Lecture Notes: Introduction to Electricity and Magnetism by Walter Lewin

Overview of Lectures

• Lectures will complement the textbook.
• Focus on concepts rather than tedious derivations.
• Aim to demonstrate the beauty of physics.

Importance of Electricity and Magnetism

• Present in everyday life: lights, clocks, microphones, TVs, computers, etc.
• Light is an electromagnetic phenomenon.
• Electricity is essential for movement (cars, planes, etc.) and bodily functions (nerve system, muscle contractions, etc.).
• Chemical reactions and vision depend on electricity.

The Atom

• Modern model: nucleus (small compared to atom size) with protons (positive) and neutrons (neutral).
• Electrons form a cloud around the nucleus.
• Neutral atoms have equal numbers of electrons and protons.
  • Removing an electron creates a positive ion.
  • Adding an electron creates a negative ion.
• Mass of an electron is much smaller than that of protons and neutrons.

Size of Atoms

• Six billion atoms lined up = 60 cm (comparable to world population).
• Size of nucleus: ~10^-12 cm; size of atom: ~10^-8 cm.

Historical Context of Electricity

• 600 B.C.: Rubbing amber attracts leaves; "electron" (Greek for amber) gives electricity its name.
• 18th century: Two types of electricity discovered (from glass and rubber/amber).
  • Charged objects interact: like charges repel, opposite charges attract.
• Benjamin Franklin's contributions:
  • Introduced the concept of "electric fluid" (positive and negative charges).
  • Established sign convention (glass = positive, rubber = negative).

Charge Conservation

• Conservation of charge: creating positive charge also creates negative charge.
• Conductors allow free movement of some electrons, while insulators do not.
• Induction: movement of electrons in response to nearby charges.

Demonstration of Charge and Induction

• Example using a positively charged glass rod and a conductor:
  • Electrons in the conductor move towards the rod, creating polarization.
  • Result: attraction between the rod and the conductor (balloon).
• Demonstration with different materials to show positive and negative charges.

Induction in Non-Conductors

• Non-conductors also show induction:
  • Electrons shift towards the charged object, creating a polarized area.
  • Non-conductors can still attract charged objects due to this polarization.

Examples of Electric Charge Through Friction

• Rubbing balloons on clothing charges them (positive or negative).
• Sparks can be generated through friction (e.g., combing hair).
• Common experiences: shocks from doorknobs, static cling from plastic wrap.

Quantitative Analysis of Charges

• Coulomb’s law describes the force between charges:
  • F = k * (|Q1 * Q2| / r^2)
  • k (Coulomb's constant) = 9 * 10^9 N m²/C².
• Comparison of electric force vs gravitational force:
  • Electric forces are much stronger (by a factor of 10^36) than gravitational forces.

Electroscopes and Charge Measurement

• Electroscope: measures charge quantitatively using conductive materials.
• Charge causes a visible reaction (e.g., aluminum foil leaves repelling each other).

Final Demonstration: Van de Graaff Generator

• Van de Graaff generator creates high voltages, demonstrating charge interaction.
• Use of confetti to show repulsion of charges.

Closing Remarks

• Students encouraged to experiment with static electricity safely at home.
• Reminder: Electric forces dominate on small scales; gravity dominates on larger scales.