Welcome to Physics 212 http online physics uiuc

Welcome to Physics 212 http: //online. physics. uiuc. edu/courses/phys 212 01 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

Physics 212 - Intro http: //online. physics. uiuc. edu/courses/phys 212 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 • • 05 Discussion Sections meet this week Labs start Thursday June 14. You must have the prelab done before! Physics 212 Lecture 1, Slide 3

How your grade will be calculated Prelectures + Checkpoints + Lectures 100 14 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

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

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

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

Homework points Immediate feedback questions performance 1. 0 Your answer has been judged correct; the exact answer is: 1. 0078643271894 E+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 11 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

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

BB Balloons 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

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 is the force by 1 on 2 (think “by-on ”) is the unit vector that points from 1 to 2. 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 q 2 If the charges have opposite sign, the force by charge 1 on charge 2 would be opposite the direction of r 12 (left) q 1 17 q 2 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. k= 9 x 109 N m 2 / BB C 2 electron charge = 1. 6 x 10 -19 Coulombs NA = 6. 02 x 1023 What will the direction of the force be? A) Attractive 17 B) Repulsive 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 k= 9 x 109 N m 2 / C 2 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/s 2)? A) Paperclip (1 g x g) B) Text book (1 kg 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

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 4, 1 F 1 q 1 F 3, 1 F 2, 1 F 4, 1 q 4 q 3 28 Physics 212 Lecture 1, Slide 17

BB Superposition ACT 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 q 2 D) Need more information to determine q 1 q 4 q 3 30 Physics 212 Lecture 1, Slide 18

The direction of all forces changes by 180 o – the magnitudes stay the same: q 2 F 4, 1 F 1 q 1 F 2, 1 F 3, 1 q 3 F 2, 1 q 3 F 3, 1 F 4, 1 33 F 2, 1 F 3, 1 q 4 F 2, 1 F 3, 1 F 4, 1 Physics 212 Lecture 1, Slide 19

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

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 3 q x C) Fx < 0 Q q q q 41 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. Assume that all charges in the problem have the same sign. BB 3 q y Q x q q q What is vertical force on Q? A) Fy > 0 47 B) Fy = 0 C) Fy < 0 Physics 212 Lecture 1, Slide 22

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

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