Lecture 24 November 17, 2004 Electrifying Stuff

Last Time We killed a cat and retained the skin. We rubbed the skin on two plastic rods. We found that the two plastic rods repelled each other. We rubbed some funny looking silk on two glass rods. We found that the two glass rods repelled each other.

From these experiments We decided something was happening to these objects when they were mechanically rubbed. We named this process “charging”. The objects were said to be “charged” We named the plastic rod’s charge negative; the glass rod’s charge positive.

ELECTROSTATIC RUBBINGS Think about this stuff dead cat

Forces Between Charges Q1Q1Q1Q1 Q2Q2Q2Q2AttractRepel ++X +-X -+X --X Q1Q1 Q2Q2 F

GIVES UP MORE ELECTRONS +-+- your hand glass your hair nylon wool fur silk paper cotton hard rubber polyester polyvinylchlor ide plastic An Aside TRIBOELECTRIC SERIES

Yesterday we also noted that The force between two charged objects increased as the objects were brought closer together. We “could have noted” … but didn’t that –The more we rubbed on the rods, the more “charged” they became and, –The greater the force.

The mathematical statement of this is known as: The force between two charged objects is directly proportional to the “sizes” of each of the charges and inversely proportional to the square of the distance between them.

Coulomb’s Law – the math The Unit of Charge is the. The Unit of Charge is the Coulomb. The constant k=9 x 10 9 N-m 2 /C 2 or k=9,000,000,000 N-m 2 /C 2 d is measured in meters.

Coulombs are Humongous! The force between two one coulomb charges separated by one meter is 9,000,000,000 Newtons (9 Billion N)

Example The force between our two rods was probably less than 1/10 N. If I did the arithmetic right.

Strange Stuff – THE 2”x4”? NEUTRAL 2 x 4

Another crazy demo Neutral Paper Bits Charged Rod If the paper bits were charged they would fly away from each other. (Like charges repel.)

Something else to see

DO BALLOON DEMO

Van der Graaff

Charging The Roller LIKE THE TAPE

SPRAY

Let’s Try it - VOLUNTEER ??? If you die I will give you an A

Let’s talk about stuff you already know Stuff is made out of molecules which are bound together by some mysterious force. –That force is primarily Coulomb’s Law. Molecules are made up of atoms. Atoms are made of electrons, neutrons and protons.

The ATOM Electron Proton Neutron

Fact Protons and neutrons are about 2000 times more massive than electrons

TWO IMPORTANT Kinds of Materials Electrical Conductors –The electrons can be easily be pushed around and will move. Electrical Insulators –The electrons are pretty much confined to the chemical bonds in the material and are very difficult to dislodge. –It is VERY hard to move these electrons

Insulators BREAK The Tape??

Close-Up View

Solids

How about a PICTURE??

EXPANDED IMAGE Silicon Dioxide Silicon

Somethings New and Somethings Old New – –The Pith Ball –Polarization Old –The Paper Bits –The 2 x 4

What’s Gonna Happen??? Neutral Light Conducting Pith Ball Negatively Charged Rod

Metals

Metals Positive charge attracts the “electronic sea” leaving a positive charge at the other side. Very mobile

SO … ATTRACTION !

What about our Volunteer??

Conductor REPULSION ! Conductor

So… Induction + Question insulator

Charging By Induction

So … now you know all about electrical Charges. Read Bolemon – Chapter 17 for the rest.