Lecture 24Lecture 24

November 17, 2004November 17, 2004

Electrifying StuffElectrifying Stuff

Last TimeLast Time

We killed a cat and retained the skin.We killed a cat and retained the skin.We rubbed the skin on two plastic rods.We rubbed the skin on two plastic rods.We found that the two plastic rods repelled We found that the two plastic rods repelled each other.each other.

We rubbed some funny looking silk on two We rubbed some funny looking silk on two glass rods.glass rods.We found that the two glass rods repelled We found that the two glass rods repelled each other.each other.

From these experiments From these experiments

We decided something was happening to We decided something was happening to these objects when they were these objects when they were mechanically rubbed.mechanically rubbed.

We named this process “charging”.We named this process “charging”.

The objects were said to be “charged”The objects were said to be “charged”

We named the plastic rod’s charge We named the plastic rod’s charge negativenegative; the glass rod’s charge ; the glass rod’s charge positivepositive..

ELECTROSTATIC RUBBINGSELECTROSTATIC RUBBINGSThink about this stuffThink about this stuff

- - - + - + - -

dead cat

Forces Between ChargesForces Between Charges

QQ11 QQ22 AttractAttract RepelRepel

++ ++ XX

++ -- XX

-- ++ XX

-- -- XX

Q1 Q2

F F

GI V

ES

UP

MO

RE

EL

EC

TR

ON

S +

-

your handglass

your hairnylonwoolfursilk

papercotton

hard rubberpolyester

polyvinylchloride plastic

An AsideAn AsideTRIBOELECTRIC SERIESTRIBOELECTRIC SERIES

Yesterday we also noted thatYesterday we also noted that

The force between two charged objects The force between two charged objects increased as the objects were brought increased as the objects were brought closer together.closer together.

We “could have noted” … We “could have noted” … but didn’tbut didn’t that that– The more we rubbed on the rods, the more The more we rubbed on the rods, the more

“charged” they became and,“charged” they became and,– The greater the force.The greater the force.

The mathematical statement of this The mathematical statement of this is known as:is known as:

The force between two charged objects isdirectly proportional to the “sizes” of each of

the charges and inversely proportional to the square of the distance between them.

Coulomb’s Law – the mathCoulomb’s Law – the math

The Unit of Charge is the The Unit of Charge is the Coulomb..

The constant k=9 x 10The constant k=9 x 1099 N-m N-m22/C/C22

or k=9,000,000,000 N-mor k=9,000,000,000 N-m22/C/C22

d is measured in meters.d is measured in meters.

221

d

QQkF

Coulombs are Coulombs are

Humongous!Humongous!

221

d

QQkF

The force between two one coulomb chargesseparated by one meter is

9,000,000,000 Newtons (9 Billion N)

ExampleExample

The force between our two rods was probably less than 1/10 N.

C 3Q

mbsmicrocoulo 3000,000,1

3

000,000,000,100

1

000,000,000,10

1.0.1Q

same) charges (Assume

2

212

21

CQ

k

kQd

QQkF

If I did the arithmetic right.

Strange Stuff – THE 2”x4”?Strange Stuff – THE 2”x4”?

--------

NEUTRAL2 x 4

Another crazy demoAnother crazy demo

NeutralPaper Bits

Charged Rod

If the paper bits were charged they would fly away from each other. (Like charges repel.)

Something else to seeSomething else to see

DO BALLOON DEMODO BALLOON DEMO

Van der GraaffVan der Graaff

Charging The RollerCharging The Roller

LIKE THETAPE

LIKE THELIKE THETAPETAPE

SPRAYSPRAY

Let’s Try it - VOLUNTEER???

If you die I will give you an If you die I will give you an AA

Let’s talk about stuff you already Let’s talk about stuff you already knowknow

Stuff is made out of molecules which are Stuff is made out of molecules which are bound together by some mysterious force.bound together by some mysterious force.– That force is primarily Coulomb’s Law.That force is primarily Coulomb’s Law.

Molecules are made up of atoms.Molecules are made up of atoms.

Atoms are made of electrons, neutrons Atoms are made of electrons, neutrons and protons.and protons.

The ATOMThe ATOM

Electron

Proton

Neutron

FactFactProtons and neutrons are Protons and neutrons are about 2000 times more about 2000 times more massive than electronsmassive than electrons

TWO IMPORTANTTWO IMPORTANTKinds of MaterialsKinds of Materials

Electrical ConductorsElectrical Conductors– The electrons can be easily be pushed The electrons can be easily be pushed

around and will move.around and will move.

Electrical InsulatorsElectrical Insulators– The electrons are pretty much confined to the The electrons are pretty much confined to the

chemical bonds in the material and are very chemical bonds in the material and are very difficult to dislodge.difficult to dislodge.

– It is VERY hard to move these electronsIt is VERY hard to move these electrons

InsulatorsInsulators

BREAKThe Tape??

Close-Up ViewClose-Up View

SolidsSolids

How about a PICTURE??How about a PICTURE??

EXPANDED IMAGEEXPANDED IMAGE

SiliconDioxide

Silicon

Somethings NewSomethings Newandand

Somethings OldSomethings Old

New –New –– The Pith BallThe Pith Ball– PolarizationPolarization

OldOld– The Paper BitsThe Paper Bits– The 2 x 4The 2 x 4

What’s Gonna Happen???What’s Gonna Happen???

-------

Neutral Light

ConductingPith Ball

Negatively Charged Rod

MetalsMetals

MetalsMetals

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

Very mobileVery mobile

---

---

---

--

++

--

SO …..SO …..

--- ++

--

--- ++

--

ATTRACTION !

What about our Volunteer??What about our Volunteer??

Conductor

REPULSION !REPULSION !

Conductor

So… Induction + QuestionSo… Induction + Question

insulator insulator

Charging By InductionCharging By Induction

So … now you know all about So … now you know all about electrical Charges.electrical Charges.

Read Bolemon – Chapter 17 Read Bolemon – Chapter 17 for the rest.for the rest.