Overview
This lecture introduces electrostatics, focusing on electric charge interactions and Coulomb’s law, which describes the force between charged objects.
Types of Electric Charge
- There are two types of charge: positive and negative.
- Like charges repel each other; opposite charges attract.
- The assignment of "positive" and "negative" is arbitrary and historical.
Charge Properties
- Charge is measured in coulombs (C), named after Charles-Augustin de Coulomb.
- One coulomb is a large quantity of charge.
- The elementary charge (charge of one electron or proton) is 1.6 × 10⁻¹⁹ C.
- Charge is conserved: it cannot be created or destroyed, only transferred.
Coulomb’s Law
- The electrostatic force between two charges decreases with the square of the distance (1/r²).
- Coulomb’s law: F = k (q₁q₂) / r², where F is force, q₁ and q₂ are charges, and r is the distance between them.
- The constant k = 8.99 × 10⁹ N·m²/C² converts units to Newtons.
- Advanced version: F = (1/4πε₀) (q₁q₂) / r², where ε₀ (epsilon naught) is the permittivity of free space, 8.85 × 10⁻¹² C²/(N·m²).
- k and ε₀ are related by k = 1/(4πε₀).
Vector Nature of Force
- Force is a vector; its direction depends on whether the charges are alike or opposite.
- The force points toward or away from the other charge based on the sign of the charges.
Key Terms & Definitions
- Charge — A property of matter that causes it to experience a force in an electric field; measured in coulombs (C).
- Coulomb (C) — The SI unit of electric charge.
- Coulomb’s Law — The law quantifying the force between two point charges.
- k (Coulomb’s constant) — 8.99 × 10⁹ N·m²/C², relates charge and force.
- Permittivity of free space (ε₀) — 8.85 × 10⁻¹² C²/(N·m²), a physical constant.
- Elementary charge — The smallest unit of charge, 1.6 × 10⁻¹⁹ C.
Action Items / Next Steps
- Review the concept of charge conservation and quantization.
- Practice applying Coulomb’s law to basic force calculations.
- Prepare for the next lesson on the direction (vector nature) of electric forces.