Electric Charge and Electric Fields

Introduction

This section introduces the foundational concepts of electric charge and electric fields, which are essential for understanding electrostatics and the broader field of electromagnetism. The material covers how objects become electrically charged, the conservation of charge, and the calculation of electric forces using Coulomb's law.

Relationship to Maxwell’s Equations

Overview of Maxwell's Equations

• Maxwell’s Equations are the fundamental laws governing all classical electromagnetism (E&M).

• This chapter focuses on the behavior of charges and electric fields in static (unchanging) situations, known as electrostatics.

The relevant Maxwell equation for electrostatics is:

• Gauss’s Law:

• Where is the electric field, is the charge density, and is the vacuum permittivity constant.

Other Maxwell equations (for reference):

Electric Charge

Nature of Electric Charge

• Electric charge is a fundamental property of matter that causes it to experience a force when placed in an electric or magnetic field.

• Electrostatics studies the interactions between electric charges at rest.

• Objects can become charged by rubbing (friction), which transfers electrons between materials.

Demonstrations of Charging

• When plastic rods are rubbed with fur, both rods acquire the same type of charge and repel each other.

• When glass rods are rubbed with silk, they also become charged and repel each other.

• A charged plastic rod attracts a charged glass rod; similarly, the plastic rod and fur attract each other, as do the glass rod and silk.

Conclusion: There are exactly two kinds of electric charge:

• Negative charge: The kind on the plastic rod rubbed with fur.

• Positive charge: The kind on the glass rod rubbed with silk.

Key Terms and Definitions

• Electrostatics: The study of electric charges at rest.

• Like charges repel, unlike charges attract.

• Positive charge is conventionally the charge acquired by glass when rubbed with silk.

• Negative charge is the charge acquired by plastic when rubbed with fur.

Examples and Applications

• Example: Rubbing a balloon on your hair transfers electrons, causing the balloon to stick to a wall due to electrostatic attraction.

• Application: Electrostatic painting uses charged paint droplets to ensure even coating on metal surfaces.

Additional info: The process of charging by friction is an example of triboelectric charging, where different materials have different tendencies to gain or lose electrons.