PHYSICS 272H Electric & Magnetic Interactions

Transcription

1 PHYSICS 7H Electic & Magnetic Inteactions Physics couse home page: Blackboad Lean:

2 Couse Content This couse deals with electic and magnetic inteactions, which ae cental to the stuctue of matte, to chemical and biological phenomena, and to the design and opeation of most moden technology. The main goal of this couse is to have you engage in a pocess cental to science: the attempt to model a boad ange of physical phenomena using a small set of poweful fundamental pinciples. The specific focus of the couse is an intoduction to field theoy, in tems of the classical theoy of electicity and magnetism (E&M). The couse also emphasizes the atomic stuctue of matte, especially the ole of electons and potons in matte.

3 Couse Components Lectues (Mon,Wed) Clicke uestions (5 points) Thid Edition Recitations (Tues, Rm 60) (00 points) Labs (i, Rm 60) (00 points) WebAssign homewok (75 points) Exams (400 points): two midtem, one final You can access you scoes in Black Boad Lean 3

4 This Week To do AS SOON AS POSSIBLE: Obtain the textbook (campus bookstoes; used books ae OK). Obtain an iclicke audience esponse emote (not a CPS pad). Registe fo WebAssign (the on-line homewok sevice) on Blackboad Lean. Registe you iclicke on Blackboad Lean. Duing the fist week you will: Attend the two scheduled lectues, Mon and Wed at :30 PM. Attend the Recitation session, on Tues (time assigned by the Regista). Attend the Lab session on i (time assigned by the Regista). 4

5 Clicke Question My majo is A) Engineeing B) Science C) Libeal Ats D) Education E) Othe 6

6 Clicke Question A) Physics B) Physics C) Physics D) Physics E) Physics My favoite subject is 7

7 Chapte 4 Electic ield 8

8 The Coulomb oce Law 4 0 Q Q "The magnitude of the electostatic foce between two point chages is diectly popotional to the magnitudes of each chage and invesely popotional to the suae of the distance between the chages." Q Q Chales-Augustin de Coulomb ( ) 0

9 Point Chages Two types: positive and negative Like chages: epel Opposite chages: attact Chage is uantized in units of e Millikan s oil dop expeiment (90-93) Point chage: Size is small compaed to the distance between it and othe objects of inteest Electic chage is an intinsic popety of the fundamental paticles that eveything is made of

10 The Coulomb oce Law 4 0 Q Q ˆ 0 = pemittivity constant + + oce epulsive oce on by + - oce attactive The foce exeted by one point chage on anothe acts along line joining the chages. The foce is epulsive if the chages have the same sign and attactive if the chages have opposite signs.

11 Units and Constants SI units of electic chage: Coulomb, C Constants: /4 0 = 9x0 9 N. m /C 0 = 8.85x0 - C /N. m pemittivity constant e =.60x0-9 C C = 6.4x0 8 elementay chages Paticle Chage electon -e positon +e poton +e antipoton -e muon +e o e pion +e o e o 0 neuton Q Q 4

12 Matte consists of atoms cm 3 : ~0 4 atoms Stuctue of Atom Nucleus: ~0 4 times smalle than electon cloud, ~0 4 times heavie than electon. Å=0-0 m Ångstöm Example: nucleus of the ion atom Size: ~0 5 m, mass: ~0-5 kg Nucleus chage = +Ze, atom with Z electons is neutal. 5

13 The Concept of Electic ield Acceleates at 9.8 m/s why? Acceleates at 0 m/s why? Thee ae many possible configuations of chages to poduce the obseved effect. 6

14 Electic ield Thee is something in space waiting fo a chaged paticle to inteact with it! This vitual foce is called electic field. An electic field ceated by chage is pesent thoughout space at all times, whethe o not thee is anothe chage aound to feel its effect. 7

15 8 Definition of Electic ield E / E

16 Electic ield E / If ou pobe chage is positive o negative due to given E will point in coect diection. E,,, E x y z t) Electic field has units of Newtons pe Coulomb: [N/C] 9

17 0 0 4 E 0 4 E E ˆ 4 0 Including diection: The Electic ield of a Point Chage + -

18 No self-foce! E 4 0 0, E Point chage does not exet field on itself!

19 Example Poblem A paticle with chage + nc ( nanocoulomb=0-9 C) is located at the oigin. What is the electic field due to this paticle at a location <-0.,-0.,-0.> m? Solution:. Distance and diection: E 4 0 ˆ obseved _ location souce _ location 0., 0., 0. 0, 0, 0 0., 0., 0. ) ) ) m 0., 0., 0. ˆ 0.57, 0.57,

20 Example Poblem. The magnitude of the electic field: E Nm C 9 0 C 0.35 m E 47 N C 4 0 ˆ 3. The electic field in vecto fom: E E Eˆ N 47 C 84, 84, , 0.57, 0.57 N C 3

21 oces due to an Electic ield Example: The electic field at a paticula location is <-300,0,0> N/C. What foce would an electon expeience if it wee placed in this location? Y E e X Solution: ee C 300,0,0 N/C ,0,0 N 4

22 The Physical Concept of ield ield: physical uantity, can be scala o vecto Examples: Tempeatue T(x,y,z,t) Gavitational field m mg E 5

23 How Stong is the Coulomb oce eleton and poton 6