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

Physics 212 Lecture 1, Slide 2 Prelectures + Preflights + Lectures Homework + 10 Quizzes250 Labs150 Hour exams (3 x 100 each)300 Final Exam200 How your grade will be calculated Prelectures: 50 Preflight's: 25 Lecture participation: 25 At the end of the semester your lecture bonus points are added to your HW/Quiz score (250 max). The lowest score is dropped and 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.

Physics 212 Lecture 1, Slide 3 Q: What are the benefits of participating ? A: You learn more Students who…Exam 1 average Viewed pre-lectures80% Blew through pre-lectures73% Data from last year’s first exam

Main Point 1 First, we introduced the central new idea that is at the core of this course, namely the need to introduce a new kind of entity into the world, electric and magnetic fields, in order to best describe a host of new phenomena. Physics 212 Lecture 1, Slide 4

Main Point 2 We then turned to the first topic of the course, the notion of electric charge, the source of these electric and magnetic fields. Electric charge comes in two kinds, positive and negative, which leads to both attractive and repulsive two- body electric forces. Physics 212 Lecture 1, Slide 5

Main Point 3 The equation describing this two-body force between two charges is called Coulomb’s Law. The direction of the force is always along the line connecting the charges and the magnitude is directly proportional to the product of the charges and inversely proportional to the square of the distance between them, as shown. Physics 212 Lecture 1, Slide 6

Main Point 4 Finally, we can obtain the force on a given charge due to an arbitrary collection of other charges by applying a principle of superposition, namely that this force is just equal to the vector sum of all the forces acting on it due to the other individual charges. Physics 212 Lecture 1, Slide 7

Physics 212 Lecture 1, Slide 8

Physics 212 Lecture 1, Slide 9 Coulomb’s Law: q1q1q1q1 q2q2q2q2 The force on a charge due to another charge is proportional to the product of the charges and inversely proportional to the separation squared. r The force is always parallel to a line connecting the charges, but the direction depends on the signs of the charges:

Physics 212 Lecture 1, Slide 10 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

Physics 212 Lecture 1, Slide 11 Coulomb’s Law q1q1q1q1 q2q2q2q2 Our notation is the force by 1 on 2 (think “by-on ”) is the force by 1 on 2 (think “by-on ”) is the vector that points from 1 to 2. is the 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) If the charges have opposite sign, the force by charge 1 on charge 2 would be opposite the direction of r 12 (left) q1q1q1q1 q2q2q2q2

Physics 212 Lecture 1, Slide 12

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 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 )

Checkpoint 1 Physics 212 Lecture 1, Slide 14 24

Physics 212 Lecture 1, Slide 15 Superposition: q1q1q1q1 q2q2q2q2 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: q3q3q3q3 q4q4q4q4 F 2,1 F 3,1 F 4,1 F1F1F1F1 F 2,1 F 3,1 F 4,1 F1F1F1F1

Physics 212 Lecture 1, Slide 16

Physics 212 Lecture 1, Slide 17 What happens to Force on q 1 if its sign is changed q1q1q1q1 q2q2q2q2 q3q3q3q3 q4q4q4q4 A)|F 1 | increases B)|F 1 | remains the same C)|F 1 | decreases D)Need more information to determine Superposition ACT Try this out with our simulation (see link on homepage)

Physics 212 Lecture 1, Slide 18 ACT +Q -Q q Case 1 +Q Case 2 +Q q Compare the magnitude of the net force on q in the two cases. A) A)|F 1 | > |F 2 | B) B)|F 1 | = |F 2 | C) C)|F 1 |< |F 2 | D) D)Depends on sign of q 33

Physics 212 Lecture 1, Slide 19 Checkpoint 2 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 q q q 3q What is the direction of horizontal force on Q? A) F x > 0 B) F x = 0 C) F x 0 B) F x = 0 C) F x < 0 x y Q 41

Physics 212 Lecture 1, Slide 20