarization

• When a charged rod is brought near a neutral conductor, the end closer to the rod becomes oppositely charged due to polarization, resulting in attraction.

• The force between the charged rod and the induced opposite charge is stronger than the force with the induced like charge, because the opposite charge is closer.

Wood Rod and Charged Rod

• Even insulators (like wood) can be attracted to a charged rod due to polarization of their electron clouds.

• This explains why packing peanuts stick to objects after rubbing (static electricity).

Touching a Polarized Electroscope

• If a polarized electroscope is touched while a charged rod is nearby, charge is transferred by induction, leaving the electroscope with a net charge after the rod is removed.

22.4 Coulomb's Law

Definition and Formula

• Coulomb's Law describes the force between two point charges:

$F_{12} = F_{21} = K \frac{|q_1 q_2|}{r^2}$

• $K = 8.99 \times 10^9 \ \text{N} \cdot \text{m}^2 / \text{C}^2$ is the electrostatic constant.

• The force acts along the line joining the two charges.

• Like charges repel; opposite charges attract.

Direction of Coulomb Force

• The force on each charge is equal in magnitude and opposite in direction (Newton's Third Law).

• For multiple charges, the net force is the vector sum of the forces from each charge.

Examples and Applications

• Calculating the force between two charges separated by a distance.

• Understanding the behavior of charged objects in everyday life (e.g., static electricity, packing peanuts sticking to surfaces).

Summary Table: Types of Materials

Type	Description	Examples
Conductor	Charges move freely	Metals, salt water
Insulator	Charges are immobile	Glass, rubber, wood

Key Concepts and Formulas

• Fundamental charge: $e = 1.60 \times 10^{-19} \ \text{C}$

• Coulomb's Law: $F = K \frac{|q_1 q_2|}{r^2}$

• Polarization: Separation of charge within a neutral object, leading to attraction by a charged object.

• Conservation of Charge: Total charge is constant in any process.

Study Tips

• Practice solving problems and conceptual questions.

• Work in groups and discuss with peers to deepen understanding.

• Review the definitions, examples, and applications of each concept.

• Understand the physical meaning behind formulas, not just how to use them.