Physics 114 – Fall 2014Prof. Martin Guthold

Office: Olin 302, Lab: Olin 202Phones: 758-4977 (office); 608-7304 (cell); 923-9902 (home)

e-mail: gutholdm@wfu.edu

OFFICE HOURSMo, We, Fr, 1:00-2:00 pm, 302 Olin

Feel free to drop by or make appointments, and I’ll try to accommodate you.

Physics 114 is the second course in a two-semester sequence in calculus-based general Physics. It does require the use of calculus and vector calculations. Calculus (Math 111) and Physics 113 (Mechanics, oscillations, waves) are a pre-requisite.

SCHEDULE

Lectures are on: Monday, Wednesday, Friday 9:00 am – 9:50 am; room Olin 101 (ch. 23 - 34)optional: Sunday, Thursday, (doodle poll); (optics, ch. 35 – 38)

Labs:All students must also enroll in one laboratory session. Labs will begin the week of Sept. 1; room Olin 104. Labs cannot be made up on other days. Attendance in the labs is required.

PHYISCS 114 SYLLABUS

TEXT AND MATERIALS

• Required text book: Physics for Scientists and Engineers, 9th ed. by Serway & Jewett vol. 2• Required: For the lab you must get the lab manual from the bookstore (~$15).• Required: Sign up for WebAssign (~ $47, more details below). • Required: i-clickers (available at bookstore (~$30), can be used for other classes, too)• Optional: Student solution manual (can help with some homework problems).

EXAMS AND GRADING

There will be one, comprehensive, 3-hour final exam and two1-hour, evening midterm exams given at the dates listed below. Homework problems will be assigned for each chapter and they will be also be graded.

1. Exam 20 %

2. Exam 20 %

Final Exam 30 %

Lab 15 %

Homework10 %

i-clickers 5%

Participation can move borderline grades.

Exams:

Exam 1: Friday, Sept. 26, 5:00 – 6:00 pm or 6:00 – 7:00 pm (Chapters 23-26)Exam 2: Friday, Nov. 7, 5:00 – 6:00 pm or 6:00 – 7:00 pm (Chapters 27-31)Final: Saturday, Dec. 13, 9:00 am – 12:00 pm (comprehensive, Chapters 23-34)

(This is the last day of finals week and I’ll probably offer one alternative date)

HOMEWORK AND PROBLEM SOLVING

Homework and problem solving is an important part of learning in a Physics course. Approximately 10-15 questions or problems per chapter will be assigned as homework. We will use WebAssign. Homework is usually due one or two lectures after it has been assigned. (Late HW – 20% reduction per day). Some homework problems may also re-appear on the exams and the final. You may collaborate on homework, but must submit your own work.

POSTINGS

Homework, practice exams, all lecture notes and all other material relating to the course will be posted on the web site for the class:

http://www.wfu.edu/~gutholdm/Physics114/phy114.html

To get ready for class: Print out lecture notes before class and bring to class. Go through notes, easy i-clicker will test reading at beginning of class). This class does not use CourseInfo or Blackboard.

WebAssign (http://www.webassign.net/) will be implemented for standard homework assignments. You have five attempts to get the answers right.Access codes to WebAssign ($47) need to be purchased from the bookstore or WebAssign.

ATTENDANCE

It is expected that students attend all scheduled classes. Attendance at the two exams and the final is required. Absence on the exams will result in a zero grade unless an official excuse is presented. Excuses should be reported to me in advance.

i-clicker gives one point for attendance, one for each correct answer.

• Demos: Understand them & and take notes. (May pop up in exam)

• Powerpoint presentations

download from http://www.wfu.edu/~gutholdm/Physics114/phy114.html, print out (e.g. three slides on a page) and bring to lecture.

• Lots of whiteboard work (bring note pad to each class; take notes!!)

• i-clickers: Concept questions and quick quizzes with immediate feedback.

Lecture format:

Labs

- The labs take place in Olin 104

- Lab manager:

Eric Chapman (Olin 110), phone: 758-5532

- Your lab teaching assistants (TAs):Xiao, Jiajie (1)

Vuong, Andy (1)Li, Wei (2)

- Need to buy lab manual

- Labs start the week of Sept. 1

PHY114 TUTOR SESSIONS

The tutor sessions in past semesters past were very successful and received high marks from many students.

All students are encouraged to take advantage of this opportunity.

Sunday Monday Tuesday Wednesday Thursday Friday

5:00 pm -7:00 pm

- Joshua Mei

Lauren Nelson

Joshua Mei Lauren Nelson

-

Pandemic Plan

• In case of pandemic or major disaster striking the University (University closing, or instructor unavailable):

• Tiered plan:– Class might be covered by other instructor (if available). – The lecture notes (ppt slides) will be distributed to you via the class

web page, e-mail or regular mail. – Short movies covering the major points may be posted on the class web

page.– You may be given a CD or DVD with all the lecture notes and exams

to be taken. – Exams will be taken on the dates indicated in the syllabus. Exams will

be taken in a location to be announced or will be sent to you via web page, e-mail or regular mail.

Material covered in this class(Chapters 23-38, Physics for Scientists and Engineers, 9th ed. vol. 2)

Electricity and Magnetism23. Electric Fields24. Gauss’s Law25. Electric Potential26. Capacitance and Dielectrics

Exam 1 (Chapters 23 - 26) Evening lectures and HW27. Current and Resistance28. Direct-Current Circuits29. Magnetic Fields30. Sources of Magnetic Field31. Faraday’s Law

Exam 2 (Chapters 27 - 34)32. Inductance33. Alternating-Current Circuits34. Electromagnetic Waves

Final exam (Chapters 23-34)

Light and Optics35. The Nature of Light, Ray Optics36. Image Formation37. Wave Optics38. Diffraction Patterns and Polarization

On average, we’ll spend about 3 lectures per chapter.

A few slides about WebAssign: Log-in

Your e-mail address: e.g. gutholdm

wfu

Set your own password

Some students who already have accounts should be able to re-use them (but still need to pay for each class

A few slides about WebAssign: What to purchase

There are two options to purchase WebAssign:

1. Purchase access code. 2. Enter access code (purchased with textbook from

bookstore).

A few slides about WebAssign: What to purchase

Lifetime of Edition (LOE)You are allowed unlimited access to WebAssign courses that use this edition of the textbook at no additional cost(you can also use for Physics 113 & 114).

The e-book is basically just a nice electronic version of the book. You don’t need it if you have the text book.

A few slides about WebAssign: Notation, significant figures

Notation (use scientific notation):

2.32 10‧ -4 2.32e-4 (in WebAssign)

Need to use three significant figures (unless otherwise stated).

Fundamental unitsTime second sDistance meter mMass kilogram kgTemperature Kelvin KCharge Coulomb C

SI Units

Derived unitsForce Newtons N kgm/s2

Energy Joule J NmPower Watt W J/sFrequency Hertz Hz s-1

Elec. Potential Volt V J/CCapacitance Farad F C/VCurrent Ampere A C/sResistance Ohm V/AMag. Field Tesla T Ns/C/mMagnetic Flux Weber Wb Tm2

Inductance Henry H Vs/A

Metric Prefixes109 G Giga-106 M Mega-103 k

kilo-110-3 m milli-10-6 micro-10-9 n nano-10-12 p pico-10-15 f femto-

Red boxes mean memorize this, not just here, but always!

Vectors• A scalar is a quantity that has a magnitude, but no direction

• Mass, time, temperature, distance• In a book, denoted by math italic font

• A vector is a quantity that has both a magnitude and a direction• Displacement, velocity, acceleration• In books, usually denoted by bold face• When written, usually draw an arrow over it

• In three dimensions, any vector can be describedin terms of its components

• Denoted by a subscript x, y, z• The magnitude of a vector is how long it is

• Denoted by absolute value symbol, orsame variable in math italic font

, , ,m t T r

, ,

, ,s v a

s v a

x

y

z

v

, ,x y zv v vv

vx

vy

vz2 2 2x y zv v v v v

Finding Components of Vectors• If we have a vector in two dimensions, it is pretty easy to compute

its components from its magnitude and directionv

x

y

v

vx

vy

cos

sinx

y

v v

v v

• We can go the other way as well2 2

1tan

x y

y

x

v v v

v

v

• In three dimensions it is harder

2 2 2x y zv v v v

Unit Vectors• We can make a unit vector out of any vector

• Denoted by putting a hat over the vector• It points in the same direction as the original vector

• The unit vectors in the x-, y- and z-direction are very useful – they are given their own names

• i-hat, j-hat, and k-hat respectively• Often convenient to write arbitrary vector in terms of these

ˆr

r r

rr

i

ˆ ˆ ˆx y zv v v v i j k

j

k

vv

Adding and Subtracting Vectors• To graphically add two vectors, just connect them head to tail• To add them in components, just add

each component• Subtraction can be done the same way

v

wv w

ˆ ˆ ˆ

x x y y z zv w v w v w v w i j k

ˆ ˆ ˆx x y y z zv w v w v w v w i j k

Multiplying VectorsThere are two ways to multiply two vectors• The dot product produces a scalar quantity

• It has no direction• It can be pretty easily computed from geometry• It can be easily computed from components

v

w

cos x x y y z zvw v w v w v w v w

• The cross product produces a vector quantity• It is perpendicular to both vectors• Requires the right-hand rule• Its magnitude can be easily computed from geometry• It is a bit of a pain to compute from components

v w

sinvw v w

ˆ ˆ ˆ

det x y z

x y z

v v v

w w w

i j k

v w

ˆ ˆ

ˆ

y z z y z x x z

x y y x

v w v w v w v w

v w v w

i j

k

Chapter 23: Electric Fields

Reading assignment: Chapter 23

Homework 23.1, due Monday, Sept. 1:

- QQ1, QQ2, 1 (all homework is submitted on WebAssign)

- Sign up (purchase access code) and check out WebAssign: http://www.webassign.net/

- Purchase i-clicker, book, lab manual

Homework 23.2, (will be posted today), due Wednesday, Sept. 3:

Homework 23.3, (will be posted today), due Friday, Sept. 5:

• Properties of electric charges

• Charging by induction

• Coulomb’s law (looks similar to Newton’s Universal law of Gravitation)

• Electric field, calculating electric field (vector field) of a charge distribution (point charges and simple extended charge distributions)

• Electric field lines

Chapter 23: Electric charge and electric field

Benjamin Franklin ( 1706-1790)

- Named positive and negative charges

Charles Coulomb (1736-1806)

- Forces between charges

Michael Faraday (1791-1867)

- Electric field

Electrostatics: Interaction of charges which are not moving

POSTIIVE CHARGE

Human Hands (usually too moist)Rabbit FurGlassHuman HairNylonWoolFurLeadSilkAluminumPaperCotton Steel (neutral)WoodAmberRubber BalloonHard RubberNickel, CopperBrass, SilverGold, PlatinumPolyesterStyrene (Styrofoam)Saran WrapPolyurethanePolyethylene (like scotch tape)PolypropyleneVinyl (PVC)SiliconTeflon (very negative)

NEGATIVE CHARGE

Triboelectric sequence:

The items on top are less attractive to electrons and become positively charged, while the items on the bottom are more attractive to electrons and become negatively charged.

Thus, on contact between any two substances shown in the column, the one appearing above becomes positively charged, the one listed anywhere below it becomes negatively charged

Franklin observed:When rubbing objects together, charges can get transferred from one object to the other.

Each transferred electron add negative charge to the silk and an equal positive charge is left on the glass rod

• Two types: positive and negative (negative charge is carried by electrons and positive charge is carried by protons (see atom model in two slides))

• Like charges repel

• Opposite charges attract

• Charge is conserved (net amount of electric charge produced in any process is zero)

• Charge is quantized (charge is always a multiple of fundamental unit of charge, e = 1.6·10-19 C)

• Unit of charge: 1 Coulomb (1C) (= 6.25·1018 electrons)

From: Physics by Giancoli

Properties of electric Charges:

i-clicker 23.1:

Three objects are brought close to each other,

two at a time. It is found that object 1 and 2

repel each other and that object 2 and 3 repel

each other. From this we can conclude that:

A. 1 and 3 carry charges of opposite sign.

B. 1 and 3 carry charges of equal sign.

C. All three carry charges of the same sign.

D. One of the objects carries no charge.

E. We need to do more experiments to determine the sign

of the charges.

From: Physics by Giancoli

The nature of matter

Atoms: Electrons, Protons, Neutrons

Electrons: -Protons: +Neutrons: 0

Nucleus

Ion: Atom +/- Electron

C+

e-

Matter: Nuclei with positive charges

Surrounded by ‘sea’ of electrons

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

+ +++++

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

- -- - -- -- -- - -- -- -- - -- -- -- - -- -- -- - -- -

White board example 23.1.

What is the charge and (average) mass of a single Na+ ion?

(Hint: Na has atomic mass 22.99; thus, 1 mole ( 6.022·1023 particles) of Na atoms have

mass 22.99 g. The atomic mass unit (1/12th the mass of carbon atom) is 1.66·10-27 kg.)

Image of sodium ion: http://faculty.clintoncc.suny.edu/faculty/michael.gregory/files/bio%20100/bio%20100%20lectures/chemistry/chemistr.htm

Insulators: Materials in which the electrons are tightly bound to

the nucleus and are not free to move through the material (glass,

rubber, plastic, dry wood are good insulators)

Conductors: Materials through which the electrons are free to

move (typically metals: silver, gold, copper, mercury)

Semiconductors: Materials with a few free electrons and the

material is a poor conductor. At higher temperatures electrons break

free and move through the material. (silicon, germanium, carbon

(graphite)).

Insulators and conductors

Some ways to charge objects

• By rubbing them together (triboelectric, tribo (greek) = to rub)• Not well understood

• By chemical reactions• This is how batteries work

• By moving conductors in a magnetic field• Get to this later

• By connecting them to conductors that have charge already• That’s how outlets work

• Charging by induction• Bring a charge near an extended conductor• Charges move in response• Separate the conductors• Remove the charge +

–––

––

+++

++

i-clicker 23.2:

Three objects are brought close to each other, two at a time. It is found

that object 1 and 2 attract each other and that object 2 and 3 repel each

other. From this we can conclude that:

A. 1 and 3 carry charges of opposite sign.

B. 1 and 3 carry charges of equal sign.

C. All three carry the charges of the same sign.

D. One of the objects carries no charge.

E. We need to do more experiments to determine the sign of the charges.

Related: How do balloons stick to a wall?

Coulomb’s Law• Like charges repel, and unlike charges attract• The force is proportional to the charges• It depends on distance rq1 q2

Notes• The r-hat just tells you the direction of the force, from 1 to 2• The Force as written is by 1 on 2• Sometimes this formula is written in terms of a

quantity0 called the permittivity of free space

1 212 2

ek q q

rF1 2

12 122ˆek q q

F rr

12 2 20

18.854 10 C /N m

4 ek

9 2 28.988 10 N m / Cek Coulomb constant

i-clicker 23.3:

Object A has a charge of +2 mC and object B has a charge of +6 mC.

Which statement is true about the electric force on the objects?

A.

B.

C.

D.

E.

3AB BAF F

AB BAF F

3 AB BAF F

3AB BAF F

3 AB BAF F

1 212 122

ˆek q qF r

r

9 2 28.988 10 N m / Cek

1 212 122

ˆG m m

F rr

11 2 26.67 10 N m /G kg

Electric force and graviational force have same functional form. Unless we have huge masses, the electric force is much larger than the gravitational force

Whiteboard problem 23.2

Three point charges are located at the corners of an equilateral triangle as shown below. Calculate the net electric force on the 7.0 mC charge.

y

x

7.0 C

2.0 C -4.0 C

0.50 m

+

+ -600

Use superposition principle and vector addition

A. angle 1 (made by q1 with the vertical) is larger than 2 (made by q2). B. angle 1 (made by q1 with the vertical) is smaller than 2 (made by q2). C. 1 = 2. D. More information is needed to answer this.

i-clicker 23.4:

In the figure below, two uncharged conductors of identical mass and

shape are suspended from a ceiling by nonconducting strings. The

conductors are then given charges q1 = Q and q2 = 3Q.

After charging

The electric field(important – will come up many times this semester)

• Many forces are ‘contact forces’, that require contact between objects (e.g. hammer and nail, friction between tires and road)

• Gravitational and electrical force act over a distance (even through vacuum) field forces

• Faraday developed the idea of a field:

An electric field extends outward from every charge (source charge) and permeates all of space.

Q

Test charge q0

The electric field of a positive point charge Q

F

Definition: The electric field, , at any point in space is defined as the force, , exerted on a tiny positive test charge, q0 at that point, divided by the magnitude of the test charge.

E

F

0

FE

q

• is independent of the tiny test charge, q0, and only depends on the source charge, Q, which produces the field.

• points away from a positive charge and points towards a negative charge.

• is a vector field, it has a direction in space everywhere.

• Unit is N/C (Newton/Coulomb) (later: also Volt/meter)

The electric field

+Q

Test charge q0

The electric field of a positive point charge Q

F

0

2ˆe

FE

q

QE k r

r

Electric field of a point charge:

E

E

E

GRAVITATIONAL FIELD ELECTRIC FIELD

Earth

20

ˆg EF M

g G rm r

EM

Source of field

Test mass

m0 +Q

Source of field

Test charge

02 2

0 0

ˆ ˆee e

F q Q QE k r k r

q q r r

Gravitational field is described by source mass, M (mass of Earth).Test mass m is a detector of gravitational field.

Electric field is generated and described by source charge +Q.Test charge q0 is a detector of electric field.

Test charge q0 <<q, so field is undisturbed.

q0

Electric Field from Discrete Distribution of Charges

The electric field at point P due to a group of source charges

can be written as the vector sum of all the individual fields:

2ˆ i

total i total e ii i i

qE E E k r

r

Calculate the total electric field at point A and at point B due to both charges, Q1 and Q2.

Use symmetry to save work, when possible.

White board example 23.3 (field of a dipole)

Electric field lines

In order to visualize the electric field we draw a series of field lines that indicate the

direction of the field at various points in space. • Lines indicate direction of field, they go from positive to negative

• Electric field points along tangent of electric field lines

• Density of lines is proportional to field strength

• Number of lines starting/ending on a charge is proportional to the magnitude of the charge.

• No two lines cross each other (Why?)

i-clicker 23.5:

Rank the magnitude of the electric field at points A, B, and C

(greatest to smallest).

A. A, B, C

B. B, C, A

C. C, A, B

D. A, C, B

E. B, A, C

Electric Field from a Continuous Charge Distribution(can get complicated, quickly…)

2 20ˆ ˆlim

i

ie i e

qi i

q dqE k r k r

r r

iq

P

E

2ˆi

i e ii i i

qE E k r

r

Electric field can come from charge spread on a line, on a surface, or throughout a volume: • Linear charge density ; units C/m

• Multiply by length• Surface charge density; units C/m2

• Multiply by area• Charge density; units C/m3

• Multiply by volume

dldq dl

dVdVdq

dAdq dA

The concept of charge density

White board example 23. 4

A rod of length l has a uniform positive charge per unit length λ

and a total charge q. Calculate the electric field at a point P that is

located along the long axis of the rod and a distance, a, from one

end. y

xPE

l

x

dx

a

dldq

Electric Field from a Continuous Charge DistributionExample: Electric field due to a charged rod

Quick Quiz: Find the electric field at the center of a uniformly charged ring.

Motion of a Charged Particle in a Uniform Electric Field

A charged particle in an electric field, E, will experience an electric force F = q·E,

and will, thus, accelerate, with a = F/m

Here, gravitational force is 15 orders of magnitude weaker than electric force. Therefore we are going to omit the gravitational force connected with electron or proton in future calculations.

White board example 23.5.

An electron (mass, me = 9.1·10-31 kg) is accelerated in the uniform electric field

(E = 5.0·104 N/C) between two parallel charged plates separated by a distance 1.5

cm. The electron is accelerated from rest near the negative plate and passes

through a tiny hole in the positive plate.

(a) Is the gravitational force important in this

problem?

(b) With what speed does the electron leave the hole?

Review: • Electric charge - positive, negative

• Charge is conserved

• Charge is multiple of e

• Conductors, Insulators

• Coulombs law

• Force between point charge distributions (know how to calculate)

• Superposition principle

• Electric field

• Electric field of a point charge distribution (know how to calculate)

• Electric field of a continuous charge distribution (know ‘simple’ cases)

• Electric field lines

• Motion of a charges in a uniform electric field