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Physics 212 Lecture 1: Coulomb's Law and Superposition

This lecture covers the concepts of Coulomb's Law and superposition in Physics 212. Make sure to complete the prelectures and checkpoints before the lecture. Bring a pen and notebook.

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Physics 212 Lecture 1: Coulomb's Law and Superposition

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  1. Welcome to Physics 212 http://online.physics.uiuc.edu/courses/phys212 This lecture is VERY full. Please sit next to someone nice. (There won’t be empty seats!). Festival seating up front !! 01

  2. Physics 212 Lecture 1 Today's Concepts: a) Coulomb’s Law b) Superposition 03

  3. Physics 212 - Intro http://online.physics.uiuc.edu/courses/phys212 • Everything can be found online – GO THERE • You have web based Prelectures and Checkpoints duebefore every lecture. • You have web based homework due every week. • Be sure to complete Prelecture 2 and Checkpoint 2 before 8 am on Thursday. • Bring pen & notebook to lecture – you will be working on problems • Register your clicker ASAP so you can see your participation grade. • Labs & Discussions • Discussion Sections meet this week • Labs start Jan. 23, must have prelab done before! 05

  4. Prelectures: 50 Checkpoints: 25 Lecture participation: 25 Your top 22 HW/Quiz scores determine your grade out of 250. Bonus Points: You can earn up to 1 extra bonus point in every lecture (for a maximum of 25 bonus points for the semester) by getting the right answers to all of the clicker questions. At the end of the semester your lecture bonus points are added to your HW/Quiz score (250 max). You can make up for several bad quizzes by answering correctly in class. How your grade will be calculated 06

  5. Q: What are the benefits of participating ? A: You learn more Data from last year’s first exam 07

  6. Big Brother is Watching • How Do We Distinguish “Viewers” from “Non-Viewers”? • We log all keystrokes !! Results from today’s PreLecture

  7. Clickers • Research shows you learn more when you talk! • 1 point for each lecture • Maximum of 25 points! • (No EX’s but there are 28 lectures) • 0.1 bonus points per correct answer (usually > 10 questions/lecture) maximum 1 point/lecture total • up to 25 points • (No EX’s but there are 28 lectures) 09

  8. Music • Who is the Artist? • Buddy Guy • RyCooder • John Lee Hooker • TajMahal • Willie Dixon Example of Krannert Center fare: Ellnora Guitar Festival (Sept. 2011) Physics 212 Lecture 1

  9. One Other Key Piece: Homework 1 1 11

  10. Homework: Delayed Feedback These questions serve as a test of your understanding of the questions posed as immediate feedback. After first deadline Delayed Feedback questions turn into “immediate feedback questions”. 80% credit can be obtained by answering these questions correctly before the second deadline. Purpose: Promote REFLECTION 11

  11. q1 q1 q1 q2 Opposite signs attract Coulomb’s Law: The force on a charge due to another charge is proportional to the product of the charges and inversely proportional to the separation squared. q2 r The force is always parallel to a line connecting the charges, but the direction depends on the signs of the charges: q2 Like signs repel q2 q1 11

  12. Balloons Take two balloons and rub them both with a piece of cloth. After you rub them they will: • Attract each-other • Repel each-other • Either – it depends on the material of the cloth 13

  13. Balloons If the same thing is done to both balloons they will acquire the same sign charge. They will repel ! + + + + + + + + 14

  14. Our notation is the force by 1 on 2 (think “by-on ”) is the unit vector that points from 1 to 2. q1 q2 q1 q2 Coulomb’s Law Examples If the charges have the same sign, the force by charge 1 on charge 2 would be in the direction of r12 (to the right) If the charges have opposite sign, the force by charge 1 on charge 2 would be opposite the direction of r12 (left) What's up with r-hat? I don't understand why it is necessary in the equation. 17

  15. Example: Coulomb Force • Two paperclips are separated by 10 meters. Then you remove 1 electron from each atom on the first paperclip and place it on the second one. k= 9 x 109 N m2 / C2 electron charge = 1.6 x 10-19 Coulombs NA = 6.02 x 1023 What will the direction of the force be? A) Attractive B) Repulsive 17

  16. Example: Coulomb Force • Two paperclips are separated by 10 meters. Then you remove 1 electron from each atom on the first paperclip and place it on the second one. k= 9 x 109 N m2 / C2 electron charge = 1.6 x 10-19 Coulombs NA = 6.02 x 1023 • Which weight is closest to the approximate force between those paperclips (recall that weight = mg, g = 9.8 m/s2)? • Paperclip (1g x g) • Text book (1kg x g) • Truck (104 kg x g) • Aircraft carrier (108 kg x g) • Mt. Everest (1014 kg x g) 22

  17. Two charges, q with +1 mC and Q with +10 mC are placed near each other as shown in the figure. Which of the following diagrams best depicts the forces acting on the charges? Checkpoint1 The forces are in opposite directions because they have like charges. The arrow on the +10 is longer because it has a greater magnitude than the +1 Determining the force of one charge on the other requires that the charges be multiplied. The order of multiplication is irrelevant. Since all other factors are constant between the two, the charges must be equal. Also, Newton's Third Law. Because the right hand sphere has a charge of +10C, it would impact the smaller sphere by -10C and visa versa. 24

  18. q2 F1 F2,1 F4,1 F3,1 F2,1 q1 F4,1 F1 F3,1 q4 q3 Superposition: If there are more than two charges present, the total force on any given charge is just the vector sum of the forces due to each of the other charges: 28

  19. Superposition ACT q2 q1 q4 q3 What happens to Force on q1 if its sign is changed • |F1| increases • |F1| remains the same • |F1| decreases • Need more information to determine 30

  20. q2 q2 F4,1 F4,1 F2,1 F2,1 q1 F1 F1 F3,1 F3,1 q4 q4 q3 q3 The direction of all forces changes by 180o – the magnitudes stay the same: q1 F1 F1 F2,1 F2,1 F3,1 F3,1 F4,1 F4,1 33

  21. Try using our Coulomb Force simulation (“simulations” link on the homepage)

  22. Checkpoint +Q +Q q q F1 +Q -Q The vector sum of the coulomb forces in case 1 results in an effectively doubled force, while the case 2 coulomb forces would cancel each other out. Case 1 Case 2 Because in Case 2, both charges have the same sign, the charge in the middle will have a net force pointing downwards, whereas in Case 1 the charge in the middle does not have a net force exerted on it. they are all the same because magnitude of force is kqq/r^2 • Compare the magnitude of the net force on q in the two cases. • |F1 | > |F2| • |F1 | = |F2| • |F1 |< |F2| • Depends on sign of q F2 = 0 33

  23. Checkpoint Four charged particles are placed on a circular ring with radius 3 m as shown below. A particle with charge Q is placed in the center of the ring y What is the direction of horizontal force on Q? A) Fx > 0 B) Fx = 0 C) Fx < 0 3q x Q q q q Great Job ! 41

  24. Checkpoint The magnitude and y component of the particle above the origin is greater than the magnitudes and y components of the particles below the origin. They are equal magnitudes opposite directions which cancel out The outer two charges exert only a fraction of their force in the y direction, therefore, the sum of the three lower charges must be less than the upper charge who's force is three times as large. 3q Four charged particles are placed on a circular ring with radius 3 m as shown below. A particle with charge Q is placed in the center of the ring y Q What is vertical force on Q? A) Fy > 0 B) Fy = 0 C) Fy < 0 x q q q 47

  25. See you Thursday! • Discussion Sections meet this week! • Be sure to complete Prelecture 2 and Checkpoint2 • Labs begin Jan. 23

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