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Welcome…

Welcome…. …to Physics 35. PHYSICS 35 College Physics II Fall 2004 Dr. Allan Pringle Course Instructor Room 122 Physics, 341‑4031 pringle@umr.edu http://www.umr.edu/~pringle/phys35. Important Note.

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  1. Welcome… …to Physics 35.

  2. PHYSICS 35 College Physics II Fall 2004 Dr. Allan Pringle Course Instructor Room 122 Physics, 341‑4031 pringle@umr.edu http://www.umr.edu/~pringle/phys35

  3. 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.”

  4. 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. 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.

  5. 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.

  6. 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.

  7. 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.

  8. The following table summarizes the points available during the course: Three Exam Scores 600 Eight Quizzes 120 Boardwork 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.

  9. 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.

  10. 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.

  11. 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.

  12. 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.

  13. 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).

  14. Prologue Things you recall from “last”* semester:  Newton’s Laws  energy and its conservation *or whenever you took your previous physics class

  15.  momentum and its conservation (linear and angular)  laws of thermodynamics well, maybe not… (we didn’t do much with thermodynamics) These “things” aren’t going to go away!

  16. 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.

  17. Electric Charge and The Electric Field Static Electricity (Demo) There are two kinds of charge. - + Properties of charges  like charges repel  unlike charges attract  charges can move but charge is conserved Law of conservation of charge: the net amount of electric charge produced in any process is zero.

  18. Although there are two kinds of charge in an atom, electrons are the charges that usuallymove 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. Charges are “quantized” (come in units of 1.6x10-19 C). 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?

  19. 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.

  20. Conductors and Insulators http://www.maps.jcu.edu.au/course/CAUTscience/elec/elec02a.html 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.

  21. Induced Charge: The Electroscope Demonstrate (if electroscope available). Coulomb’s Law We’ve seen attractive and repulsive electrical forces at work. Coulomb’s law quantifies the magnitude of the electrostatic force. 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. OSE: attractive for unlike

  22. 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 1/40, where 0=8.85x10-12 C2/N·m2. 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! OSE: attractive for unlike I have to do it this way because we aren’t using unit vectors. Later I’ll show a better way to write it.

  23. r - + + - + - + - 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!).

  24. 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. 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. OSE: attractive for unlike

  25. y Q3=+65C 30 cm 60 cm =30º x Q1=-86C Q2=+50C 52 cm Solving Problems Involving Coulomb’s Law and Vectors You may wish to review vectors. Example 16-4: Calculate the net electrostatic force on charge Q3 due to the charges Q1 and Q2.

  26. 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.

  27. F32 F31 y Q3=+65C 30 cm 60 cm =30º x Q1=-86C Q2=+50C 52 cm 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.

  28. F32 F31 y Q3=+65C 30 cm 60 cm =30º x Q1=-86C Q2=+50C 52 cm Step 2: OSE <complaining> ”Do I have to put in the absolute value signs?” Yes.

  29. F32 F31 y Q3=+65C 30 cm 60 cm =30º x Q1=-86C Q2=+50C 52 cm Step 3: Replace Generic Quantities by Specifics (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.

  30. F32 F31 y Q3=+65C 30 cm 60 cm =30º x Q1=-86C Q2=+50C 52 cm Step 3 (continued) (+ sign comes from diagram) (- sign comes from diagram) Can you put numbers in at this point? OK for this problem. You would get F31,x = +120 N and F31,y = -70 N.

  31. F32 F3 F31 y Q3=+65C 30 cm 60 cm =30º x You know how to calculate the magnitude F3 and the angle between F3 and the x-axis. (If not, holler!) Q1=-86C Q2=+50C 52 cm Step 4: Complete the Math The net force is the vector sum of all the forces on Q3. F3x = F31,x + F32,x = 120 N + 0 N = 120 N F3y = F31,y + F32,y = -70 N + 330 N = 260 N

  32. 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 - + 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: attractive for unlike OSE: Q2 r12 Much more satisfying! Q1

  33. 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…

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