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Welcome to Phys 208!Welcome to Phys 208!

• Duration: 5 Sep-Dec 15 15 Weeks• Lectures: Tu, Th 12:05-12:55 pm• Honor lectures: Fri 12:05-12:55• Room: 2103 Chamberlin

Visit the Web site: http://uw.physics.wisc.edu/~rzchowski/phy208/

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Welcome to Phys 208!Welcome to Phys 208!

Contents• Electricity and Magnetism (Ch 23-31)• Electromagnetic Waves and Optics (Ch 34-38)• Modern Physics (Ch 41-46)• Friday Honor Lectures (modern physics and physics

applied to biology and other human sciences) Honor students must attend Friday Lecture, optional for

others (start next week)

Text Book: Serwey Jewett, Physics for Scientists andEngineers with Modern Physics, 6th edition

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Meet the lecturersMeet the lecturers

• Mark Rzchowski rzchowski@physics.wisc.edu Office: 5114 Chamberlin Hall Tel.: 265-2876

Office Hrs: Wed 1-2:30 pmResearch: Condensed Matter Physics

• Teresa Montaruli tmontaruli@icecube.wisc.edu Office: 4112 Chamberlin Hall Tel.: 890-0901

Office Hrs: Tue 2-3:30 pm

Personal WEB page: http://icecube.wisc.edu/~tmontaruliResearch: Research: High Energy Particle Physics and Astrophysics

IceCube Project

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Meet your Disc/Lab instructorsMeet your Disc/Lab instructorsLarry Watson, Faculty Assoc., is helping us for Labs, Disc, HWsYour TAs:• David Felker dfelker@wisc.edu• Peter Hyland pohyland@wisc.edu   • Ye Li  yli32@wisc.edu • Siddharth Malu ssmalu@wisc.edu • Daniel Passmore dpassmore@wisc.edu

Meet them in discussions NEXT WEEK!!

Shusaku Horibe will talk to you, distribute surveys to know youropinion

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Discussions and LabsDiscussions and Labs

• Discussions M&W. Start next week!

Extremely useful for your HWs and Exams

• Labs start next week (3 hrs long, 3254Chamberlin).

- All Labs must be completed to pass the course.

- Missed labs need an approved excuse.

- They can be eventually made up during an otheralternate period or during an exam week

A lot of Opportunities for interpersonalinteractions

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HomeworkHomework

• Homework is assigned each Thursday and is due thefollowing Thursday at midnight (!).

• First assignment of HW1 on Sep. 7 due on Sep. 14 atmidnight

We use an excellent online homework system:Mastering Physics by Pearson/Addison-Wesley

• Access for this semester is FREE - you don’t have to buyanything You have been emailed an access code that you will use to register. Follow the instructions in the email to add yourself to the

appropriate section. Homework assignments will be done on this web site. All your grades will also be available at this site.

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Meet your colleagues!Meet your colleagues!

• 161 students enrolled,mostly Bachelor ofScience, NaturalSciences,

Bachelor of Arts

• Will continue mainly inBiological Sciences,Agriculture, Math,Physics, Medicine

• Some in Engineering

We need to improveconnections betweenPhysics and other HumanSciences such as Biology!

Try to have examples to usephysics methods tounderstand processeshappening in Humans andNature!

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Exam scheduleExam schedule

• Exam 1: Oct 4 1:30 hrs 5:30-7 pm• Exam 2: Nov 1 1:30 hrs 5:30-7 pm• Exam 3: Nov 29 1:30 hrs 5:30-7 pm• Final Exam: Dec 20 7:25-9:25 pm

• Remember: exams are the main determiningfactor in the grade (65%)!!!

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GradingGrading

Mon & Wed10%DISC

1:30 hrs

1:30 hrs

1:30 hrs

2:00 hrs

15 %15%15%20%

Exams:

MTE1

MTE2

MTE3

Final

Mastering Physics10% HW

1 assigned at the end ofLabs

7%Lab Write up

9/10 required to pass thecourse

8% (-30% if 1 lost)Labs

CommentsWeight for yourfinal grade

Part of the exam work

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P208 SuggestionsP208 Suggestions

• Not everything you need for exams is in lectures! Lectures present: Concepts, Connections, Motivations

• However, help yourselves with Textbook (Read it - before & after the lecture) Attend Discussions + and do your Homeworks Hands-On: Labs

• Taking Notes Lecture notes available in the WEB site. Augment with your own notes.

• Understanding Ask questions in lectures, discussions sessions! Discuss problems with others, but try to solve problems

on your own.

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Electric ChargesElectric Charges

• There are two kinds of electric charges Called positive and negative (after B. Franklin 1706-1790)

• Negative charges are electrons• Positive charges are protons• Matter is globally neutral: atoms are made of

A positively charged central nucleus r~10-15 m Negatively charged electrons orbit around the nucleus r~10-10 m

Example: Lithium

3 protons in nucleus + 3 electrons(also neutrons in nucleus) 12

+ and - charges can be separated+ and - charges can be separated

• Triboelectric Charge is transferred as a result of

mechanical (frictional) action

• Conduction charge transfer by contact (spark)

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Conservation of Electric ChargeConservation of Electric Charge

Triboelectric - glass rod rubbed with silk ⇒ positive (electrons

transferred from glass rod to silk) - rubber rod rubbed with fur ⇒ negative

(electrons transferred from fur to robber rod)

Charge is always conserved!!

It cannot be created or

destroyed.

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Sign of ChargeSign of Charge

Charges of the same sign repel one another andcharges with opposite signs attract one another

Demo: stir-up + rods

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Quantization of Electric ChargesQuantization of Electric Charges

• The SI unit of charge is Coulomb (C )• The electric charge, q, is said to be quantized

(R. Millikan, 1868-1953) quantized = it is some integer multiple of a

fundamental amount of charge e q = Ne

• N is an integer• e is the charge of an electron = 1.6 x 10-19 C• Electron: q = -e• Proton: q = +e

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ConductorsConductors

• Electrical conductors are materials in which some of theelectrons are free and can move through the material

• Good conductors: copper, iron, aluminum and silver

• Earth: conductor with infinite storage capability

• Usually good electrical conductors are also good thermalconductors (free electron motion is main mechanism for heatconduction)

• When a good conductor is charged in a small region, thecharge readily distributes itself over the entire surface of thematerial

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Insulators or DielectricsInsulators or Dielectrics• Electrical insulators: all electrons are tightly bound to atoms

• Good insulators: glass, rubber and wood

• Air is a good insulator except when molecules become ionized(dielectric breakdown = a spark is produced betweenconducting surfaces, eg. lightning may discharge by transfer tothe Earth )

• Usually good electrical insulators are also good thermalinsulators

• When a good insulator is charged in a small region, the chargeis unable to move to other regions of the material 18

SemiconductorsSemiconductors

• The electrical properties of semiconductors aresomewhere between those of insulators andconductors

• Examples of semiconductor materials includesilicon and germanium

• Used for chips in computers, cellular phones,…

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SolutionsSolutions

• Mixture of a solvent (most abundant component) and solutes

• Solutions with ions (charged atoms or molecules) conductelectricity

• Giving up electron leaves a + charge (cation)

• Taking on electron produces a - charge (anion)

• Biological examples of liquid solutions: blood, intracellular fluid

• Pure Water is a good insulator but the normal high conductivityof water is due to the presence of salts and metal ions

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SolutionsSolutions

•Na+ and K+ cations (monovalent = 1 charge): carryelectricity in excitable cells, such as nerves andmuscles

•Substance which donates H+ ions to solution = acid•Substance which donates OH- ions to solution = base•Physiologists have devised a logarithmic unit: pH = - log [H+], where [H+] is the H ion

concentration in moles/liter •high pH: few H+ low pH: many H+

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Charging by conduction (touching)Charging by conduction (touching)

Neutralmetal

+ + + + + + ++ + + + ++

+++

+- --

-

-

Positively charged rod(too few electrons)

+ + + + + + +

Less positively charged rod

+

+++

+

Positivelychargedmetal

+ + + + + + ++ + + + +

+

+++

+- --

-

-

electron flow

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Interactions between chargesInteractions between charges

attractive force between positive and negativecharges.

repulsive force between two positive or two negativecharge

Why did the electrons flow?

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Charging conductors by InductionCharging conductors by Induction

• Charging by induction requires no contact with theobject inducing the charge

a) We start with aneutral metallic sphere

b) The rod does not touch thesphere. The electrons in theneutral sphere areredistributed

charged rubber rod

a) b)

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Charging by Induction, 2Charging by Induction, 2

If the sphere is grounded, some

electrons can leave the sphere

through the ground wire

When the ground wire is removed

there is still a net positive chargeon the sphere ⇒ positive charge

has been induced in the sphere.

When rod removed, electrons will

redistribute in the sphere.

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ElectroscopeElectroscope

The positively charged rod attractsnegative charges to the top ofthe electroscope.

This leaves positive charges onthe leaves.

The like-charges on the leavesrepel each other.

Electroscope: next Lab 26

Quick QuizQuick Quiz

A negatively charged rod is usedto charge an electroscope byinduction. What is the resultingnet charge on the electroscopeleaves?

1) Positive.2) Zero.3) Negative.

If negative charged rod were used, negative charges from thetop of the electroscope are repelled and drift to the leaves whichend up with a net negative charge, while the top of theelectroscope is positive.

Positivecharged

rod resultsin positive

leaves.

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Charge Rearrangement in InsulatorsCharge Rearrangement in Insulators

• A process similar toinduction can takeplace in insulators

• The charges withinthe molecules of thematerial arerearranged

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Coulomb’s LawCoulomb’s Law

• Electrical force between two stationary charged particles

• The SI unit of charge is the coulomb (C ), µC = 10-6 C

• 1 C corresponds to 6.24 x 1018 electrons or protons

• ke = Coulomb constant ≈ 9 x 109 N.m2/C2 = 1/(4"εo) εo = permittivity of free space = 8.854 x 10-12 C2 / N.m2

Gravitational force: FG=GM1M2/ r2

G=6.7x10-11 Nm2/kg2

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Vector Nature of Electric ForceVector Nature of Electric Force

a)The force is repulsive if charges are of like sign

b)The force is attractive if charges are of opposite sign

The force is a conservative force

Electrical forces obey Newton’s Third Law:

F21 = -F12

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Hydrogen Atom ExampleHydrogen Atom Example

• The magnitude of the electric force between the electron andproton: Fe = ke|e| |-e| / r2 = 8.2 x 10-8 N, r ~ 0.53 x 10-10 m

• The gravitational force between the electron and the proton Fg = Gmemp / r2 = 3.6 x 10-47 Nme = 9.11 x 10-31 kg, mp = 1.67 x 10-27 kgG = 6.67 x 10-11 N m2/kg2

Fe/Fg ~ 2 x 1039 N!!!+ -

r = 0.53x10-10 m

Mp=1.67x10-27 kg Me = 9.11 x10-31 kg

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+ --

1) Positive charge is attracted (force to left)

2) Negative charge is repelled (force to right)

3) Positive charge is closer so force to left is larger.

A positive and negative charge with equal magnitude are connected bya rigid insulating rod, and placed near a negative charge. What is thenet force on the two connected charges?

1) Left 2) Zero 3) Right

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rqkq

F =

Quick Quiz 1Quick Quiz 1

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Quiz: Induced DipoleQuiz: Induced Dipole

-

Electrons on A move far from B towards C• Negative charge attracts + repels -

An uncharged conducting sphere is hung between a negativelycharged sphere and a grounded sphere and held midway betweenthe two. What happens when the uncharged sphere is released?

+A

B C

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Induced DipoleInduced Dipole

An uncharged conducting sphere is hung between a charged sphereand a grounded sphere and held midway between the two. Whathappens when the uncharged sphere is released?

1) Negative charge attracts + repels -2) Since + is closer, attractive force is strongest

A- + -

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Induced DipoleInduced Dipole

An uncharged conducting sphere is hung between a charged sphereand a grounded sphere and held midway between the two. Whathappens when the uncharged sphere is released?

1) Negative charge attracts + repels –2) Since + is closer, attractive force is strongest3) Once they touch both are negatively charged, and thenrepel1) Touches grounded sphere and becomes neutral again

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Question?Question?

If the conducting sphere is replaced by an unchargedinsulating sphere, what will happen when it is released? Sticks to charged sphere. Sticks to grounded sphere. Nothing. Swings back and forth between charged and grounded

spheres.

Charges can not separate in insulator, so net force iszero

-

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Ben Franklin’s door bellBen Franklin’s door bell

++ +

+ ++ +

+---

-