Chapter 32 Electrostatics

Introduction Static  Lightening build-up  Sparks on carpet  Hold atoms together to form molecules Flow  Controls all sorts of devices Lamps to computers

Knowledge of Electricity Brings prosperity Not controlled until the late 1800’s Changed our world

Electrostatic Electricity at rest Involves  Electric charge  Forces between the charges  Behavior in materials Future chapters  Electric fields  Electric currents  Magnetism

Electric Forces and Charges Gravity – familiar force that attracts you to the Earth Imagine a force:  Billions of times stronger  Capable of compressing you into a thin piece of paper  With a repelling force that counteracts  No noticeable effect on you  THIS FORCE IS ELECTRICAL FORCE

Where does this force come from? Arise from particles in atoms  Rutherford and Bohr Positive nucleus and negative cloud Protons “hold” electrons in orbit Electrons are attracted to protons Protons repel protons, electrons repel electrons Attraction and repulsion = CHARGE

Facts on atoms All e- are identical – same charge and mass Nucleus – protons + neutrons (except hydrogen – only p+)  All protons are identical  All neutrons are identical  A proton is 4000X heavier than an e- Atoms usually have equal numbers of p+ and e-

Why do charges attract or repel? It is a basic fundamental behavior Beyond the scope of this class Fundamental rule –  Like charges repel, opposite charges attract

Facts on atoms Every atom has a positive nucleus and a negative electron cloud All electrons (e-) are identical – same charge and mass Nucleus = protons (p+) and neutrons (nº) (except Hydrogen – only proton)  All protons are identical  All neutrons are identical Atoms usually have equal number of p+ and e-

Why do electrons repel and attract? It is a basic fundamental behavior Beyond the scope of this class Phenomena Rule:  Like charges repel, opposite charges attract

History of “Opposites Attract” Old days  Public lecturers would travel around on horseback  Used PITH balls (from a spongy plant) covered with Aluminum paint Used two types of wands  Rubber rubbed with cat hair  Glass rubbed with silk

Pith Balls Balls would become attracted or repelled by the wands or each other ging_pith.gif&imgrefurl= s/content/e_and_m/pith_balls/pith_balls.html&h=20 7&w=424&sz=10&hl=en&start=4&tbnid=w7- qwB3A1SGinM:&tbnh=62&tbnw=126&prev=/image s%3Fq%3Dpith%2Bballs%26gbv%3D2%26svnum %3D10%26hl%3Den%26sa%3DG

Conservation of Charge Neutral atom has equal numbers of p+ and e-  Atom is Neutral When an electron is removed, atoms becomes positive

Ions Cation – positive ion Anion – negative ion Imbalance caused by adding or removing e-

Electrons Inner electrons are bound tightly to nucleus Outermost electrons  very loosely held  Easy to dislodge Energy to remove varies between substances

Electrons and Pith Balls Rubber – Fur RRubber holds electrons tight FFur hold loose Glass/Silk GGlass loosely SSilk tight

Conservation of Charge Electron are not created nor destroyed Cornerstone of physics Change is always in whole numbers of the charge of electrons (never break an electron)

Coulomb’s Law Electric force between objects is inverse square relationship Discovered by physicist Charles Coulomb ( ) For charges particles or objects that are small compared with the distance between them, the force between the charges varies directly as the product of the charges and inversely as the square of the distances between them

Coulomb’s Law F = k q 1 q 2 d² F (E) = Force (Newtons) d = distance between charged particles q 1 = quantity charge on one particle (coulombs) q 2 = quantity charge on other particle k = proportionality constant = 9 x 10^ 9 Nm²/C²

Coulombs SI unit of charge (C)  1 coulomb = 6.24 x 10^18 electrons  Amount of electrons that pass through a 100-W bulb for 1 second

Comparison of Gravity to Coulombs CoulombCharge between bodies Large quantity Attraction or repulsion GravityForce between bodies Small quantity Only attraction

Gravity vs. Electric Force Earth and Moon  No measurable difference between them electrically  Weak gravitational force Electron and Proton  Gravity is small ( 3.7 x 10^-47 N)  Electrical force is larger (8.2 x 10^-8 N)  Electric force is 10^39 times stronger!!

Conductors vs. Insulators Conductor - allows e- to roam freely in the material Insulator – electrons are tightly bond

Electron flow Rather flow through conductors  Goes through kilometers of wire than jump centimeters of insulation on the poles  Goes through meters of wire than jump millimeters of insulation in a cord

Semiconductors Good insulators when pure Conduct when one atom per million is impure Can be made to behave in both ways Used to make transistors  “transfer resistor”  Sandwich layers of semi-conducting material  controls flow of electrons in a circuit

Superconductors At temperatures near absolute zero, some metals acquire infinite conductivity  Zero resistance to electron flow  In 1987 Found a “high-temp” superconductor  Conducts at -273 ºC (100 K) Electrons flow indefinately

Electric charging by Friction Examples  Stroke cat’s hair  Comb your hair  Scuff shoes on rug  Getting out of a parked car

Electron transfer Touching Charging by contact  Conductor – spread charge out  Insulator – charge stays in one place

Charging by Induction Bring charged object NEAR a conducting surface without physical contact AB Rod is brought close and induces a charge in the balls INDUCTION – moves balls apart from each other Balls have opposite charge

Another example Net charge = Zero Net charge still zero Induction Touching removes electrons by contact Net charge on ball is positive Electrons move to sphere by touch Net charge on ball is negative

Thunderstorms Charge by induction occurs in T- storms

Ben Franklin Inducted a charge in a key Lucky that he wasn’t electrocuted

Other BF accomplishments Coined the terms “positive” and “negative” Lightning rods  Discovered charges leave from sharp points  Point collect e- from air, prevents a large build-up by induction  Prevents lightning  Gives a short circuit to ground if it is hit

Charge Polarization In an insulator – atoms and molecules rearrange to shift charge Polarized electrically

Charge Polarization Explains why neutral items are attracted to a charged object  Comb and paper  Balloon in hair  Balloon to wall  Water

Water Charged molecule Electric dipole Small stream will bend towards a charged stick O H H + + -

Microwave cooking Water molecules oscillate in different magnetic fields Food is cooked by “friction” of the moving water molecules Waves do not effect nonpolar molecules in foam, paper or ceramics

Summary Charging occurs in 3 ways  Friction – transfer of electrons from one object to another (static electricity)  Contact – transferred by direct contact without rubbing (transfer static to another item)  Induction – electrons gather or disperse by presence of nearby charge without physical contact (static in hair from electrical storm)