PHY 114 A General Physics II 11 AM-12:15 PM TR Olin 101. Plan for Lecture 1:

Transcription

1 PHY 4 A General Physics II AM-:5 PM TR Olin Plan for Lecture :. Welcome & overview. Class structure & announcements 3. Electrical charges and forces /8/0 PHY 4 A Spring 0 -- Lecture

2 PHY 4 A General Physics II AM-:5 PM TR Olin Instructor: Natalie Holzwarth (Olin 300) Course Webpage: Webpage for Webassign system: You will also need an i-clicker device for each class /8/0 PHY 4 A Spring 0 -- Lecture

3 Where to find information about the course on the web: /8/0 PHY 4 A Spring 0 -- Lecture 3

4 Course organization: /8/0 PHY 4 A Spring 0 -- Lecture 4

5 /8/0 PHY 4 A Spring 0 -- Lecture 5

6 Tentative schedule subject to change (after vote) /8/0 PHY 4 A Spring 0 -- Lecture 6

7 i-clicker exercise: When would you prefer to have scheduled office hours? (Note additional meetings with your instructor can always be arranged.) A. Before class (9-:30 AM TR) B. After class (:5-:30 PM TR) C. Evenings 6-7 PM TR D. Evenings 6-7 PM MWF E. Other /8/0 PHY 4 A Spring 0 -- Lecture 7

8 PHY 4 Labs will start Monday Jan. 3 rd and run through the week of April 6 th. PHY 4 Tutorial sessions will also start next week. /8/0 PHY 4 A Spring 0 -- Lecture 8

9 General advice on how to learn physics: Physics is built on basic principles ( laws ) from which all results can be derived (deduced) Focus your attention on understanding the basic principles Practice using the basic principles to solve problems Take care to formulate your questions and make sure they are answered Note: In general, the best way to develop your problem solving skills is to practice solving problems. There is less (no??) need to memorize. As you are working problems, you may wish to construct a sheet of paper containing the basic equations you need to consult in order to solve the problems. You can take this same basic equation sheet to use during the in-class exams. /8/0 PHY 4 A Spring 0 -- Lecture 9

10 Overview -- Important concepts from General Physics I F = m a (force acceleration) K f + U f = K i + U i + W (other) (energy relationships) Important concepts in General Physics II General Physics I is still true Electricity & Magnetism Light Modern Physics /8/0 PHY 4 -- Lecture

11 Charging ahead... (Chap. 3 of S & J) Most matter is made up of charged particles labeled + (proton: q p = x -9 C) labeled - (electron: q e = x -9 C) Coulomb s law describes the electrical force between two charged particles: F = k e r r or equivalently : F = k e qq r q q ( r r ) r rˆ 3 r r /8/0 PHY 4 -- Lecture

12 Coulomb s law (continued) F = k e q q r rˆ r /8/0 PHY 4 A Spring 0 -- Lecture

13 Coulomb s law (continued) F = k e q q r rˆ r k k ε c e e 0 µ 0 4πε 0 = 8.99 = 8.85 = 4π = 3.00 µ 0c 4π N m C / C / N m (permittivity of free space) T m/a (permeability of free space) m/s (speed of light in vacuum) /8/0 PHY 4 A Spring 0 -- Lecture 3

14 i-clicker exercise: Consider a proton (m p =.67x -7 kg; q p =+.60x -9 C) and an electron (m e =9.x -3 kg; q e =-.60x -9 C) separated by 5.9x - m. The particles are attracted to each other by both the force of gravity and by Coulomb s law force. Which of these has the larger magnitude? A. Gravitational force B. Coulomb s law force Gm Gravity : F = r k Coulomb : F = e p q r m p e q e 6.67 = 8.99 = 9.67 ( 5.9 ) ( 5.9 ) = 3.6 = 8. We will mostly ignore gravitational effects when we consider electrostatics N N /8/0 PHY 4 A Spring 0 -- Lecture 4

15 Problem solving steps. Visualize problem labeling variables. Determine which basic physical principle(s) apply 3. Write down the appropriate equations using the variables defined in step. 4. Check whether you have the correct amount of information to solve the problem (same number of knowns and unknowns). 5. Solve the equations. 6. Check whether your answer makes sense (units, order of magnitude, etc.). /8/0 PHY 4 -- Lecture 5

16 Problem example from Webassign: /8/0 PHY 4 A Spring 0 -- Lecture 6

17 /8/0 PHY 4 A Spring 0 -- Lecture 7

18 9.e-4 Problem solving steps. Visualize problem labeling variables. Determine which basic physical principle(s) apply 3. Write down the appropriate equations using the variables defined in step. 4. Check whether you have the correct amount of information to solve the problem (same number of knowns and unknowns). 5. Solve the equations. 6. Check whether your answer makes sense (units, order of magnitude, etc.). F keq q = r = = 9. ( 5) N /8/0 PHY 4 A Spring 0 -- Lecture 8

19 Where do the charges q i come from? All matter is composed of nuclei (q N = Ze) and electrons (q e = -e) and generally there are an equal number of positive and negative charges. Insulating material Metallic material Cl - Cu + Na + e - /8/0 PHY 4 A Spring 0 -- Lecture 9

20 Note: Coulomb s law applies both at the atomic level and at the macroscopic scale Example on the macroscopic scale: /8/0 PHY 4 A Spring 0 -- Lecture 0

21 T k e q x mg T sin θ = mg T cos θ = k e x q tan θ = x / L /8/0 PHY 4 A Spring 0 -- Lecture

22 Coulomb s law for multiple charges F F 3 F F = F = k + e F 3 q q d + k e ( d + d ) q q 3 /8/0 PHY 4 A Spring 0 -- Lecture

23 i-clicker exercise Consider the configuration of 4 charges at the 4 corners of a square shown in the diagram. Which vector a,b,c,d, or e (pointing into the screen) indicates the direction of net force on the charge? +Q /8/0 PHY 4 A Spring 0 -- Lecture 3