PHYS 1441 Section 002. Lecture #3

Transcription

1 PHYS 1441 Section 00 Chapte 1 Lectue #3 Wednesday, Sept. 6, 017 Coulomb s Law The Electic Field & Field Lines Electic Fields and Conductos Motion of a Chaged Paticle in an Electic Field Electic Dipoles Today s homewok is homewok #, due 11pm, Monday, Sept. 11!! 1

2 Announcements Thee ae still two of you who haven t egisteed fo homewok system It will be vey had to pass this couse without the homewok! I stongly suggest you to egiste ASAP! 1 st Tem exam In class, Wednesday, Sept. 0: DO NOT MISS THE EXAM! CH1.1 to what we lean on Monday, Sept Appendices A1 A8 You can bing you calculato but it must not have any elevant fomula pe-input BYOF: You may bing a one 8.5x11.5 sheet (font and back) of handwitten fomulae and values of constants fo the exam No deivations, wod definitions, o solutions of any poblems! No additional fomulae o values of constants will be povided! Colloquium today at 4pm! Physics faculty expo! Submit the special poject now!

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4 Electic Chage in an Atom It has been undestood though the past centuy that an atom consists of A positively chaged heavy coe ç What is the name? This coe is the nucleus and consists of neutons and potons. Many negatively chaged light paticles suound the coe ç What is the name of these light paticles? These ae called electons How many of these in an atom? As many as the numbe of potons in the nucleus!! So what is the net electical chage of an atom? Zeo!!! Electically neutal!!! Can you explain what happens when a comb is ubbed on a towel? Electons fom the towel get tansfeed to the comb, making the comb negatively chaged while leaving positive ions on the towel. These chages eventually get neutalized pimaily by wate molecules in the ai. 4

5 Insulatos and Conductos Let s imagine two metal balls of which one is chaged What will happen if they ae connected by A metallic object? Some chage is tansfeed. These objects ae called conductos of electicity. An wooden object? No chage is tansfeed These objects ae called nonconductos o insulatos. Metals ae geneally good conductos wheeas most othe mateials ae insulatos. Thee ae thid kind of mateials called, semi-conductos, like silicon o gemanium è conduct only in cetain conditions Atomically, conductos have loosely bound electons while insulatos have them tightly bound! 5

6 Induced Chage When a positively chaged metal object is bought close to an unchaged metal object If two objects touch each othe, the fee electons in the neutal one ae attacted to the positively chaged object and some will pass ove to it, leaving the neutal object positively chaged è Chaging by conduction If the objects get close, the fee electons in the neutal one still move within the metal towad the chaged object leaving the opposite side of the object positively chaged. The chages have been induced in the opposite ends of the object. 6

7 Induced Chage gound We can induce a net chage on a metal object by connecting a wie to the gound. The object is gounded o eathed. Since the Eath is so lage and conducts, it can give o accept chage. The Eath acts as a esevoi of electic chage. If negative chage is bought close to a neutal metal Positive chage will be induced towad the negatively chaged metal. The negative chages in the neutal metal will be gatheed on the opposite side, tansfeing though the wie to the Eath. If the wie is cut, the metal ba has net positive chage. An electoscope is a device that can be used fo detecting chage and signs. How does this wok? 7

8 Coulomb s Law Chages exet foce to each othe. What factos affect the magnitude of this foce? Any guesses? Chales Coulomb figued this out in 1780 s. Coulomb found that the electic foce is Popotional to the multiplication of the two chages If one of the chages doubles, the foce doubles. If both the chages double, the foce quaduples. Invesely popotional to the squae of the distances between them. Electic chage is a fundamental popety of matte, just like mass. How would you put the above into a fomula? 8

9 Coulomb s Law The Fomula Q F Q Q 1 Fomula 1 Is Coulomb foce a scala quantity o a vecto quantity? Unit? A vecto quantity. The unit is Newtons (N)! The diection of electic (Coulomb) foce is always along the line joining the two objects. If the two chages ae the same: foces ae diected away fom each othe. If the two chages ae opposite: foces ae diected towad each othe. Coulomb foce is pecise to 1 pat in Unit of chage is called Coulomb, C, in SI. The value of the popotionality constant, k, in SI unit is 9 k = N m C Thus, 1C is the chage that gives F~9x10 9 N of foce when placed 1m apat fom each othe. QQ F = k 9

10 Electic Foce and Gavitational Foce QQ F = k 1 Extemely Simila MM F = G 1 Does the electic foce look simila to anothe foce? What is it? Gavitational Foce What ae the souces of the foces? Electic Foce: Electic chages, fundamental popeties of matte Gavitational Foce: Masses, fundamental popeties of matte What else is simila? Invesely popotional to the squae of the distance between the souces of the foce è What is this kind law called? Invese Squae Law What is the diffeence? Gavitational foce is always attactive. Electic foce depends on the type of the two chages. (must pay good attention to the signs due to the sign of the chage and the vecto foce diections!!) 10

11 The Elementay Chage and Pemittivity Elementay chage, the smallest chage, is that of an 19 electon: e= C Since electon is a negatively chaged paticle, its chage is e. Object cannot gain o lose faction of an electon. Electic chage is quantized. It changes always in intege multiples of e. The popotionality constant k is often witten in tems of anothe constant, ε 0, the pemittivity* of fee space. They 1 ae elated k = 14πε0 and ε = πk = C N m. Thus the electic foce can also be witten as: Note that this foce is fo point chages at est F 1 4πε 0 QQ 1 *Miiam-Webste, Pemittivity: The ability of a mateial to stoe electic potential enegy unde the influence of an electic field = 11

12 Example on Coulomb Foce Electic foce on electon by poton. Detemine the magnitude of the electic foce on the electon of a hydogen atom exeted by the single poton (Q =+e) that is its nucleus. Assume the electon obits the poton at its aveage distance of =0.53x10-10 m. Using Coulomb s law F 1 QQ = = k 4πε 1 1 Each chage is Q1 = e= C 0 QQ and Q =+ e= C So the magnitude of the foce is QQ F = k 1 9 = N m 8 = N Which diection? Towad each othe C ( 19 )( C C) ( m) 1