Static Electricity Basics

Sep 30, 2025

Overview

This lecture introduces static electricity, explains how electrical charges interact, and details the fundamental principles and equations governing electrostatic forces.

Static Electricity and Charge Interactions

  • Static electricity is caused by an imbalance of positive and negative electric charges on an object.
  • Objects seek to return to electrical equilibrium by redistributing excess charge.
  • Like charges repel; opposite charges attract.
  • Everyday examples include shocks from doorknobs, hair standing with balloons, and lightning.

Charging Methods

  • Friction: Rubbing two materials transfers electrons, creating opposite charges (e.g., cloth and glass rod).
  • Contact: Touching a charged object to a neutral object allows electrons to flow and equalize charge distribution.
  • Induction: Bringing a charged object near a neutral conductor redistributes charges without direct contact; grounding can then leave a net charge.

Conductors, Insulators, and Free Electrons

  • Conductors (e.g., copper) have free electrons that move easily, allowing charge flow.
  • Insulators (e.g., wood) hold electrons tightly, preventing charge movement.
  • Free electrons exist in the outer shell of atoms and can be transferred by external forces.

Conservation and Grounding

  • The law of conservation of electric charge states charge cannot be created or destroyed, only transferred.
  • Grounding connects a charged object to Earth, allowing excess charge to dissipate and neutralize the object.

Quantifying Charge and Coulomb’s Law

  • Electric charge is measured in Coulombs (C), denoted by “q”.
  • The elementary charge (e) is 1.6 × 10⁻¹⁹ C (proton is +e, electron is -e).
  • Coulomb’s Law: The force (F) between two charges is F = k * (q₁q₂) / r², where k = 9 × 10⁹ N·m²/C².
  • The force can be attractive or repulsive based on the sign of q₁q₂.
  • The magnitude and direction of the net force can be determined using vector addition.

Key Terms & Definitions

  • Static electricity — Electric charge imbalance in an object.
  • Conductor — Material that allows electrons to move freely.
  • Insulator — Material that resists electron movement.
  • Free electron — Electron in an atom’s outer shell that can move or transfer.
  • Grounding — Connecting an object to Earth to neutralize charge.
  • Coulomb’s Law — Equation calculating the force between two point charges.

Action Items / Next Steps

  • Review the processes of charging by friction, contact, and induction.
  • Prepare for the next lecture on electric fields and their visualizations.