Lecture 3.7 ELECTRICITY. Electric charge Coulomb s law Electric field

Transcription

1 Lectue 3.7 ELECTRICITY Electic chage Coulomb s law Electic field

2 ELECTRICITY Inteaction between electically chages objects Many impotant uses Light Heat Rail tavel Computes Cental nevous system Human body made up of electic chages. Atoms contain positive and negative chages Atoms bound togethe by electic foces» molecules Molecules inteact to poduce bones, blood, skin, etc

3 Histoical ELECTRICITY 6 th centuy B.C., Geeks noticed spaks wee poduced when the fossilized tee esin called ambe was ubbed with fu. Geek wod fo ambe is elekton fom which the wod electicity is deived. Majo discovey 1861: Maxwell s equations. Unified electic and magnetic phenomena (electomagnetism). End of the 19 th and ealy 0 th centuy: undamental discoveies concening the electonic stuctue of the atom wee made.

4 Electic chage and the atom Electic chage is a chaacteistic of subatomic paticles. Simple View An atom is composed of 3 kinds of paticles: potons, electons and neutons. Nucleus Atom Neutons Potons e e e Paticle Chage Value (SI unit) poton +e 1.6 x Coulomb (C) electon e 1.6 x Coulomb (C) neuton none

5 Electic chage and the atom Cabon Atom Nucleus 6 potons: chage +6e 6 neutons: (no chage) 6 electons: chage 6e Atoms ae electically neutal Total positive chage of the nucleus total negative chage of the electons aound the nucleus. These paticles ae, in geneal, neithe ceated no destoyed, but electons can be displaced fom one atom to an othe. Electon emoved esult positive ion Electon added esult negative ion

6 Electic chage and the atom Electic chage is a basic physical popety of subatomic paticles, potons and electons. 3 Popeties of chage 1. Two types of chages, positive and negative. Chage is conseved. Cannot be ceated o destoyed. Chages can be sepaated. 3. Like chages epel and unlike changes attact Electostatic foces esult fom the sepaation of positive and negative chages.

7 Electic chage Electically chaged mateials Many examples Almost any two nonconducting substances when ubbed togethe will become chaged Plastic comb un though you hai comb will then attact bits of pape Balloon and wool ubbed togethe: balloon becomes + negatively chaged iction associated with ubbing does not ceate the chage Chage tansfeed by movement of electons Chage is conseved Neithe ceated o destoyed Total amount of chage in univese: constant

8 Electic chage Basic unit of positive chage: +e 1.6 x Coulomb Basic unit of negative chage: e 1.6 x Coulomb (C) Any chaged object: Total chage is always a multiple of e Chage can only have values ±e, ±e, ±3e ±.. Chage is said to be quantised Neve factional chage

9 Coulomb s Law undamental quantity of chage found in matte is that associated with a poton (+e) and electon (e) Example. How many electons ae equied to make up a negative chage of one Coulomb? Chage on an electon 1.6x10 19 C theefoe 1C 1.6x10 19 C 6.5x10 18 electons ae equied to make up a chage of 1 Coulomb.

10 Electic chage Types of Mateials Conductos Example: metals, coppe etc. chages ae fee to move. Insulatos Example: Rubbe, plastic etc chages ae not fee to move. Semiconductos Example: Silicon, Gemanium movement of chages can be contolled by tempeatue o doping of the mateial. Application: electonic devices Photoconductos: Example: Selenium In dakness: Insulato Exposed to light: conducto Application: photocopie, lase pinte

11 Electic chages and foces Coulomb s law Mathematical law that descibes how like chages epel unlike chages attact Chales Coulomb ( ) ench physicist, Unlike chages Like chages q 1 + q q 1 q qq 1 q 1 q + + Coulomb s law: the foce between two point chages is popotional to the poduct of thei chage and invesely popotional to the squae of thei sepaation Diection of the foce: along line joining the point chages.

12 qq 1 Coulomb s Law qq k 1 SI unit of chage is called the Coulomb oce is known as the Coulomb foce o electostatic foce and its units ae Newtons distance is in metes Hence units of k ae Nm C The constant k is detemined by expeiment to be 9x10 9 Nm C (in a vacuum) k The constant k is fequently witten as 1 4πε 0 whee ε 0 is called the pemittivity of fee space ε CN m 1 1 Coulomb is a vey lage quantity of chage

13 Coulomb s Law Example: Two chages, each of one Coulomb, ae a distance of 1 mete apat. What is the foce between them? 1 4πε 0 qq 1 1C 1C 4π 8.85x10 1 C N 1 m 1m 1m 9x10 9 N 9 billion Newtons Coulomb foce is vey lage compaed with gavitational foce

14 Coulomb oce Example Compae electostatic epulsive foce between two electons held one mete apat in a vacuum and the gavitational foce of attaction between them? e C N 1 m mass of an electon kg G N m kg Coulomb foce.3 x 10 8 N 1 4πε 0 qq x10 19 C 1.6x10 19 C 4π 8.85x10 1 C N 1 m 1m 1m Gavitational foce mm G 1 ( ) x N

15 Coulomb s Law Electon and poton, a distance apat, ae simultaneously eleased fom est. Whee do they collide? + Poton q +e Electon q e Newton 3 d law, foces ae equal and opposite Collision at midpoint?? acceleation a m Newton nd law ma electon mass kg poton mass kg Mass of poton 1833 times mass of electon acceleation of electon 1833 times geate

16 1 x v t at Coulomb s Law x e at e xp at p 1 x x e p a m e x a / e me p p xp / mp me x x e p x e kg kg x p + Collide vey nea initial position of poton

17 Electic ield Electostatic foce and gavitational foce can both act though space even when thee is no physical contact between the objects involved. Gavitational field: g m mass m expeiences a foce: mg Potential Enegy U mgh ield lines show the diection of the foce and indicate its elative magnitude In the case of chaged paticles, what tansmits the foce between them? An electic field exists in a egion of space aound a chaged object.

18 Electic field lines Electic ield + Electic field nea a negative chage is diected adially into the chage as shown Electic field nea a positive chage is diected adially out fom the chage + Double the chage Double the numbe of field lines Simila to gavitational field lines Electic field lines show diection of the foce indicate its elative magnitude

19 Electic ield Conside positive test chage q 0 (fictitious) at A Test chage: helpful in detemining foces geneated by othe chages Assume test chage is small and does not affect any othe chages Conside foce on positive test chage q 0 q + A + 0 A + Electic field epesents the electic foce a stationay positive chage expeiences.

20 Electic ield The electic field E is said to exist in the egion of space aound a chaged object. Example: Q Test chage q 0 + is the foce exeted on q 0 by Q Electic field E due to chage Q at location of small test chage q 0 is given by; E q 0 SI unit of electic field Electic foce pe Coulomb Newton pe coulomb (NC 1 )

21 Electic ield E q 0 qe Analogous to mg in gavitational field 0 1 Qq 4πε 0 E 1 4πε 0 0 Q Eq 0 Electic field at a given point depends only on the chage Q on the object setting up the field and the distance fom the object to the specific point in space

22 Electic ield Example Detemine the electic field at A, a distance of 40cm fom a positive chage Q of x10 3 Coulombs Q + A E ind field E at point A 40 cm fom Q. E Q k E 9 x 10 9 N.m C ( x 103 ) C (0.4m) E 11.5 x10 6 NC 1

23 Electic chages Example How many electons must be emoved fom an object so that it is left with a chage of 8 x10 10 C Total chage 8 x C Chage on electon 1.6 x C Theefoe numbe of electons emoved 8 x C 5 x 10 9 electons 1.6 x C

24 Example Coulomb foce Detemine the minimum distance apat two identical chages of 0.1 C must be in ode that the Coulomb foce between them is less than 500 N. k (9 x 10 9 ) Nm C qq k 1 500N ( ) 500 Nm C C C ( ) ( ) m

25 Electic chages Example An object has a total chage of x10 6 C. How many excess electons does the object have. Total chage x 10 6 C Chage on electon 1.6 x C Theefoe numbe of excess electons x 10 6 C 1.5 x electons 1.6 x C