Sources of Electric Charge

Atomic Structure and Charge

All matter is composed of atoms, which consist of a small, massive nucleus surrounded by an electron cloud. The nucleus contains protons and neutrons, while electrons orbit the nucleus.

• Proton: Positively charged particle, charge =

• Electron: Negatively charged particle, charge =

• Neutron: Electrically neutral particle, charge = $0$

The SI unit of electric charge is the Coulomb (C).

• Magnitude of the charge on an electron:

Example: Atomic Model

In a simple atomic model, the nucleus (containing protons and neutrons) is at the center, with electrons distributed in a cloud around it.

Coulomb's Law

Fundamental Law of Electric Force

Coulomb's Law describes the electric force between two point charges. The magnitude of the force is given by:

• where is Coulomb's constant, and are the charges, and is the distance between them.

Key Points:

• Like charges ( or ) repel each other.

• Opposite charges () attract each other.

Example: Charged Spheres

When two spheres are given positive charges, the electric force each exerts on the other is repulsive and directed away from the other sphere. If one sphere has three times the charge of the other, the force magnitudes are equal and opposite, but the direction is always away from each other.

Example: Electric Force and Acceleration

Consider two ping-pong balls separated by , with charges and , and each with mass .

• Electric force:

• Substitute values:

• Acceleration:

Example: Protons in a Helium Nucleus

Two protons separated by inside a helium nucleus:

• Electric force:

• Substitute values: (repulsive, away from each other)

• Acceleration:

Application: Everyday Electrostatics

Static electricity can build up on a child at the bottom of a plastic playground slide due to the transfer of electrons during sliding. This is a practical demonstration of charge transfer and Coulomb's law in action.

Summary Table: Properties of Subatomic Particles

Summary Table: Coulomb's Law Parameters

Key Points to Remember

• Opposite charges attract; like charges repel.

• Coulomb's law quantifies the force between two point charges.

• Charge transfer (usually electrons) is responsible for static electricity in everyday life.

• Forces and resulting accelerations can be calculated using and Coulomb's law.

Additional info: The notes above expand on the brief points in the slides, providing definitions, formulas, and context for each concept. Tables have been added for clarity and comparison.