Welcome to Physics 212 http://online.physics.uiuc.edu/courses/phys212 01

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

Physics 212 - Intro Everything can be found online Labs & Discussions http://online.physics.uiuc.edu/courses/phys212 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

Prelectures + Checkpoints + Lectures How your grade will be calculated Prelectures: 50 Checkpoints: 25 Lecture participation: 25 Prelectures + Checkpoints + Lectures 100 14 Homework + 10 Quizzes 250 Labs 150 Hour exams (3 x 100 each) 300 Final Exam 200 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

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

Big Brother is Watching How Do We Distinguish “Viewers” from “Non-Viewers”? We log all keystrokes !! Results for today’s lecture

2. Hold ON/OFF until POWER light flashes 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

Homework Immediate feedback questions points performance 11 1.0 Your answer has been judged correct; the exact answer is: 1.0078643271894E+00 11

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

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 q1 r The force is always parallel to a line connecting the charges, but the direction depends on the signs of the charges: q1 q2 Opposite signs attract q2 q1 Like signs repel q2 q1 11

Balloons BB 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

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

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 r12 (to the right) q1 q2 If the charges have opposite sign, the force by charge 1 on charge 2 would be opposite the direction of r12 (left) q1 q2 17

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 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 F = 9e9 x (1.6e-19 * 3e22)^2 / 100 = 2 e 15 Newtons Equivalent to weight of 2e14 kg object near surface of the earth paperclip 1e-3 kg textbook 1 kg person 200 kg car 2000 kg Aircraft carrier 97000 tons = 1e5 x 1e3 = 1e 8 kg Mt Everest 9000 m height,. Estimate volume as 1/3 h^3 = 1/3 * (9e3)^3 = 243 e 9 m^3. (if density = 1000 kg/m^3) = 2.43e14 kg. 17

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 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) F = 9e9 x (1.6e-19 * 3e22)^2 / 100 = 2 e 15 Newtons Equivalent to weight of 2e14 kg object near surface of the earth paperclip 1e-3 kg textbook 1 kg person 200 kg car 2000 kg Aircraft carrier 97000 tons = 1e5 x 1e3 = 1e 8 kg Mt Everest 9000 m height,. Estimate volume as 1/3 h^3 = 1/3 * (9e3)^3 = 243 e 9 m^3. (if density = 1000 kg/m^3) = 2.43e14 kg. 22

Checkpoint 1 24

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: q2 F2,1 F3,1 F4,1 F1 F4,1 F2,1 q1 F1 F3,1 q4 q3 28

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

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

BB Checkpoint +Q -Q q Case 1 Case 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 F1 F2 = 0 33

Checkpoint What is the direction of horizontal force on Q? 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

Checkpoint What is vertical force on Q? BB 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. Assume that all charges in the problem have the same sign. y Q x q q q What is vertical force on Q? A) Fy > 0 B) Fy = 0 C) Fy < 0 Next do calculation 47

Discussion Sections meet this week. Be sure to complete Prelecture 2 and Checkpoint2. Labs begin June 14

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