Electricity - Chapters 15-17 Summary

Electric Charge

• Matter consists of electric charges that exert forces on each other.

• Two types:

  • Positive (+): Protons, positive ions

  • Negative (-): Electrons, negative ions

• Like charges repel, opposite charges attract.

• Charge is represented by "q" in Coulombs (C).

• Smallest charge (fundamental charge): q = 1.609 × 10^{-19} C

• Atoms:

  • Contain a positive nucleus and electron cloud.

  • Electrically neutral due to balanced charges.

  • Nucleus contains neutrons (no charge).

• Particle properties:

  • Electron: q = -1.609 × 10^{-19} C, m_p = 9.11 × 10^{-31} kg

  • Proton: q = 1.609 × 10^{-19} C, m_p = 1.673 × 10^{-27} kg

  • Neutron: q = 0 C, m_n = 1.675 × 10^{-27} kg

Conductors and Insulators

• Conductors: Allow free flow of charge (e.g., metals).

• Insulators: Block the flow of charge (e.g., rubber).

• Both can accumulate charge; static charge accumulates on the exterior of conductors.

Triboelectric Charging

• Materials charge positively or negatively when rubbed together, based on their position in the triboelectric series.

Electric Force

• Magnitude is directly related to the product of two charges: F \propto q1 · q2

• Inversely proportional to the square of the distance between charges.

• Coulomb's Law: Fq = k \frac{|q1||q_2|}{r^2}, where k = 8.99 × 10^9 \frac{N m^2}{C^2}

• Similar to Newton's Law of Gravitation but electric force can attract or repel.

Solving Electric Force Problems

• Determine force components for each charge pair using Coulomb's Law and rules of attraction/repulsion; vector sum forces.

Electric Fields

• Electric field explains how electric force acts over a distance, similar to gravitational fields.

• Direction: Away from positive, toward negative charges.

• Electric Field Strength: E = \frac{F}{q_0}

Electric Fields Rules

• Field lines point in the direction of the electric field (from + to -).

• Lines start at positive charges or infinity.

• Lines end at negative charges or infinity.

• Line density indicates field magnitude.

• Number of lines proportional to charge magnitude.

Uniform Electric Field

• Field is uniform in direction and magnitude, created by parallel plates with opposite charges (parallel-plate capacitor).

Conductors and Electric Fields

• Excess charge on a conductor moves to its outer surface; electric field inside is zero (shielding).

• Electric field lines contact conductors at right angles.

Charging by Induction

• Charging without direct contact: bring a charged rod near a conductor, ground the conductor, remove ground, then remove rod.

Electric Potential Energy and Electric Potential

• Analogy between gravitational and electrical potential energy.

• Electric Potential Energy: PE_q = qEd (J)

• Electric Potential: V = Ed (Volts)

• Work: W = Fd=qEd and W = -q\Delta V

  • Electric Field E = - \frac{V_{ba}}{d}