Lect01 - Welcome to Physics 212...

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!). Festival seating up front !! 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 Physics http://online.physics.uiuc.edu/courses/phys212 Everything can be found online – GO THERE Everything • You have web based pre-lectures and pre-flights due before every before lecture. lecture. • You have web based homework due every week. • Be sure to complete prelecture 2 and preflight 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 • • 05 Discussion Sections meet this week Labs start August 30th, must have prelab done before! Physics 212 Lecture 1, Slide 3 How your grade will be calculated Prelectures + Preflights + Lectures 100 14 Homework + 10 Quizzes 250 Labs 150 Hour exams (3 x 100 each) 300 Final Exam Prelectures: 50 50 Preflights: 25 25 Lecture participation: 25 Lecture participation 200 Your top 22 HW/Quiz scores determine your grade out of 250. 250. Bonus Points: You can earn up to 1 extra bonus point iin every n You extra lecture (for a maximum of 25 bonus points for the semester) by getting the right answers to all of the clicker questions. getting At the end of the semester your lecture bonus points are At added to your HW/Quiz score (250 max). You can make up for several bad quizzes by answering correctly in class. 06 06 Physics 212 Lecture 1, Slide 4 Q: What are the benefits of participating ? A: You learn more 30 Viewer Non-Viewer 25 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… Exam 1 average Viewed pre-lectures Blew through pre-lectures 07 80% 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 IMPORTANT: The class next door (at 2pm) also is using i>clickers We need to run at a different frequency 1. Power On 2. Hold ON/OFF until POWER light flashes 09 3. Press BB Physics 212 Lecture 1, Slide 7 Music BB Who is the Artist? A) B) C) D) E) Eric Clapton Bill Frisell Bill Frisell Jimmy Page Jeff Beck Buddy Guy Why? My vote for most memorable performance at My New Orleans Jazzfest by a big name! Jazzfest Somewhere over the Rainbow People Get Ready Amazing…. Physics 212 Lecture 1, Slide 8 One Other New Thing: Homework points Immediate feedback questions 11 performance Physics 212 Lecture 1, Slide 9 Homework: Delayed Feedback Purpose: Purpose: Promote REFLECTION REFLECTION These questions serve as a These test of your understanding of the questions posed as immediate feedback. After first deadline After Delayed feedback questions Delayed 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 10 10 Coulomb’s Law: The force on a charge due to another charge is proportional The 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, The but the direction depends on the signs of the charges: but q2 q2 q1 q2 11 q1 q1 Opposite signs attract Like signs repel Physics 212 Lecture 1, Slide 11 11 BB Balloons Take two balloons and rub them Take both with a piece of cloth. After you rub them they will: you A) Attract each-other B) Repel each-other C) Either – it depends on the material of the cloth Either 13 Physics 212 Lecture 1, Slide 12 12 Balloons If the same thing is done to both balloons If same thing they will acquire the same sign charge. same sign They will repel ! They + ++ + 14 + ++ + Physics 212 Lecture 1, Slide 13 13 Coulomb’s Law Our notation Our kq1q2 ˆ F1,2 = 2 r1,2 r1,2 F1,2 is the force by 1 on 2 (think “by-on ”) by on (think on ˆ r12 is the unit vector that points from 1 to 2. from to 2. Examples 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 r1,2 q2 F1,2 If the charges have opposite sign, the force by charge 1 on charge 2 would be opposite the direction of r12 (left) q1 F1,2 17 17 q2 r1,2 Physics 212 Lecture 1, Slide 14 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 q1 q2 F = k 2 r12 ˆ r12 k= 9 x k= 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? What A) Attractive 17 B) Repulsive B) Physics 212 Lecture 1, Slide 15 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. q1 q2 F = k 2 r12 ˆ r12 BB k= 9 x 109 N m2 / C2 k= 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) Text C) Truck (104 kg x g) kg D) Aircraft carrier (108 kg x g) (10 Aircraft E) Mt. Everest (1014 kg x g) kg 22 Physics 212 Lecture 1, Slide 16 16 Preflight 1 “The bigger charge will push the smaller charge harder than The the smaller will push the larger. “ 70 60 50 40 “According to Newton's Third Law, the charges exert equal According and opposite forces on each other. “ 30 20 10 0 A 24 B C “The forces of two objects are always equal and opposite. The both equasions involve the charge of both particles in the same manner, therefore they are the same. .” Physics 212 Lecture 1, Slide 17 17 Superposition: If there are more than two charges present, the total force If on any given charge is just the vector sum of the forces due vector of to each of the other charges: to q2 F2,1 F4,1 F1 q1 F3,1 F1 F3,1 F2,1 F4,1 q4 q3 F1 = F2,1 + F3,1 + F4,1 + ... 28 28 Physics 212 Lecture 1, Slide 18 Physics BB Superposition ACT What happens to Force on q1 if its sign is changed A) |F1| increases B) |F1| remains the same C) |F1| decreases q2 D) Need more information to determine q1 q4 q3 30 Physics 212 Lecture 1, Slide 19 Physics The direction of all forces changes by 180o – the magnitudes stay the same: The direction the magnitudes q2 q2 F4,1 F4,1 F1 q1 F2,1 F1 F3,1 q3 F2,1 F2,1 F3,1 q4 q3 F1 q1 q4 F2,1 F3,1 F3,1 F4,1 F1 = F2,1 + F3,1 + F4,1 + ... 33 33 F1 F4,1 − F1 = − F2,1 − F3,1 − F4,1 − ... Physics 212 Lecture 1, Slide 20 Physics Try using our Coulomb Force simulation (“simulations” link on the homepage) Physics 212 Lecture 1, Slide 21 Physics BB Preflight +Q Compare the magnitude of the net force on q in the two cases. A) |F1 | > |F2| B) |F1 |< |F2| D) Depends on sign of q q q |F1 | = |F2| C) +Q F2 = 0 F1 -Q +Q Case 1 Case 2 70 60 50 40 30 20 10 0 A B C D In case one, both of the forces are working in the same direction (the downward direction) resulting in a greater net force as opposed to case two where the repelling forces are working in opposite directions in turn canceling each other out Both cases have a charged placed equidistant from charges of the same magnitudes so no matter charge q has the forces at this instant are the same. 33 There is no way to tell if the forces on the center charge are repulsive or attractive without Physics 212 Lecture 1, Slide 22 Physics knowing the sign of the center charge. Preflight 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 What horizontal force on Q? horizontal A) Fx > 0 B) Fx = 0 3q x C) Fx < 0 Q 100 80 60 40 q q 20 0 q Great Job ! 41 Physics 212 Lecture 1, Slide 23 Physics BB Preflight 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 60 50 40 30 20 10 0 A B C Fy would equal 0 if all 3 of the bottom charges were on top of each other, but because they are pulling at an angle, they will not equal the force of the 3q charge that is pulling only in the +Y direction. The y-direction components also cancel each other out since there is a charge or force of 3Q pointing both upward and downward. The two q charges on the side also have an x component so their Fy is not as great thus the sum of the force from the bottom charges in smaller than the top charge. The forces are repulsive so the charge will have a negative Fy. Physics 212 Lecture 1, Slide 24 Physics 47 See you Thursday! • Discussion Sections meet this week! • Be sure to complete prelecture 2 and preflight 2 • Labs begin August 30th Physics 212 Lecture 1, Slide 25 Physics ...
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