Physics 212 Lecture 1, Slide 1Welcome to Physics 212http://online.physics.uiuc.edu/courses/phys212This lecture is VERY full. Please sit next to someone nice. (There won’t be empty seats!).01Festival seating up front !!Physics 212 Lecture 1, Slide 2Physics 212Lecture 1Today's Concepts:a) Coulomb’s Lawb) Superposition03Physics 212 Lecture 1, Slide 3Everything can be found online – GO THERE• You have web based Prelectures and Checkpoints duebefore every lecture.• You have web based homework due every week.• Be sure to complete Prelecture 2 and Checkpoint 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.Physics 212 -IntroLabs & Discussions• Discussion Sections meet this week• Labs start August 29, must have prelab done before!http://online.physics.uiuc.edu/courses/phys21205Physics 212 Lecture 1, Slide 4Prelectures + Preflights + Lectures 10014 Homework + 10 Quizzes 250Labs 150Hour exams (3 x 100 each) 300Final Exam 200How your grade will be calculatedPrelectures: 50Checkpoints: 25Lecture participation: 25At the end of the semester your lecture bonus points are added to your HW/Quiz score (250 max). 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.You can make up for several bad quizzes by answering correctly in class. 06Physics 212 Lecture 1, Slide 5Q: What are the benefits of participating ?A: You learn more05101520253043 48 52 57 61 65 70 74 78 83 87 91 96 100Exam ScoreViewerNon-ViewerStudents who… Exam 1 averageViewed PreLectures 80%Blew through PreLectures 73%Data from last year’s first exam07Physics 212 Lecture 1, Slide 6Big Brother is Watching• How Do We Distinguish “Viewers” from “Non-Viewers”?– We log all keystrokes !!Results from today’s PreLecturePhysics 212 Lecture 1, Slide 7Clickers• 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 for correct answers – up to 25 points– (No EX’s but there are 28 lectures)09MusicWho is the Artist?A) Buddy GuyB) Ry CooderC) John Lee HookerD) Taj MahalE) Willie DixonWhy?He’s coming to Krannert Center for the Ellnora Guitar Festival!!Check out the Lineup at:http://www.ellnoraguitarfestival.com/Physics 212 Lecture 1Physics 212 Lecture 1, Slide 9One Other Key Piece: Homework1111Physics 212 Lecture 1, Slide 10Homework: Delayed Feedback11Purpose: Promote REFLECTIONThese questions serve as a test of your understanding of the questions posed as immediate feedback. After first deadlineDelayed 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 11Physics 212 Lecture 1, Slide 12Coulomb’s Law:q1q2The force on a charge due to another charge is proportional to the product of the charges and inversely proportional to the separation squared. 1 22q qFr∝rThe force is always parallel to a line connecting the charges, but the direction depends on the signs of the charges:q1q2q1q2Opposite signs attractq1q2Like signs repel11Physics 212 Lecture 1, Slide 13BalloonsTake two balloons and rub them both with a piece of cloth. After you rub them they will:A) Attract each-otherB) Repel each-otherC) Either – it depends on the material of the cloth13Physics 212 Lecture 1, Slide 14If the same thing is done to both balloons they will acquire the same sign charge. They will repel !++++++++Balloons14Physics 212 Lecture 1, Slide 15Coulomb’s Lawq1q21,2r1 21,2 1,221,2ˆkq qF rr=17Examples 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)1,2FIf the charges have opposite sign, the force by charge 1 on charge 2 would be opposite the direction of r12(left)q1q21,2r1,2FOur notation is the force by1 on2 (think “by-on ”) is the unit vector that points from1 to2. 1,2F12ˆrPhysics 212 Lecture 1, Slide 16Example: 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. 1 212212ˆq qF k rr=k= 9 x 109N m2/ C2electron charge = 1.6 x 10-19CoulombsNA= 6.02 x 1023What will the direction of the force be? A) Attractive B) Repulsive17Physics 212 Lecture 1, Slide 17Example: 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. 1 212212ˆq qF k rr=k= 9 x 109N m2/ C2electron charge = 1.6 x 10-19CoulombsNA= 6.02 x 1023Which 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 (104kg x g)D) Aircraft carrier (108kg x g)E) Mt. Everest (1014kg x g)22Physics 212 Lecture 1, Slide 18The forces are in opposite directions because they have like charges. The arrow on the +10 is longer because it has a greater magnitude than the +1Determining the force of one charge on the other requires that the charges be multiplied. The order of multiplication is irrelevant. Since all other factors are constant between the two, the charges must be equal. Also, Newton's Third Law.Because the right hand sphere has a charge of +10C, it would impact the smaller sphere by -10C and visa versa.24Checkpoint 1Two charges, q with +1 µC and Q with +10 µC are placed near each other as shown in the figure. Which of the following diagrams best depicts the forcesacting on the charges?Physics 212 Lecture 1, Slide 19Superposition:q1q2If 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:q3q4F2,1F3,1F4,1F1F2,1F3,1F4,1F12,1 3,1 4,1...F F F= + + +  1F28Physics 212 Lecture 1, Slide 20What happens to Force on q1if its sign is changedq1q2q3q4A) |F1| increasesB) |F1| remains the sameC) |F1| decreasesD) Need more information to determineSuperposition ACT30Physics 212 Lecture 1, Slide 21q1q2The direction of all forces changes by 180o– the magnitudes stay the