Introduction to Electricity and Magnetism

Introduction

• Lecturer: Walter Lewin
• Lectures will complement the book and vice versa.
• Focus will be on concepts, not tedious derivations (those are in the book).
• Aim: To show the beauty of physics and foster appreciation.

Importance of Electricity

• **Everyday Applications: **
  • Electric lights, clocks, microphones, calculators, TVs, VCRs, radios, computers.
  • Light and radio waves are electromagnetic phenomena.
  • Colors of the rainbow and blue sky are due to electrical phenomena.
  • Cars, planes, and trains operate due to electricity.
  • Muscles and nerves operate electrically.
  • Atoms, molecules, and chemical reactions depend on electricity.
  • Vision and heartbeat depend on electrical mechanisms.

Atomic Structure

• Components:
  • Nucleus: Contains positively charged protons and neutral neutrons.
  • Electrons: Orbit in a cloud around nucleus; negatively charged.
• Charge Details:
  • Neutral atom: Equal number of protons and electrons.
  • Ion formation: Gaining or losing electrons.
• **Mass and Size: **
  • Proton/Neutron: ~1.67x10^-27 kg.
  • Electron: ~1/1830 mass of proton (negligible for most purposes).
  • Atom diameter: ~10^-8 cm; Nucleus diameter: ~10^-12 cm.
  • 6 billion atoms lined up: ~60 cm (shows small size).

Historical Context

• Origins of electrical knowledge:
  • 600 BC: Rubbing amber attracts dry leaves; 'electron' is Greek for amber.
  • 16th century: More substances showing similar behavior found (glass, sulfur).
  • 18th century: Franklin's experiments defined two types of electricity.
• Types of Electrical Charges:
  • Rubbing glass vs rubber creates different charges (A and B types).
  • Franklin's concept of 'electric fluid'; positive/negative definitions.
  • Conservation of charge: Cannot create charge without counter-charge.

Conductors and Induction

• **Conductors vs Non-conductors: **
  • Conductors (metals) have free electrons that can move.
  • Non-conductors have electrons bound to atoms.
• Induction:

  • Attraction of electrons in conductors by a charged rod results in polarization.

  • Illustration: Helium-filled balloon attracted/repelled by charged rod.

  • Non-conductors also show induction at atomic level (polarization of atoms).

Demonstrations

• **Rubbing a Glass Rod: **
  • Charge induction: Balloon approaches glass rod.
  • Rubber rod demonstration with cat fur shows different charge attraction.
• **Everyday Observations: **
  • Friction causes electric charge (comb, removing nylon shirt in darkness).
  • Inducing charge results in static electrical phenomena (e.g., saran wrap).
• Volunteer Experiment:
  • Charging nylon parka using cat fur.
  • Neon flash tube lights up upon contact.

Quantitative Exploration

• Coulomb's Law:
  • Force (F) between two charges (Q1 and Q2) is proportional to the product of charges divided by the square of the distance (R) between them.
  • Formula: F = k * (Q1 * Q2) / R^2
  • K in SI units: 9 x 10^9 N m^2 / C^2
• Superposition Principle:
  • Total force on a charge is the vector sum of forces from other charges.

Gravity vs Electricity

• Comparisons:
  • Electric forces are vastly more powerful than gravitational forces.
  • Example: Electric force between protons is 10^36 times stronger than gravitational force.
• Relevance:
  • On atomic level, electric forces dominate.
  • On large scales (planets, stars), gravity dominates due to small charge per mass ratio.

Measuring Charge

• **Electroscope: **
  • A simple instrument with a conducting rod and tinsel/mechanical parts to detect charge.
  • Demonstration: Charged bodies repel equally charged leaves or tinsel.

Conclusion and Fun Experiment

• Vandegraaff Machine:
  • Demonstrates large voltage effects.
  • Used to illustrate electric forces with confetti and tinsel on subject.
• Closing Encouragement:
  • Experiment with friction-induced charge at home (nylon shirt in darkness).
  • Emphasis on the engaging and observable nature of electrical phenomena.