Overview
This lecture introduces static electricity, explores how electric charges are transferred and redistributed, and explains Coulomb’s Law for calculating electrostatic force.
Static Electricity and Charge
- Static electricity is created by an imbalance of positive or negative electric charges in an object.
- Like charges repel and opposite charges attract.
- Examples include doorknob shocks, balloon hair tricks, and lightning.
Structure of Atoms and Charge Movement
- Atoms have positive protons and negative electrons; neutral atoms have equal numbers of each.
- In solids, protons are fixed; only electrons (especially free or valence electrons) can move.
- Conductors (e.g., copper) allow electrons to move freely; insulators (e.g., wood) do not.
Methods of Charging
- Charging by Friction: Rubbing two objects transfers electrons, creating net charges on both.
- Charging by Contact: Touching a charged object to a neutral one transfers charge until both share the same sign.
- Charging by Induction: Bringing a charged object near (without touching) a neutral conductor causes redistribution of charge, potentially creating a net charge.
- Grounding: Connecting an object to the Earth allows excess charge to dissipate, neutralizing the object.
Conservation of Charge
- The law of conservation of electric charge states that total charge is constant; charges can only move, not be created or destroyed.
Quantifying Charge and Electrostatic Force
- Electric charge (q) is measured in Coulombs (C), which can be positive or negative.
- An electron’s charge is –1.6 × 10⁻¹⁹ C, called the elementary charge (e).
- Coulomb’s Law calculates the force (F) between two charges:
( F = k \frac{q_1 q_2}{r^2} )
- k (Coulomb’s constant) ≈ 9 × 10⁹ N·m²/C² in air or vacuum.
- The electrostatic force can be attractive or repulsive depending on the sign of the charges.
- Vector addition is used to find net force when multiple charges are present.
Key Terms & Definitions
- Static Electricity — Electric charge imbalance on an object.
- Conductor — Material that allows electrons to move freely.
- Insulator — Material that resists electron movement.
- Charging by Friction/Contact/Induction — Methods of transferring or redistributing electric charge.
- Grounding — Connecting to Earth to neutralize excess charge.
- Coulomb (C) — SI unit of electric charge.
- Elementary Charge (e) — Smallest unit of electric charge (charge of one electron/proton).
- Coulomb’s Law — Equation relating force between two point charges.
Action Items / Next Steps
- Review Coulomb’s Law and practice calculating forces between charges.
- Prepare for next lesson on electric fields and their visualization.