General Physics 2 (Lesson 4) - Electric Charge, Coulomb's Law, Electric Fields, and Electric Flux

Conductors and Insulators

  • Conductivity: Measure of ease at which an electric charge moves.
  • Conductors: Materials allowing easy charge flow (e.g., metals).
  • Insulators: Materials resisting charge flow (e.g., ceramics).
  • Semiconductors: Intermediate conductivity (e.g., resistor, capacitor, diode, transistor).
  • Superconductors: Offer almost zero resistance below critical temperatures.
  • Charging by Rubbing: Transfer of charge between materials. Triboelectric series helps predict charge.

Coulomb's Law

  • Defines electric force FE between two charged particles: FE = k \frac{|q1q2|}{r^2}, where:
    • k is Coulomb's constant (approximately 9 × 10^9 N⋅m^2/C^2)
    • q1 and q2 are charges in coulombs.
    • r is the distance in meters.
  • Similar in form to Newton's law of universal gravitation: FG = G \frac{m1m_2}{r^2}
  • Superposition Principle: Total force on a charge is the vector sum of individual forces from other charges.

Electric Field

  • Electric field intensity is the strength of the electric field at a point.
  • Defined as force per unit charge: E = \frac{FE}{q0}
  • Electric field due to a point charge: E = k \frac{|q|}{r^2}
  • Units: newton/coulomb (N/C)
  • Electric field is a vector quantity and follows the superposition principle.
  • Force on a charge in an electric field: F_E = E|q|. Direction is opposite for negative charges.

Electric Flux

  • Measure of the number of field lines passing through a surface.
  • Φ = E ⋅ A = EA cos θ, where θ is the angle between the electric field and the area vector.
  • Electric flux is a scalar quantity with units of N⋅m^2/C.

Gauss's Law

  • Total electric flux through a closed surface relates to the enclosed charge:
    Φ{total} = EA cos θ = \frac{q{total}}{∈0} where ∈0 is the permittivity of free space (8.8542 × 10^{-12} C^2/N⋅m^2).
  • In integral form: Φ = ∫E⋅dA

Charge Distribution

  • Gauss's law can be used to compute electric fields for symmetrical charge distributions.
  • Linear charge density: λ (charge per unit length).
  • Surface charge density: σ (charge per unit area).
  • Volume charge density: ρ (charge per unit volume).