Lecture Notes by Walter Lewin

Introduction

• Lectures will complement the textbook rather than repeat it.
• Focus on concepts and the beauty of physics.
• Importance of attending every class; Course 802 is challenging.

Relevance of Electricity and Magnetism

• Electricity is ubiquitous in modern life: lights, clocks, radios, computers, etc.
• Key roles in muscle contractions, nerve functions, chemical reactions, and life itself.

Modern Atom Model

• Structure:
  • Nucleus (small relative to atom size) contains protons (positive) and neutrons (neutral).
  • Electrons are in a cloud around the nucleus.
  • Atoms are neutral when the number of protons equals electrons.
  • Charging an atom creates ions (positive or negative).
• Mass Details:
  • Proton/neutron mass: ~6.67 x 10^-27 kg.
  • Electron mass: ~1/1836 of proton mass (negligible).
• Atom size:
  • 6 billion atoms in a row = ~60 cm (size perspective).

Historical Context of Electricity

• Term "electricity" comes from the Greek word for amber (electron), known to attract dry leaves since 600 BC.
• In the 16th century, more substances (glass, sulfur) exhibited similar properties.
• Benjamin Franklin's Experiments:
  • Identified two types of electricity (A and B) through rubbing materials.
  • Introduced the concept of electric fluid (positive and negative charge convention).

Principles of Charge

• Charges interact:
  • Like charges repel; opposite charges attract.
  • Conservation of charge: creating one charge type results in the other.
• Conductors vs. Insulators:
  • Conductors (metals) have mobile electrons; insulators do not.
  • Induction occurs when a charged object is brought near a conductor, leading to charge polarization.

Demonstrations of Charge

• Induction Demonstrations:
  • Example: Rubbing a glass rod and bringing it close to a conductor (like a balloon).
  • Demonstrating how balloons respond to different charges (positive and negative).

Understanding Electrons and Polarization

• Nonconducting objects can still exhibit polarization:
  • Electron cloud shifts toward a charged object, creating localized charge distribution.

Practical Applications

• Friction causes charging:
  • Everyday examples: balloons sticking to surfaces after rubbing, hair standing on end after taking off a nylon shirt.

Electroscope as Measurement Tool

• Simple device to measure charge:
  • Conducting rod with tinsel that spreads apart with charge.

Coulomb's Law

• Relationship for electrical forces:
  • Force proportional to the product of two charges divided by the square of the distance between them.
  • Expressed as: F = k * (Q1 * Q2) / r^2
  • K = Coulomb's constant (~9 x 10^9 N m²/C²).

Comparison with Gravity

• Electric forces vs gravitational forces:
  • Electric forces are much stronger (by ~10^36) than gravitational forces between protons.
  • On the scale of atomic nuclei, electric forces are critical; on larger scales, gravity dominates.

Conclusion

• Importance of the experiments and concepts covered.
• Encouragement to explore electricity through practical applications (e.g., weekend experiments).

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Upcoming lecture will delve deeper into concepts and applications of charge.
Remember to bring materials for hands-on experiments next class!

End of notes.