PHYSICS 272 Electric & Magnetic Interactions

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1 PHYS 7: Matte and Inteactions II -- Electic And Magnetic Inteactions PHYSICS 7 Electic & Magnetic Inteactions Lectue 3 Chaged Objects; Polaization of Atoms; Induced Dipoles [EMI ] Fall Pof. Yong Chen (yongchen@pudue.edu) Pof. Michael Manfa (mmanfa@pudue.edu) Lectue 3 Slide

PHYS 7: Matte and Inteactions II -- Electic And Magnetic Inteactions http://www.physics.pudue.

2 PHYS 7: Matte and Inteactions II -- Electic And Magnetic Inteactions ReCap Last Time Supeposition pinciple (multiple chages) Electic dipole & dipole moment Electic field due to dipole Dipole in Electic field Fall Pof. Yong Chen Pof. Michael Manfa Lectue 3 Slide

3 PHYS 7: Matte and Inteactions II -- Electic And Magnetic Inteactions Dipole Moment x: y, z: E E s p,, 3 s p,, 3 >>s The electic field of a dipole is popotional to the Dipole moment: p = s p s, diection fom to + Dipole moment is a vecto pointing fom negative to positive chage Fall Pof. Yong Chen (yongchen@pudue.edu) Pof. Michael Manfa (mmanfa@pudue.edu) Lectue 3 Slide 3

4 PHYS 7: Matte and Inteactions II -- Electic And Magnetic Inteactions F Dipole in a Unifom Field E Foces on + and have the same magnitude but opposite diection F net E E It would expeience a toue about its cente of mass. What is the euilibium position? Electic dipole can be used to measue the diection of electic field. Fall Pof. Yong Chen (yongchen@pudue.edu) Pof. Michael Manfa (mmanfa@pudue.edu) Lectue 3 Slide

5 PHYS 7: Matte and Inteactions II -- Electic And Magnetic Inteactions Choice of System Multipaticle systems: Split into objects to include into system and objects to be consideed as extenal. To use field concept instead of Coulomb s law we split the Univese into two pats: the chages that ae the souces of the field the chage that is affected by that field Example: Oscilloscope Chages on metal plates ae the souces of an unifom E field (fist 3 minutes) Fall Pof. Yong Chen (yongchen@pudue.edu) Pof. Michael Manfa (mmanfa@pudue.edu) Lectue 3 Slide 5

com/piezo_theoy/piezo_theoy.pdf Coulomb s law is not completely coect it does not contain time t no speed of light c.

!! Do Fundamental paticles (neutons, electons ) have electic dipole moments (EDM)?

6 PHYS 7: Matte and Inteactions II -- Electic And Magnetic Inteactions Some Food fo Thought Dipoles and Piezoelectic Mateials Coulomb s law is not completely coect it does not contain time t no speed of light c. [ etadation effect no instant field] F ˆ E ˆ v<<c!!! Do Fundamental paticles (neutons, electons ) have electic dipole moments (EDM)? [elated to matte-antimatte asymmety in univese] Fall Pof. Yong Chen (yongchen@pudue.edu) Pof. Michael Manfa (mmanfa@pudue.edu) Lectue 3 Slide 6

7 PHYS 7: Matte and Inteactions II -- Electic And Magnetic Inteactions Question (Chap. ) What is the diection of the electic field at location X, due to the dipole? - + B C D X iclicke Feuency Code: CD (befoe each lectue) A E Fall Pof. Yong Chen (yongchen@pudue.edu) Pof. Michael Manfa (mmanfa@pudue.edu) Lectue 3 Slide 7

8 PHYS 7: Matte and Inteactions II -- Electic And Magnetic Inteactions Question (Chap. ) Locations and ae euidistant fom the cente of the dipole. At which location is the magnitude of the electic field lage? d - + A. at location d B. at location C. magnitudes ae the same Fall Pof. Yong Chen (yongchen@pudue.edu) Pof. Michael Manfa (mmanfa@pudue.edu) Lectue 3 Slide 8

9 PHYS 7: Matte and Inteactions II -- Electic And Magnetic Inteactions Chapte 5 Matte and Electic Fields Fall Pof. Yong Chen (yongchen@pudue.edu) Pof. Michael Manfa (mmanfa@pudue.edu) Lectue 3 Slide 9

10 PHYS 7: Matte and Inteactions II -- Electic And Magnetic Inteactions Net Chage Matte is made out of atoms. Atom contains chaged paticles: electons (-e), potons (+e) Neutal atom: numbe of electons and potons is eual: Example: Hydogen atom: poton, electon net chage = (+e) + (-e)= Sodium atom: potons, electons Sodium atom (Na) can lose an electon: Sodium ion (Na + ): (+e) + (-e) = +e Odinay matte is electically neutal. Howeve, can be chaged by adding/emoving chaged paticles Fall Pof. Yong Chen (yongchen@pudue.edu) Pof. Michael Manfa (mmanfa@pudue.edu) Lectue 3 Slide

11 PHYS 7: Matte and Inteactions II -- Electic And Magnetic Inteactions Consevation of Chage The net chage of a system and its suoundings cannot change If one object gets chaged positively, thee must be an object which gets chaged negatively. The net electic chage is conseved in any physical pocess. Chage can be tansfeed fom one object to anothe. Annihilation : e e Fall Pof. Yong Chen (yongchen@pudue.edu) Pof. Michael Manfa (mmanfa@pudue.edu) Lectue 3 Slide

epel, unlike chages attact The electic foce: acts along a line between the chages, deceases apidly as the distance between the chages inceases, is popotional to the amounts of both chages.

12 PHYS 7: Matte and Inteactions II -- Electic And Magnetic Inteactions You will be doing expeiments with chaged tapes in the Lab session Conclusions of expeiments with U and L tapes: Thee ae two kinds of chages (+, -) Like chages epel, unlike chages attact The electic foce: acts along a line between the chages, deceases apidly as the distance between the chages inceases, is popotional to the amounts of both chages. Ou obsevations of U and L tapes seem to be consistent with a desciption of the electic inteactions between chaged objects. We conclude that U and L tapes ae electically chaged, and have unlike chages. Fall Pof. Yong Chen (yongchen@pudue.edu) Pof. Michael Manfa (mmanfa@pudue.edu) Lectue 3 Slide

13 PHYS 7: Matte and Inteactions II -- Electic And Magnetic Inteactions Detemining Sign of the Chage How would we detemine the sign of the chage? Comb (o plastic pen) gets negatively chaged when ubbed though hai, fu o wool; glass od becomes positively chaged. Why get chaged? Beaking lage molecules, tansfe ions Tansfe electons Why positive o negative? Don t know Fall Pof. Yong Chen (yongchen@pudue.edu) Pof. Michael Manfa (mmanfa@pudue.edu) Lectue 3 Slide 3

(chaged) tapes with othe objects: Plastic pen Pape Hand Metal Net chage is zeo!

14 PHYS 7: Matte and Inteactions II -- Electic And Magnetic Inteactions Inteaction of Chages and Neutal Matte Inteaction of U and L (chaged) tapes with othe objects: Plastic pen Pape Hand Metal Net chage is zeo! Why does it inteact? Fall Pof. Yong Chen (yongchen@pudue.edu) Pof. Michael Manfa (mmanfa@pudue.edu) Lectue 3 Slide

15 PHYS 7: Matte and Inteactions II -- Electic And Magnetic Inteactions The Stuctue of an Atom Hydogen - m ( Å) Nucleus, ~ -5 m Chage of electon cloud euals that of nucleus neutal atom. If the electon cloud is centeed on the nucleus electic field poduced by electons exactly cancels the field poduced by nucleus. Fall Pof. Yong Chen (yongchen@pudue.edu) Pof. Michael Manfa (mmanfa@pudue.edu) Lectue 3 Slide 5

16 PHYS 7: Matte and Inteactions II -- Electic And Magnetic Inteactions Polaization of Atoms + E - + Foce due to E ceated by positive chage shifts electon cloud and nucleus in opposite diections: electic dipole. An atom is said to be polaized when its electon cloud has been shifted by the influence of an extenal chage so that the electon cloud is not centeed on the nucleus. Fall Pof. Yong Chen (yongchen@pudue.edu) Pof. Michael Manfa (mmanfa@pudue.edu) Lectue 3 Slide 6

17 PHYS 7: Matte and Inteactions II -- Electic And Magnetic Inteactions Induced Dipole An applied electic field ceates induced dipoles! E it is not a pemanent dipole an induced dipole is ceated when a neutal object is polaized by an applied electic field Fall Pof. Yong Chen (yongchen@pudue.edu) Pof. Michael Manfa (mmanfa@pudue.edu) Lectue 3 Slide 7

18 PHYS 7: Matte and Inteactions II -- Electic And Magnetic Inteactions Polaization Amount of polaization p in most mateials is popotional to the magnitude of the applied electic field: p E α - polaizability of a mateial In an induced dipole, is the distance between the chages fixed? The distance is popotional to the stength of the applied field. Fall Pof. Yong Chen (yongchen@pudue.edu) Pof. Michael Manfa (mmanfa@pudue.edu) Lectue 3 Slide 8

19 PHYS 7: Matte and Inteactions II -- Electic And Magnetic Inteactions A typical atomic polaizability is α= - C m/(n/c). If =e (poton chage), what is the chage sepaation by applying a field E=3 6 N/C? p E 3 6 (C m) 3 3 C m p s Example s p m 5 m Shift is five odes of magnitude smalle than the atom itself! Fall Pof. Yong Chen (yongchen@pudue.edu) Pof. Michael Manfa (mmanfa@pudue.edu) Lectue 3 Slide 9

20 PHYS 7: Matte and Inteactions II -- Electic And Magnetic Inteactions A Neutal Atom and a Point Chage. Chage ceates field E at the location of the atom E ˆ Fall Pof. Yong Chen (yongchen@pudue.edu) Pof. Michael Manfa (mmanfa@pudue.edu) Lectue 3 Slide

21 PHYS 7: Matte and Inteactions II -- Electic And Magnetic Inteactions A Neutal Atom and a Point Chage ) E ˆ. Field E polaizes the atom ceating a dipole p E ˆ Fall Pof. Yong Chen (yongchen@pudue.edu) Pof. Michael Manfa (mmanfa@pudue.edu) Lectue 3 Slide

22 PHYS 7: Matte and Inteactions II -- Electic And Magnetic Inteactions A Neutal Atom and a Point Chage ) ) E p ˆ ˆ 3. Dipole ceates field E at the location of E p 3 3 E 5 ˆ Fall Pof. Yong Chen (yongchen@pudue.edu) Pof. Michael Manfa (mmanfa@pudue.edu) Lectue 3 Slide

Fall Pof. Yong Chen (yongchen@pudue.edu) Pof. Michael Manfa (mmanfa@pudue.

23 Fall Pof. Yong Chen Pof. Michael Manfa Lectue 3 Slide 3 PHYS 7: Matte and Inteactions II -- Electic And Magnetic Inteactions Induced dipole exets foce F on the chage: E ˆ E F ˆ 5 p ˆ E ˆ 5 ) ) 3) A Neutal Atom and a Point Chage

24 Fall Pof. Yong Chen Pof. Michael Manfa Lectue 3 Slide PHYS 7: Matte and Inteactions II -- Electic And Magnetic Inteactions 5. The chage exets foce F on the dipole (ecipocity): E ˆ F F ˆ 5 p ˆ E ˆ 5 F ˆ 5 ) ) 3) ) A Neutal Atom and a Point Chage

25 Fall Pof. Yong Chen Pof. Michael Manfa Lectue 3 Slide 5 PHYS 7: Matte and Inteactions II -- Electic And Magnetic Inteactions E ˆ p ˆ E ˆ 5 F ˆ 5 ) ) 3) ) F ˆ 5 5) Neutal atoms ae attacted by chages! Inteaction stength ~ / 5 A Neutal Atom and a Point Chage

PHYS 7: Matte and Inteactions II -- Electic And Magnetic Inteactions http://www.physics.pudue.edu/academic_pogams/couses/phys7/ Execise Atom A is easie to polaize than atom B.

26 PHYS 7: Matte and Inteactions II -- Electic And Magnetic Inteactions Execise Atom A is easie to polaize than atom B. Which atom would expeience a geate attaction to a point chage a distance away? A - + F A B - + F B F 5 ~ Fall Pof. Yong Chen (yongchen@pudue.edu) Pof. Michael Manfa (mmanfa@pudue.edu) Lectue 3 Slide 6

27 PHYS 7: Matte and Inteactions II -- Electic And Magnetic Inteactions Detemining the Chage of an Object Suppose tape is negatively chaged, and you ub a wooden pencil on a wool sweate and bing it nea the tape If tape swings towad the pencil, does it show that the pencil had been chaged positively? NOT NECESSARILY! Attaction: can happen fo like-chaged objects! Repulsion: can happen only fo like-chaged objects! Fall Pof. Yong Chen (yongchen@pudue.edu) Pof. Michael Manfa (mmanfa@pudue.edu) Lectue 3 Slide 7

28 PHYS 7: Matte and Inteactions II -- Electic And Magnetic Inteactions Electic Field Though Intevening Matte F F The field appeas to be weake in pesence of intevening object Supeposition pinciple: the pesence of matte does not affect the electic field poduced by a chaged object. Intevening matte does not block the E field The esulting field is a supeposition of two fields: Field of the othe chage plus the field of induced dipoles. Fall Pof. Yong Chen (yongchen@pudue.edu) Pof. Michael Manfa (mmanfa@pudue.edu) Lectue 3 Slide 8

29 PHYS 7: Matte and Inteactions II -- Electic And Magnetic Inteactions Recap Review: Electic Field (pemanent) Dipole This time: Electic Field induced Dipole Example: electical inteaction/foce b/t chage and neutal atom (induced dipole) Fall Pof. Yong Chen Pof. Michael Manfa Lectue 3 Slide 9

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