p = q s Previously.Electric Fields Point charge: Lecture 3 Uniformly charged sphere: Chapter 14. Matter & Electric Fields Dipole: for r>>s : 1 E = y

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s,, 3 at <,,> at <,,> Lectue 3 Chate. Matte & lectic ields *. Chaged Paticles *. lectic Inteactions between Chaged Objects *.

1 Peviously.lectic ields Point chage: Diole: fo >>s : y z s + Diole moment: x s Unifomly chaged shee: Q shee fo >R (outside) fo <R (inside) shee s 3,, at <,,> s,, 3 s,, 3 at <,,> at <,,> Lectue 3 Chate. Matte & lectic ields *. Chaged Paticles *. lectic Inteactions between Chaged Objects *.3 Inteaction of Chage & Neutal Matte Matte Boh Model Quantum Physics Net Chage Hydogen atom: oton, electon Sodium atom: otons, electons s s 6 3s htt://static.howstuffwoks.com/gif/ atomuantum.jg If a Sodium atom (Na) loses an electon: /9/ 3 Odinay matte is electically neutal. Howeve, matte can be chaged by adding/emoving chaged aticles

Consevation of Chage The net chage of a system lus its suoundings cannot change.

Glass od becomes ositively chaged when ubbed with silk The net electic chage is conseved in any hysical ocess.

Annihilation : e + + e γ + γ Why is one ositively chaged not negatively chaged? htt://svs.gsfc.nasa.gov/vis/a/a/a8/a8.

How would we detemine the sign of the chage?

2 Consevation of Chage The net chage of a system lus its suoundings cannot change. Pai oduction How a solid object becomes chaged Plastic od gets negatively chaged when ubbed with fu. Glass od becomes ositively chaged when ubbed with silk The net electic chage is conseved in any hysical ocess. Chage can be tansfeed fom one object to anothe. Why do they become chaged? Annihilation : e + + e γ + γ Why is one ositively chaged not negatively chaged? htt://svs.gsfc.nasa.gov/vis/a/a/a8/a8.mg DMO Obseving lectic Inteactions How can we decide if a iece of invisible tae is electically chaged? How would we detemine the sign of the chage? Chage ceates field that can be detected by anothe chaged object: foce has the same oeties as xeiments with Tae Does the tae have nonzeo net chage?. Inteaction of two Ue ieces of tae Reulsion, attaction o no inteaction? Is it an electic inteaction? diection distance deendence deendence on the amount of chage

Unlike Chages How would we make two taes with oosite chages?. Inteaction of Ue and Lowe ieces of tae Reulsion, attaction o no inteaction? Is it an electic inteaction?

3 Unlike Chages How would we make two taes with oosite chages?. Inteaction of Ue and Lowe ieces of tae Reulsion, attaction o no inteaction? Is it an electic inteaction? diection distance deendence deendence on the amount of chage xeiments with Tae Conclusions of exeiments with Ue & Lowe ieces of tae: Thee ae two kinds of chages (+, ) Like chages eel, unlike chages attact The electic foce: acts along a line between the chages, deceases aidly as the distance between the chages inceases, is ootional to the amounts of both chages. Ou obsevations of the Ue and Lowe ieces of tae seem to be consistent with a descition of the electic inteactions between chaged objects. We conclude that the Ue and Lowe ieces of tae ae electically chaged, and have unlike chages. Inteaction of Chages and Neutal Matte The Stuctue of an Atom Hydogen DMO: x m ( Å) Nucleus, ~ m Chage of electon cloud euals that of nucleus neutal atom. If the electon cloud is centeed on the nucleus electic field oduced by electons exactly cancels the field oduced by nucleus. 3

4 Polaization of Atoms Induced Diole An alied electic field ceates induced dioles! + + it is not a emanent diole an induced diole is ceated when a neutal object is olaized by an alied electic field Polaization Amount of olaization in most mateials is ootional to the magnitude of the alied electic field: xamle A tyical atomic olaizability is C m/(n/c). If e (oton chage), what is the chage seaation by alying a field 3 6 N/C? olaizability of a mateial In an induced diole, is the distance between the chages fixed?

5 . Chage ceates field at the location of the atom A Neutal Atom and a Point Chage. ield olaizes the atom ceating a diole ) A Neutal Atom and a Point Chage 3. Diole ceates field at the location of ) ) A Neutal Atom and a Point Chage. Induced diole exets foce on the chage: ) ) 3) A Neutal Atom and a Point Chage

6 6. The chage exets foce on the diole (eciocity): ) ) 3) ) A Neutal Atom and a Point Chage ) ) 3) ) ) Neutal atoms ae attacted by chages! Inteaction stength ~ / Summay: A Neutal Atom and a Point Chage Suose tae is negatively chaged, and you ub a wooden encil on a wool sweate and bing it nea the tae. If tae swings towad the encil, does it show that the encil had been chaged ositively? NOT NCSSARILY! Detemining the Chage of an Object The field aeas to be weake in esence of intevening object Sueosition incile: the esence of matte does not affect the electic field oduced by a chaged object. Intevening matte does not block the field The esulting field is a sueosition of two fields: ield of the othe chage lus the field of induced dioles. lectic ield Though Intevening Matte

Electric Field. y s +q. Point charge: Uniformly charged sphere: Dipole: for r>>s :! ! E = 1. q 1 r 2 ˆr. E sphere. at <0,r,0> at <0,0,r>

Electric Field. y s +q. Point charge: Uniformly charged sphere: Dipole: for r>>s :! ! E = 1. q 1 r 2 ˆr. E sphere. at <0,r,0> at <0,0,r> Electic Field Point chage: E " ˆ Unifomly chaged sphee: E sphee E sphee " Q ˆ fo >R (outside) fo >s : E " s 3,, at z y s + x Dipole moment: p s E E s "#,, 3 s "#,, 3 at