George Cross Electromagnetism Charge Model Lecture26 (2)

Antelope Valley CollegeMath & Sciences Dept

George Cross

General PhysicsPhysics 120

Chapter 26: ELECTRICCHARGES & FORCES

ANNOUNCEMENTS

TODAYS LECTURE

• Introductions

• How this Class Will Run

• Syllabus

• Class Safety

• Chapter 26

Physics 120

TEXT: PHYSICS for Scientists and Engineers

Second Edition: A Strategic Approach; Knight

Lecture: 6:00 – 10:05 PM Monday, SC3 168

Office: 6:00 – 6:50 PM Wednesday, SC2 136

Lab: 7:00 – 10:05 PM Wednesday, SC2 136

Course Goals – Learn and Understand•Electrostatics, electric fields and potentials

•Resistance, capacitance, current, DC circuits

•Magnetic fields, electromagnetic induction

•Electromagnetic fields and waves, AC circuits

•Help you to Love Physics (Well, at least like it) as much as I do

•Have Fun while we learn

How We Learn• Repetition (minimum of 7 times before it is

locked into memory)– Read Chapter Before Coming to Class– Listen in Class– Discuss in Study Groups– Do Your Homework

• Tell it to Someone Else– Participate in Study Groups– Explain it to Someone

• Participation– Participate in the Class Discussion & Activities– Participate in Lab

How This Class Will Run

• It won’t work to sit in class and expect me to pour knowledge into you. You must follow the way we learn. Therefore:– Read Chapter Before You Come to Class This will

help you to:• Remember the material• Understand the lecture• Participate in the class and group discussion• Pass the Chapter Quiz at the beginning of class

– Participate in Class– This Class Will not be a Typical Lecture– This Class Will be a Combination of Lecture,

Demonstration, Discussion, Group Work with a Workbook and so Forth that is Coupled with a Lab

Why Study Physics? What Good Will it Be in My Life or Career?

• Physics is the science of how things work

• Physics plays a part in every technology that has been devised by man

• Physics is infused into every aspect of your life– What you physically see/observe– What you physically experience– The consequences to your physical actions or

inaction– What happens to the objects around you

Physics Can Help You Work In State Of The Art Technological Laboratories Like This

Physics Can Help Prevent You From Doing Stupid Things With Serious Health Consequences

Inertia Works!

TODAYS LECTURE

• Introductions


• Syllabus

• Class Safety

• Chapter 26

Warning About Disobeying Rules of Conduct

TODAYS LECTURE

• Introductions


• Syllabus

• Class Safety

• Chapter 26

WARNING: Carrying Calculators, Rulers etc.

Los Angeles County Teacher Arrested in New York Airport

A public school teacher was arrested today at John F. Kennedy International Airport ashe attempted to board a flight in possession of a ruler, a protractor, a compass, a sliderule, and a calculator. At a morning press conference, the Attorney General said hebelieves the man is a member of the notorious Al-Gebra Movement.

He did not identify the man, who has been charged by the FBI with carrying weapons ofMath Instruction.

“Al-Gebra is a problem for us,” the Attorney General said. “They derive solutions bymeans and extremes, and sometimes go off on tangents in search of absolute values.” They use secret code names like “X” and “Y” and refer to themselves as “unknowns.”Their variable nature makes everyone leery of their actions as sometimes the equationdoes not add up. We have determined that these operatives belong to a commondenominator with coordinates in every country. An examination of the matrix of theiroperation, will allow the FBI to determine their function, and plan methods to FOIL theirmaneuvers.

When asked to comment on the arrest, President Obama said, “If God had wanted us tohave better Weapons of Math Instruction, He would have given us more fingers andtoes.”

TODAYS LECTURE

• Introductions


• Syllabus

• Class Safety

• Chapter 26

CHAPTER 26 QUIZ

1. Which of the following is not true? The electric force___ 1. decreases with the inverse of the square of the distance between twocharged particles.___ 2. between an electron and a proton is much stronger than the

gravitationalforce between them.___ 3. between two protons separated by a distance d is larger than that

betweentwo electrons separated by the same distance d.___ 4. may be either attractive or repulsive.___ 5. Is the coolest new sci-fi tech TV show.

2. A material that permits electric charge to move through it is called a(n)___ 1. insulator.___ 2. conductor.___ 3. capacitor.___ 4. incapacitator.___ 5. inductor.

3. When the electric charge on each of two charged particles is doubled, the

electric force between them is___ 1. doubled.___ 2. quadrupled.___ 3. the same.___ 4. totally awesome!___ 5. none of the above

4. In any reaction involving charged particles, the total charge before and

after the reaction is always the same. This relationship is known as___ 1. quantization of charge.___ 2. conservation of charge.___ 3. status quo law of charge___ 4. the law of induction.___ 5. not covered in the reading assignment

5. The net charge on any charged object is always an integer multipleof the charge of the electron (e =1.6 x 1019 Coulombs). This relationship is known as___ 1. quantization of charge.___ 2. Coulomb’s Law.___ 3. the law of induction.___ 4. conservation of charge.___ 5. Murphy’s Law___ 6. not covered in the reading assignment

1. Which of the following is not true? The electric force___ 1. decreases with the inverse of the square of the distance between twocharged particles.___ 2. between an electron and a proton is much stronger than the

gravitationalforce between them._X_ 3. between two protons separated by a distance d is larger than that

betweentwo electrons separated by the same distance d.___ 4. may be either attractive or repulsive._X_ 5. Is the coolest new sci-fi tech TV show.

2. A material that permits electric charge to move through it is called a(n)___ 1. insulator._X_ 2. conductor.___ 3. capacitor.___ 4. incapacitator.___ 5. inductor.

3. When the electric charge on each of two charged particles is doubled, the

electric force between them is___ 1. doubled._X_ 2. quadrupled.___ 3. the same.___ 4. totally awesome!___ 5. none of the above

4. In any reaction involving charged particles, the total charge before and

after the reaction is always the same. This relationship is known as___ 1. quantization of charge._X_ 2. conservation of charge.___ 3. status quo law of charge___ 4. the law of induction.___ 5. not covered in the reading assignment

5. The net charge on any charged object is always an integer multipleof the charge of the electron (e =1.6 x 1019 Coulombs). This relationship is known as_X_ 1. quantization of charge.___ 2. Coulomb’s Law.___ 3. the law of induction.___ 4. conservation of charge.___ 5. Murphy’s Law___ 6. not covered in the reading assignment

Einstein said,

He also said that the development of the

theory of electromagnetismwas “the most importantevent in physics since

Newton’s time.”

ELECTRICITY & MAGNETISM• Electric Charges & Forces• Electric Field• Gauss’s Law• Electric Potential• Potential & Field• Current & Resistance• Fundamentals of Circuits• Magnetic Field• Electromagnetic Induction• Electromagnetic Fields & Waves• AC Circuits

TODAYS LECTURE

• Developing a Charge Model

• Charge

• Insulators & Conductors

• Coulomb’s Law

• The Field Model

• Electricity Powers Your Modern Life– Entertainment

• Audio• Video

– Communication• TV• Radio• Cell phones & Land Lines

– Transportation– Light & heat

Developing a Charge Model• Rubbing things together produces charges

& forces

• Three states of charge: a) positive, b) negative and c) neutral (equal mix of positive and negative charges)

• Like charges repel, opposites attract

• Force between charged objects is a long distance force that decreases with distance

• Charge can be transferred by contact

Electric Properties of Materials• Charge can be transferred by contact

between objects

• Two types of materials: a) conductors (conduct charge easily), b) insulators (do not conduct charges easily)

• Both conductors and insulators can be charged

• Conductors give up charge easily

• Insulators do not give up charge easily

Charge• Positive charge & negative charge –

named by Benjamin Franklin• Franklin named the charge on the glass

rod positive• Any charge that repels the glass rod is

also positive and any that attracts the glass rod is negative

• By convention, electrons are negatively charged and protons are positively charged

• Developed without knowledge of atoms

Electron orbital frequencies~ 1015 rev/minute

~10-14 m

•Protons•1.67x10-27 kg•+e

•Electrons•9.11x10-31 kg•-e

e is called the fundamental charge

This atom is neutral in chargesince it has the same numberof protons andelectrons.

The Micro/Macro Connection

• Electrons & protons – basic charges of ordinary matter– Particles– Follow Newton’s Laws of Motion

• Charge represented by symbol “q”

• Object is charged if the total number of electrons and protons are unequal

• Objects charge will always be an integer multiple of e (charge quantization)

The Micro/Macro Connection

• Objects acquire charge by gaining or loosing (ionization) electrons– Atoms missing electrons – positive ions– Atoms with extra electrons – negative ions

• Friction causes charging (ionization)– Molecular bonds broken at surface– One part of molecule looses an electron– One part of molecule gains an electron– Ions are on opposing objects

• Fur (positive) & rubber rod (negative)• Silk (negative) & glass rod (positive)

Charge Conservation• Laws of Conservation

– Conservation of Energy– Conservation of linear and rotational momentum– Conservation of charge

• Law of Conservation of Charge:

Charge is neither created or destroyed. Charge can be transferred from one object to another as electrons and ions move about, but the total amount of charge remains constant.

(Net charge of fur and rubber rod is zero)

Example of charge diagrams. Draw a simple two-dimensional cross-sectionOf the object/s. Draw only the net charge (neutral should show no charges). Charge on conductor will only be at the surface. Conserve charge from onediagram to the next if multiple diagrams are needed.

Insulators & Conductors

• Insulator - Electrons tightly bound to nucleus and cannot move about

• Conductor – Outer electrons (valence electrons) are weakly bound. When atoms come together to form a solid, these valence electrons are shared and are then free to move about or wander through the entire solid. Similar to an array of positive ion cores with negatively charged fluid moving about in between (sea of electrons)

In a metal the mobile valenceelectrons are called charge carriers.

Electrons in a metal arehighly mobile.

Charging an insulatorBy friction leaves patches of charge on surface. These patches of chargeare not mobile and the charge does not spreadout.

Insulators & Conductors – Cont.

• Conductors– Metals – charge conductors are electrons– Ionic solutions – charge conductors are

positive and negative ions– Plasma – charge conductors are electrons

and nuclei

Charging

Metal objects usually cannot be charged by rubbing, but by coming into contact with a charged object.

Electrons in a conductor are free to move.When charge is transferred, electrons repel each other and rapidly move to redistribute themselves accordingly (entire sea shifted to side and then redistributes). This responseIs virtually instantaneous.

Other than this brief adjustment period whencharging is happening, the charges in a conductor are in static equilibrium. This meansthat the charges are at rest because there is nonet force acting on any of them. “Electrostatic Equilibrium”

In an isolated conductor, any excess charge islocated on the surface of the conductor.

Electroscope

Since the electroscope is metal, all excess charges move as farapart as possible. As a result, the leaves become charged as wellAnd repel each other since they are of like charge.

Discharging• Charging happens as a result of friction• Discharging happens by contact (touching)

– A conductor, or reasonable conductor touching a charged object removes or conducts away charge

– Things that can remove charge• Water – most water contains ions making it a good

conductor• Humans – mostly salt water – therefore, good

conductor• Grounding straps• Air – poor conductor but some charged ions which

will gradually discharge an object exposed to the air

• Earth – “grounded”

Human – fair conductor. Charges willquickly spread over entire conductive area which now includes the human. Not much charge will be left on the metalwhen person removes their hand.

Insulators will not give up their chargeeasily since the charges are not free tomove. They may, but in the small area in contact with the discharging object.

An object connected to the earth will shareits charge with entire earth and willbecome completely discharged.

Grounding objects prevents the build up ofexcess charge in electric and electroniccircuits.

Charge Polarization• Charging happens as a result of friction• Discharging happens by contact (touching)

– A conductor, or reasonable conductor touching a charged object removes or conducts away charge

– Things that can remove charge• Water – most water contains ions making it a good

conductor• Humans – mostly salt water – therefore, good

conductor• Grounding straps• Air – poor conductor but some charged ions which

will gradually discharge an object exposed to the air

• Earth – “grounded”

Charge Polarization

Charge PolarizationCharge polarization is the slight separation of positive and negative charges

in a neutral object.

Charge Polarization

• Equilibrium position for electrons is such that force of nearby charged object and force from positive nuclei is balanced– Nuclei exert restoring force on electrons– Actual position shift is typically < 10-15m

Polarization force arises due to separation of charge in the metal

Polarization Force

The Electric Dipole

Why does a charged object attract neutral insulators?

This slight separation of charge is called an electric dipole.

Also an example of polarization force.

•Polarizes atoms in paper•Exerts attractive polarizationforce on each atom

A charged object can pick up paper because it:

Charging by Induction

Coulomb’s Law

• Basic law of electric force

• Explains why– Adding more charge increases the

repulsive/attractive force– Increasing the distance reduces the

repulsive/attractive force

• Some debate as to whether he discovered the inverse square law or made a good guess, but he was correct

K is the electrostatic constant

Law also says that the force is an action/reaction pair acting along the line joining the two charges and that the force is repulsive For like charges and attractive for opposites.

Coulomb’s Law – Cont.

Similar to Newton’s Law of Gravitationbut:Charges can be positive or negativetherefore the absolute value signs giveus the magnitude. The second part ofCoulomb’s Law tells us the direction ofthe forces on the respective charges.

Coulomb’s Law & Units of Charge• Coulomb had no unit of charge so he

could not define the electrostatic constant (numerical value depends on units of distance and charge)

• Fundamental unit of charge: e = 1.60x10-19 C (C is for Coulomb) or… 1C = 6.25x1018 protons• Typical charge on rubber rod is 10-9C to

10-7C or an excess of 1010 or 1012 electrons

• K = 8.99x109 Nm2/C2

Coulomb’s Law – Cont.

• We can make Coulomb’s Law more useful for later chapters by rewriting it– Define permittivity constant o (epsilon zero)

– o = 1/4K = 8.85x10-12 C2/Nm2

Now let’s use Coulomb’s Law!

The Field Model• Electric & magnetic forces are long range forces• Despite the success of Newton’s Law of

Gravitation, scientists of the time still had great difficulty with forces at a distance

• Faraday postulated that the object producing the field changed the space around it in some way and that the behavior of a charged object was in response to the change in the space & affects all space

• Faraday also postulated that the effects of field took a finite time (not instantaneous) to propagate

The Field Model – Cont.• Faraday was not taken seriously at first –

many scientists thought it was just a pictorial gimmick

• Over time scientists realized that the concept of field was required to understand the behavior of electromagnetics

• Put on a mathematical basis in 1865 by James Clerk Maxwell– Explained all known electromagnetic behavior

in four equations known as “Maxwell’s Equations” (Chapter 35)

Gravity is a Field Force

•r is measured from the center of mass to center of mass•Force acts as if all mass concentrated at center of mass•Action reaction force pair•Force is in direction of center of mass of object exerting the force


All objects of mass aregravitational sinks - field lines are in towardCenter of mass


This is approximately true for all locations near theEarth’s surface.

The Electric Field Model• We will call some charges source charges

(electric field source) and others sink charges

• A separate charge in the electric field experiences a force exerted by the field

• Force proportional to strength of field

• “E” is called the electric field strength

How Do We Know There is an Electric Field?

Put a point charge at location ofinterest and see if there is a forceon it

The Electric Field Model

• From any given charge there is an electric field vector defined for every point in space

• If q of the test charge is positive, the force will be in direction of field vector at that point

• Electric field does not depend on size of test charge – force dependant upon q, so field is independent.

• Fon q = qE

Point charge q creates field.Point charge q’ to test the field made by q.By measuring the force on q’ we can determine the direction of E at that point.We note that no matter where we place q’, the field will always point out away from q.

F = (1/4o)qq’/r2

E = F/q’ = (1/4o)q/r2

Positive Charge: Field Source

Notice how the arrows forthe field get smaller the Farther out they go. ThisIs due to the fact that the field for a point charge drops with the square of the distance.

Arrows are only a representation of the field. The field exists at all points. The lengths arerepresentative of the field strength at that point (Iwherethe dot is).

The Electric Field of a Point Charge

• We will use vector notation to make our equation a little more useful and allow use of positive or negative charges

Position (Location) Vectors

Electric Field Vectors

Negative Charge: Field Sink

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George Cross Electromagnetism Charge Model Lecture26 (2)

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