Lectures_Lect01

Lectures_Lect01 - Welcome to Physics 212...

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Unformatted text preview: 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!). 01 Festival seating up front !! Physics 212 Lecture 1, Slide 1 Physics 212 Lecture 1 Today's Concepts: a) Coulomb's Law b) Superposition 03 Physics 212 Lecture 1, Slide 2 http://online.physics.uiuc.edu/courses/phys212 Physics 212 Intro Everything can be found online GO THERE You have web based Prelectures and Checkpoints due before every lecture. You have web based homework due every week. Be sure to complete Prelecture 2 and Checkpoint 2 before 8 am on Tuesday. 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 Thursday June 16. You must have the prelab done before! Physics 212 Lecture 1, Slide 3 05 How your grade will be calculated Prelectures + Preflights + Lectures 14 Homework + 10 Quizzes Labs Hour exams (3 x 100 each) Final Exam 100 250 150 300 200 Your top 22 HW/Quiz scores determine your grade out of 250. Prelectures: 50 Checkpoints: 25 Lecture participation: 25 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. 06 Physics 212 Lecture 1, Slide 4 Q: What are the benefits of participating ? A: You learn more 30 Viewer 25 Non-Viewer Data from last year's first exam 20 15 10 5 0 43 48 52 57 61 65 70 74 78 83 87 91 96 100 Exam Score Students who... Viewed prelectures Blew through prelectures 07 Exam 1 average 80% 73% Physics 212 Lecture 1, Slide 5 Big Brother is Watching How Do We Distinguish "Viewers" from "NonViewers"? We log all keystrokes !! Results for today's lecture Physics 212 Lecture 1, Slide 6 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) Maximum 1 bonus point/lecture 1. Power On 2. Hold ON/OFF until POWER light flashes 3. Press BB 09 Physics 212 Lecture 1, Slide 7 One Other New Thing: Homework points Immediate feedback questions 1.0 performance 1.0 1.0 Your answer has been judged correct; the exact answer is: 1.0078643271894E+00 11 Physics 212 Lecture 1, Slide 8 Homework: Delayed Feedback These questions serve as a test of your understanding of the questions posed as immediate feedback. Purpose: Promote REFLECTION 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. Physics 212 Lecture 1, Slide 9 11 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 q1 r q1q2 F 2 r The force is always parallel to a line connecting the charges, but the direction depends on the signs of the charges: q2 q2 q2 11 q1 q1 q1 Opposite signs attract Like signs repel Physics 212 Lecture 1, Slide 10 Balloons Take two balloons and rub them both with a piece of cloth. After you rub them they will: A) Attract eachother BB B) Repel eachother C) Either it depends on the material of the cloth 13 Physics 212 Lecture 1, Slide 11 Balloons If the same thing is done to both balloons they will acquire the same sign charge. They will repel ! + + + + + + + + 14 Physics 212 Lecture 1, Slide 12 Coulomb's Law Our notation F1,2 is the force by 1 on 2 (think "byon ") ^ r12 is the unit vector that points from 1 to 2. kq1q2 ^ F1,2 = 2 r1,2 r1,2 Examples If the charges have the same sign, the force by charge 1 would be in the direction of r12 (to the right) q1 on charge 2 r1,2 q2 F1,2 If the charges have opposite sign, the force by charge 1 would be opposite the direction of r12 (left) on charge 2 q1 F1,2 17 r1,2 q2 Physics 212 Lecture 1, Slide 13 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. BB q1q2 F = k 2 r12 ^ r12 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 Physics 212 Lecture 1, Slide 14 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. BB q1q2 F = k 2 r12 ^ r12 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)? A) Paperclip (1g x g) B) Text book (1kg x g) C) Truck (104 kg x g) D) Aircraft carrier (108 kg x g) E) Mt. Everest (1014 kg x g) 22 Physics 212 Lecture 1, Slide 15 Checkpoint 1 24 Physics 212 Lecture 1, Slide 16 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: Superposition: q2 F4,1 F1 F2,1 F4,1 F1 F3,1 q1 F3,1 F2,1 q4 q3 F1 = F2,1 + F3,1 + F4,1 + ... 28 Physics 212 Lecture 1, Slide 17 BB Superposition ACT What happens to Force on q1 if its sign is changed A) B) C) D) |F1| increases |F1| remains the same |F1| decreases Need more information to determine q2 q1 q4 q3 30 Physics 212 Lecture 1, Slide 18 The direction of all forces changes by 180o the magnitudes stay the same: q2 F4,1 F1 F4,1 q2 q1 F3,1 F2,1 q1 F3,1 F2,1 F1 q4 F2,1 F4,1 F1 F3,1 q4 F2,1 F4,1 F1 F3,1 q3 q3 F1 = F2,1 + F3,1 + F4,1 + ... 33 - F1 = - F2,1 - F3,1 - F4,1 - ... Physics 212 Lecture 1, Slide 19 Try using our Coulomb Force simulation ("simulations" link on the homepage) Physics 212 Lecture 1, Slide 20 BB Checkpoint +Q +Q Compare the magnitude of the net force on q in the two cases. A) B) C) D) |F1 | > |F2| |F1 | = |F2| |F1 |< |F2| Depends on sign of q q F1 -Q q F2 = 0 +Q Case 1 Case 2 33 Physics 212 Lecture 1, Slide 21 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 41 Physics 212 Lecture 1, Slide 22 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. Assume that all charges in the problem have the same sign. q BB 3q Q y x q q What is vertical force on Q? A) Fy > 0 B) Fy = 0 C) Fy < 0 47 Physics 212 Lecture 1, Slide 23 Discussion Sections meet this week. Be sure to complete Prelecture 2 and Checkpoint2. Labs begin June 16 Physics 212 Lecture 1, Slide 24 ...
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