Welcome to Physics 272

Transcription

1 Welcome to Physics 272 Lectue 1 Electic Chage and Coulombs Law Bob Mose mose@phys.hawaii.edu GO TO THIS SITE FOR ALL COURSE INFORMATION

2 Phys-272 Bob Mose GO TO THIS SITE TO REGISTER FOR MASTERING PHYSICS I-CLICKERS To do: Sign into Masteing Physics phys-272 webpage Registe i-clickes (you i-clicke ID to you name on class-list)

3 Physics 272 Motivation: Physics of E&M allows wide ange of technologies: Radios and TV Computes Cell phones ipods, iphones Had dives Washing machines Micowave ovens Light bulbs Lase copies MRIs etc. Even a Pius Impotant to undestand. e.g Physics Nobel Pize: Fibe Optics and CCD sensos e.g Physics Nobel Pize: Giant Magneto-Resistance

4 How to lean physics Read the book (eally!) But ead actively (not passively). Engage and Wok Poblems Reseach in physics education shows that pue lectuing is not vey effective in developing conceptual undestanding: Some new appoaches: ead the book befoe class, clicke questions and pe-flights and pe-lectues

5 Electic Chage Souce of electic and magnetic phenomena. Will study E&M much of the semeste. A.) Geneating: ubbing tansfes chage. glass with silk lucite with fu Two types of chage (+ and -). Ben Fanklin ( ): chage on glass od is +. Like chages epel. Unlike chages attact. Demo: Pith ball +q -q What if both +?

6 Electic Chage B. Chage is quantized. Not continuous as Fanklin thought. chage of electon = -e chage of poton = +e e is fundamental unit of chage Can wite Q = ± Ne (Q is any chage in natue; no isolated quaks found) e = 1.6 x C (SI unit - Coulomb) Coulomb defined in tems of Ampee (cuent). C. Chage is conseved. Glass chaged by tansfe of chage; no chage is ceated. γ e + e - pai poduction γ e + single positon poduction not allowed pai poduction

7 Conductos and Insulatos Insulatos chage (electons) not fee to move. Examples: glass, pocelain. Conductos electons fee to move. Example: Coppe 1 fee electon pe atom. Thee ae no pefect insulatos o conductos! Demo: Electoscope Demo: Chaging by induction

8 Coulomb s Law Chales Coulomb ( ) Applies to point chages. magnitude diection: F = k F is a vecto! q q q1q2 = 2 2 4πε 0 k = 8.99 x 10 9 N m 2 /C 2 9 x 10 9 N m 2 /C 2 ε 0 = 8.85 x C 2 /Nm 2 along line between two chages attactive if unlike chages epulsive if like chages F is a vecto! F q 1 F G = Looks like: Gm1m 2 Diffeence? q 2 2 F Note foces ae equal and opposite (Newton s Thid Law)

9 Example: How Stong is the Coulomb Foce? Two papeclips ae sepaated by 10 metes. Then you emove 1 electon fom each atom on the fist papeclip and place it on the second one. qq F = k ˆ k= 9 x 10 9 N m 2 / C 2 electon chage = 1.6 x Coulombs N A = 6.02 x Which weight is closest to the appoximate foce between those papeclips (ecall that weight = mg, g = 9.8 m/s 2 )? A) Papeclip (1g x g) B) Text book (1kg x g) C) Tuck (10 4 kg x g) D) Aicaft caie (10 8 kg x g) E) Mt. Eveest (10 14 kg x g)

10 Coulombs Law What if moe than one chage? Use supeposition. Example 1: Thee point chages on x-axis. Foce on q 0? y x q 1 = 25 nc x 1 = 0 q 2 = -10 nc x 2 = 2 m q 0 = 20 nc x 0 = 3.5 m k x 25*20 (nc) 2 /(3.5 m) 2 k x -10*20 (nc) 2 /(1.5 m) 2 To the ight To the left -48 x * k= -4.3 x 10-7 N to the left

11 Example of Coulomb s Law Example 2: Foce on q 3? q 2 = 3.0 µc y 2 = 6 m y q 1 = 3.0 µc x 1 = y 1 =0 x q 3 = 2.0 µc x 3 = 8 m Now let s beak F 23 into components. F(x comp) = N F(y comp) = N F 23 (x comp) = F 23 8/[sqt( )] = N F 23 (y comp) = - F 23 6/[sqt( )]= N F 13 (magnitude)= k*3.0*2.0*(µc) 2 /(8 2 ) F 13 = N F 23 (magnitude)= k*3.0*2.0*(µc) 2 /( ) F 23 = N

12 Electic Field Coulomb s Law Poblem: Spooky Action at a distance: (speed of popagation not consideed) Anothe appoach: 1. Chage in space ceates electic field. 2. Field acts on 2 nd chage. q 1 F E q 2 F q 2 F 12 q 1 F 21 Field popagates though space with speed of light.

13 Electic fields We must intoduce the concept of the electic field. What is a Field? A FIELD is something that can be defined anywhee in space A field epesents some physical quantity (e.g., tempeatue, wind speed, foce) that is a function of 3-D spatial position (x,y,z) It can be a scala field (e.g., tempeatue field) It can be a vecto field (e.g., foce field o electic field)

14 A Scala Field These isolated tempeatues sample the scala field (you only lean the tempeatue at the point you choose, but T is defined eveywhee (x, y)

15 A Vecto Field It may be essential to know which way the wind is blowing That would equie a vecto field (you must specify both wind speed and diection)

16 E Electic Field How to detemine? F q Put a small test chage at point in space. Fq 0 E = units: N/C q 0 q 0 E Diection? What if q 0 is negative? Why a small test chage? Moe accuately: E = limq 0 0 F q q 0 0

17 Electic Field Can place test chage at many points to map Example: Electic Field aound chage Q. E E Q Foce on chage at point in electic field? = F q qe q F q E

18 E Electic Field E fo point chage q? E Can calculate: qq 0 = 2 Fq 4πε 0 q 0 = = 2 q0 q0 4πε 0 magnitude = q 4πε 0 2 ˆ magnitude and diection. defined fom q! q ˆ E F 0 q 0 Example: a.) E at P? q 1 = 60 nc y 1 = 3 cm y P x x = 4 cm