OCW/Electricity and Magnetism/Lecture 1

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Contents

Introduction

  • Presence of electricity in almost all places.

Atom structure

  • nucleus: protons (positively charged), and neutrons (no charge).
  • electronic cloud.

Historical evidences of electric nature

600 BC 
Rubbing amber (named elektron in greek), it can attract dry leaves.
18th century 
2 types of electricity (glass, rubber/amber). One repels the same, and the same attract.
Franklin 
All substances have some special "electric fluid", "electrical fire". Too much "fire", positively charged, and viceversa. Rubbing with glass = positive. Also noted that a body could be more charged, and that the closer the object, the stronger the forces. Also noted that some substances conduct the "fluid": conductors.

Demo: induction with a conducting balloon

Rub a rod with silk, becomes positively charged. Electrons can move in the conductor. If the rod is close to the conductor, some positive charges will go away, but the negative ones will come closer to the rod.

Demo: induction with a non conducting balloon

Electrons in non conductors, can't scape from its atom. But electrons are moving inside its atom, and if you bring a charged body close to it, electrons will move differently, and the shape of the electronic cloud will change: electrons will try to come close to a positively charged body, or far away from a negatively charged one.

That means that even in non conductors you can have induction. In the demo, the non conducting glass comes close to the positively charged body, but goes away from the negatively charged one. That's because the balloon was (unintentionally) negatively charged. If it weren't charged at all, it should come close to both bodies. However, it's hard to get the charge out of the balloon, because it's a non conductor.

Demo: beat with cat fur

Walter Lewin "beats" a student with cat fur, to show how the student becomes charged with one type of charge, and Walter with another type of charge.

Van der Graaf generator

The Van der Graaf, is a tool with a mechanism that allows it to become charged easily with a great amount of charge. This demonstration shows how a bunch of confetti spreads when put on top of the Van der Graaf.

Coulomb's Law

\vec F_{1,2} = K \frac{q_1 q_2}{r^2} \hat r_{1,2}

Electroscope

Made with of conduction material: a metallic rod ended in two pieces of aluminum foil. Gives an idea of whether a body is charged or not.

Living electroscope

If you have long hair, each of your hair can act as the pieces of an electroscope. Walter Lewin shows it for a great final demonstration.

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