Welcome to Physics 212

Transcription

1 Welcome to Physics Physics 212 Lecture 1, Slide 1

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

3 Physics Intro Everything can be found online 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 14. You must have the prelab done before! 05 Physics 212 Lecture 1, Slide 3

4 How your grade will be calculated Prelectures + Checkpoints + Lectures Homework + 10 Quizzes 250 Labs 150 Hour exams (3 x 100 each) 300 Final Exam 200 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. 06 Physics 212 Lecture 1, Slide 4

5 Q: What are the benefits of participating? A: You learn more Viewer Non-Viewer Data from last year s first exam Exam Score Students who Exam 1 average Viewed pre-lectures 80% Blew through pre-lectures 73% 07 Physics 212 Lecture 1, Slide 5

6 Big Brother is Watching How Do We Distinguish Viewers from Non-Viewers? We log all keystrokes!! Results for today s lecture Physics 212 Lecture 1, Slide 6

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) 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

8 Homework Immediate feedback questions points performance Your answer has been judged correct; the exact answer is: E Physics 212 Lecture 1, Slide 8

9 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. 11 Physics 212 Lecture 1, Slide 9

10 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. q 2 r q 1 F qq r The force is always parallel to a line connecting the charges, but the direction depends on the signs of the charges: q 2 q 1 Opposite signs attract q 2 q 2 q 1 q 1 Like signs repel 11 Physics 212 Lecture 1, Slide 10

11 Balloons BB Take two balloons and rub them both with a piece of cloth. After you rub them they will: A) Attract each-other B) Repel each-other C) Either it depends on the material of the cloth 13 Physics 212 Lecture 1, Slide 11

12 Balloons If the same thing is done to both balloons they will acquire the same sign charge. They will repel! Physics 212 Lecture 1, Slide 12

13 Coulomb s Law kq q 1 2 1,2 2 1,2 Our notation r1,2 is the force by 1 on 2 (think by-on ) F 1,2 ˆr 12 F is the unit vector that points from 1 to 2. rˆ Examples If the charges have the same sign, the force by charge 1 on charge 2 would be in the direction of r 12 (to the right) q 1 r q 1,2 2 F 1,2 If the charges have opposite sign, the force by charge 1 on charge 2 would be opposite the direction of r 12 (left) F 1,2 q 1 q 2 17 r 1,2 Physics 212 Lecture 1, Slide 13

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. qq F k rˆ r k= 9 x 10 9 N m 2 / C 2 electron charge = 1.6 x Coulombs N A = 6.02 x BB What will the direction of the force be? A) Attractive B) Repulsive 17 Physics 212 Lecture 1, Slide 14

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. qq F k rˆ r k= 9 x 10 9 N m 2 / C 2 electron charge = 1.6 x Coulombs N A = 6.02 x Which weight is closest to the approximate force between those paperclips (recall that weight = mg, g = 9.8 m/s 2 )? A) Paperclip (1g x g) B) Text book (1kg x g) C) Truck (10 4 kg x g) D) Aircraft carrier (10 8 kg x g) E) Mt. Everest (10 14 kg x g) BB 22 Physics 212 Lecture 1, Slide 15

16 Checkpoint 1 24 Physics 212 Lecture 1, Slide 16

17 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: q 2 F 1 F 2,1 F 3,1 F 4,1 q 1 F 2,1 F 4,1 F 1 F 3,1 q 4 q 3 F 1 F2,1 F3,1 F4, Physics 212 Lecture 1, Slide 17

18 Superposition ACT BB What happens to Force on q 1 if its sign is changed A) F 1 increases B) F 1 remains the same C) F 1 decreases D) Need more information to determine q 2 q 1 q 4 q 3 30 Physics 212 Lecture 1, Slide 18

19 The direction of all forces changes by 180 o the magnitudes stay the same: q 2 q 2 F 4,1 F 4,1 q 1 F 2,1 q 1 F 2,1 F 1 F 1 F 3,1 q 4 F 3,1 q 4 q 3 F 1 F 2,1 F 3,1 q 3 F 1 F 2,1 F 3,1 F 4,1 F 4,1 F 1 F2,1 F3,1 F4,1... F 1 F2,1 F3,1 F4, Physics 212 Lecture 1, Slide 19

20 BB Checkpoint Compare the magnitude of the net force on q in the two cases. A) F 1 > F 2 +Q q +Q q B) F 1 = F 2 C) F 1 < F 2 D) Depends on sign of q -Q F 1 F 2 = 0 +Q Case 1 Case 2 33 Physics 212 Lecture 1, Slide 20

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? 3q x A) F x > 0 B) F x = 0 C) F x < 0 Q q q q 41 Physics 212 Lecture 1, Slide 21

22 Checkpoint 3q BB 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 y x q q q What is vertical force on Q? A) F y > 0 B) F y = 0 C) F y < 0 47 Physics 212 Lecture 1, Slide 22

23 Discussion Sections meet this week. Be sure to complete Prelecture 2 and Checkpoint2. Labs begin June 14 Physics 212 Lecture 1, Slide 23

24 Try using our Coulomb Force simulation ( simulations link on the homepage) Physics 212 Lecture 1, Slide 24