Overview
This lecture introduces the fundamentals of static electricity, electric charge, methods of charging, and Coulomb’s Law, focusing on how electrostatic forces work between particles.
Static Electricity and Electric Charge
- Static electricity results from an object gaining a net positive or negative charge, creating an imbalance.
- Like charges repel and opposite charges attract due to electrostatic forces.
- Atoms contain equal numbers of positive protons and negative electrons, making them neutral unless charges are unbalanced.
- Free electrons (valence electrons) move more easily in conductors (e.g., copper) than in insulators (e.g., wood).
Methods of Charging
- Charging by Friction: Rubbing two materials transfers electrons, leaving one with a positive charge and the other with a negative charge.
- Law of Conservation of Charge: Total charge is conserved; charges only move between objects, not created or destroyed.
- Charging by Contact: Touching a charged object to a neutral one transfers electrons until charges are balanced.
- Charging by Induction: Bringing a charged object near a neutral conductor rearranges charges within, leading to polarization and net charge after grounding.
- Grounding: Connecting an object to Earth allows excess charge to flow away, neutralizing the object.
Quantifying Electric Charge and Force
- Charge is measured in Coulombs (C), symbolized by “q”, and can be positive or negative.
- The elementary charge (e) is ±1.6 × 10⁻¹⁹ C for protons and electrons.
- The force (F) between two charges is given by Coulomb’s Law:
( F = k \frac{|q_1 q_2|}{r^2} )
- The Coulomb’s Law constant (( k )) is 9 × 10⁹ N·m²/C² in air or vacuum.
- Electrostatic force can be attractive or repulsive, depending on the signs of the charges.
- Vector addition allows calculation of net forces from multiple charges.
Key Terms & Definitions
- Static Electricity — Charge imbalance resulting in observable forces.
- Conductor — Material allowing free movement of electrons.
- Insulator — Material restricting electron movement.
- Charging by Friction — Electron transfer via rubbing materials.
- Charging by Contact — Electron transfer through physical touch between objects.
- Charging by Induction — Redistribution of charges without direct contact.
- Grounding — Neutralizing charge by connecting to Earth.
- Coulomb (C) — SI unit of electric charge.
- Elementary Charge (e) — Basic unit of charge carried by a proton (+e) or electron (−e).
- Coulomb’s Law — Equation describing force between two charges.
Action Items / Next Steps
- Review examples involving Coulomb’s Law calculations.
- Prepare to study electric fields and their visualization in the next lecture.