Welcome…

…to Physics 35.

PHYSICS 35College Physics II

 

Fall 2004 

Dr. Allan PringleCourse Instructor

Room 122 Physics, 341‑4031

pringle@umr.eduhttp://www.umr.edu/~pringle/phys35

Important Note

The next few slides summarize important information contained in the course syllabus. Please refer to the syllabus for details.

If there is any discrepancy between these notes and the syllabus, the syllabus is the “official word.”

Text

The text is Physics, fifth edition, by Douglas Giancoli, Prentice-Hall, publisher. This book is written for students taking an introductory course in physics, and uses algebra and trigonometry but not calculus.

Course Description

Physics 35 is a 3-hour, algebra-based introductory physics course. This syllabus covers the second semester of the two-semester sequence. You will be introduced to the fundamental ideas of physics, including electricity, magnetism, and light. The prerequisite for this course is a grade of C or better in a Math 6 or its equivalent. Students lacking the prerequisite may enroll with consent of the instructor.

Course Schedule Physics 35 meets from 8:00-8:50, Monday, Wednesday, and Friday. A preliminary set of homework assignments through the first exam will be handed out along with this syllabus.

There are three scheduled hour examinations and a final: 

Exam 1 -- Friday, September 17.Exam 2 -- Friday, October 15.Exam 3 -- Friday, November 12.

Comprehensive Final Examination. Tuesday, December 14, 8:00 a.m.-10:00 a.m.

The hour exam dates may be changed, if appropriate.

Hour Examinations and Final Exam The 4 exams will be worth 200 points each.

The exams will cover concepts and definitions, assigned problems with minor numerical changes, and problems similar to those assigned but requiring a deeper understanding of concepts or more complex calculations.

You will be provided with an equation sheet and you may also use one 3"x5" card and any calculator containing any information you want. Note that assigned text material not covered in lecture is testable.

Course Grades The lowest of the four exam scores will be dropped. (The Final can be dropped only if you attend 2/3 of end-of-semester classes!) There will be ten 15-point quizzes during the semester. The quizzes will cover recently-assigned reading material and problems. Your two lowest quiz scores will be dropped. A number of class periods will be devoted in part to student presentation of their homework at the blackboard. A maximum of 80 points will be given for boardwork.

The following table summarizes the points available during the course:

Three Exam Scores 600Eight Quizzes 120Boardwork 80

Total 800

Letter grades for Physics 35 will be assigned as follows:

716 - up A (89.50%)636 - 715 B (79.50%)556 - 635 C (69.50%)476 - 555 D (59.50%)Below 476 F

There is no limit to the number of A's, B's, etc.

Make-Up Exam Policy

There will be no special make-ups in this course. The dropping of the lowest score is intended to accommodate students who miss one exam due to hospitalization, illness, family emergencies, mental stress, athletic events, etc. This also has the side effect of benefiting students who under perform on one exam, and students who are satisfied with their grade and wish to skip the final exam.

See the syllabus for procedures for incompletes and for taking an exam if you are out of town on an official university event.

Important Dates Student Council Free Day is Friday, October 1, 2004.

The last day to drop this class without a withdrawal showing on your transcript is Tuesday, October 5, 2004.

The last day to drop this class is Tuesday, November 16, 2004.

Drop Policy

Any student who has missed a total of 4 graded assignments of any kind and has an average score of less than 69.50% on graded assignments may be dropped at any time.

Homework and the PLC Homework help will be available at the Physics Learning Center (PLC). You may be able to excel in the course without ever setting foot in the PLC. You may need to spend many hours in the PLC every week just to receive a passing grade.

Physics 35 students may wish to organize their own PLC. The PLC is open from 2:30-5:00 p.m. and 6:00-8:30 p.m. The PLC operates in rooms 129-130 of Physics, with Physics 24 (and 35) help available on Mondays and Wednesdays. Individual tutors are also available. Visit the web site http://www.umr.edu/~tutors for up-to-date information.

Regrade Requests If you want a quiz or exam problem regraded, please write the reason for the request on a sheet of paper, staple it to the exam or quiz, and return it to me within one week from the time at which the exam or quiz started.

Specify which problem you want regraded, and provide a detailed written statement as to why the original work which appeared on the exam deserves more points. Don't wait until the day the final grades are due and ask for Exam 1 to be regraded. Don't wait until after the final exam and ask that grade cutoffs be lowered by 1 point so you can get the next higher grade; it won’t happen. However, scoring mistakes (points added up wrong, score recorded incorrectly) can be corrected at any time.

E-Mail 

You can send e-mail to me at pringle@umr.edu. Students frequently send me e-mail from accounts which are not their UMR e-mail accounts (e.g., a friend's computer, a Hotmail account). If you send e-mail from such an account and want a reply to your UMR account, be sure to include you UMR e-mail address.

Unresolved Complaints

It is hoped that any complaints about the course can be resolved in a collegial manner through discussions between student and instructor. However, if there are any complaints that cannot be resolved, you may take them up with the Physics Department Chairman, Dr. Paul Parris (parris@umr.edu) or the Dean of the College of Arts and Sciences, Dr. Paula Lutz (plutz@umr.edu).

Prologue

Things you recall from “last”* semester:

*or whenever you took your previous physics class

Newton’s Laws

F ma

energy and its conservation

21KE mv

2 2

spring

1U ks

2gravU mgy

fi other i f

E E W E K U

laws of thermodynamics

momentum and its conservation (linear and angular)

p mv z zL I

SSSSSSSSSSSSSSSSSSSSSSSSSSSS

fiP P z,f z,iL L

well, maybe not… (we didn’t do much with thermodynamics)

These “things” aren’t going to go away!

This semester we study electromagnetic forces and their consequences.

These forces are responsible for holding together living and man-made things, so I suppose they are worth studying…

…not to mention the fact that the technology that dominates your life depends on electromagnetic forces.

like charges repel unlike charges attract

charges can move but charge is conservedLaw of conservation of charge: the net amount of electric charge produced in any process is zero.

There are two kinds of charge.

+ -

Electric Charge and The Electric Field

Static Electricity (Demo)

Properties of charges

Although there are two kinds of charge in an atom, electrons are the charges that usually move around.

A proton is roughly 2000 times more massive than an electron.

The charge of an electron is -1.6x10-19 coulombs.

The charge of a proton is +1.6x10-19 coulombs.

Nitpicking: electric charge is a property of matter, not a “thing” in itself. It is “not good” to say “like charges repel.” It is “good” to say “like-charged particles repel.” I choose the “not good” terminology here to be consistent with your text.And who’s the clown who decided electrons have negative charges?

Charges are “quantized” (come in units of 1.6x10-

19 C).

And yes, he really flew the kite in the thunderstorm. See here.Franklin’s experiments showed him that there were two “kinds” of charge, which he named “positive” and “negative.” More than a century later we learned that negative charges are associated with electrons.

Oh, and the next two people who tried the kite experiment were killed in the process.

Conductors and Insulators

There is no such thing as a perfect conductor.

A superconductor has no resistance to the flow of current, but is not the same as a perfect conductor.There is also no such thing as a perfect insulator.

http://www.maps.jcu.edu.au/course/CAUTscience/elec/elec02a.html

Induced Charge: The Electroscope

Demonstrate (if electroscope available).

Coulomb’s Law

1 2212

Q QF k ,12 r

attractive for unlike OSE:

Coulomb’s law gives the force between charges Q1 and Q2 (in coulombs), where r is the distance in meters between the charges, and k=9x109 N·m2/C2.

We’ve seen attractive and repulsive electrical forces at work.

Coulomb’s law quantifies the magnitude of the electrostatic force.

To make this into a “really good” OSE I should specify “repulsive for like,” but that makes it too wordy. You’ll just have to remember that!

1 2212

Q QF k ,12 r

attractive for unlike OSE:

Force is a vector quantity, so the equation by itself gives the magnitude of the force. Note how the direction is specified. If the charges are opposite in sign, the force is attractive; if the charges are the same in sign, the force is repulsive. Also, the constant k is equal to 1/40, where 0=8.85x10-12 C2/N·m2.

I have to do it this way because we aren’t using unit vectors. Later I’ll show a better way to write it.

The equation as written is valid for point charges. If the charged objects are spherical and the charge is uniformly distributed, r is the distance between the centers of the spheres.

If more than one charge is involved, the net force is the vector sum of all forces (superposition—let’s not make that a required OSE). For objects with complex shapes, you must add up all the forces acting on each separate charge (turns into calculus!).

+ -

r

++

+-

--

We could have agreed that in this formula, the symbols Q1 and Q2 stand for the magnitudes of the charges. In that case, the absolute value signs would be unnecessary.

1 2212

Q QF k ,12 r

attractive for unlike OSE:

However, in later OSE’s the sign of the charge will be important, so we really need to keep the magnitude part.

On your homework diagrams, show both the magnitudes and signs of Q1 and Q2.

Solving Problems Involving Coulomb’s Law and Vectors

You may wish to review vectors.

x

y

Q2=+50C

Q3=+65C

Q1=-86C

52 cm

60 cm

30

cm

=30º

Example 16-4: Calculate the net electrostatic force on charge Q3 due to the charges Q1 and Q2.

Step 0: Think!

This is a Coulomb’s Law problem (all we have to work with, so far).

We only want the forces on Q3. Don’t worry about other forces.

Forces are additive, so we can calculate F32 and F31 and add the two.

If we do our vector addition using components, we must resolve our forces into their x- and y-components.

Step 1: Diagram

Draw a representative sketch—done.

Draw and label relevant quantities—done.

Draw axes, showing origin and directions—done.Draw and label forces (only those on Q3).Draw components of forces which are not along axes.

x

y

Q2=+50C

Q3=+65C

Q1=-86C

52 cm

60 cm

30

cm

=30º

F31

F32

Step 2: OSE

1 2212

Q QF k , attractive/ repulsive12 r

<complaining> ”Do I have to put in the absolute value signs?”

x

y

Q2=+50C

Q3=+65C

Q1=-86C

52 cm

60 cm

30

cm

=30º

F31

F32

Yes.

Step 3: Replace Generic Quantities by Specifics

3 2232

Q QF k ,32 r

repulsive

3 2232

Q QF k32,y r

F 032,x(from diagram)

Can you put numbers in at this point? OK for this problem. You would get F32,y = 330 N and F32,x = 0 N.

x

y

Q2=+50C

Q3=+65C

Q1=-86C

52 cm

60 cm

30

cm

=30º

F31

F32

Step 3 (continued)

3 1231

Q QF k ,31 r

attractive

3 1231

Q QF k cos31,x r

Can you put numbers in at this point? OK for this problem. You would get F31,x = +120 N and F31,y = -70 N.

(- sign comes from diagram) 3 1231

Q QF k sin31,y r

(+ sign comes from diagram)

x

y

Q2=+50C

Q3=+65C

Q1=-86C

52 cm

60 cm

30

cm

=30º

F31

F32

Step 4: Complete the Math

F3x = F31,x + F32,x = 120 N + 0 N = 120 N

F3y = F31,y + F32,y = -70 N + 330 N = 260 N

You know how to calculate the magnitude F3 and the angle between F3 and the x-axis. (If not, holler!)

F3The net force is the vector sum of all the forces on Q3.

x

y

Q2=+50C

Q3=+65C

Q1=-86C

52 cm

60 cm

30

cm

=30º

F31

F32

Remember I said I’d show you a better way to write Coulomb’s law? I have to write the OSE like this because you are not supposed to have to use unit vectors:

1 2212

Q QF k ,12 r

attractive for unlike OSE:

If you define as the unit vector (a vector of unit length) pointing from Q1 to Q2, and F12 is the force on Q1 due to Q2, and you include the correct signs in Q1 and Q2, then

r̂12

1 2122

12

Q QˆF k r12 r

+

-

r12

Q1

Q2

Much more satisfying!

I did a sample Coulomb’s law calculation using three point charges.

How do you apply Coulomb’s law to objects that contain distributions of charges?

We need another tool to do that…