From Last Time. Quick Quiz. The electric dipole. Force on an electric dipole. F = r. Exam 1 Mon. Sep. 29, 5:30-7 pm

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Brianne Newton
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1 Fom Last Time Magnitude of the electic foce Diection of the electic foce Exam Mon. Sep. 9, 5:07 pm Coves Chap..57,,.4,.7, 4.5, 67 lectue, lab, discussion, HW 8 / x handwitten note sheet (both sides) allowed Students with scheduled class conflicts: stay afte lectue Tuesday Sep to aange time Review Goup/Quiz (solutions on website). Review lab question sheets. Review sample exams on website. Fom Last Time Magnitude of the electic foce Diection of the electic foce Equal but opposite chages connected by a igid insulating od, placed nea a negative chage as shown. What diection is the net foce on the two connected chages? ) Left ) Right C) Up D) Down E) Zeo kq q F = 4 The electic dipole Foce on an electic dipole Can all be appoximated by electic dipole. Two opposite chages magnitude q sepaated by distance s What is the diection of the foce on the electic dipole fom the positive point chage?. Up. Down C. Left p Dipole moment Vecto p Points fom chage to chage Has magnitude qs 5 D. Right E. Foce is zeo How does the magnitude of the foce depend on p? 6

2 Induced dipoles (chage edistibution) chaged ubbe od Unusual dipoles: Electogenic fish Dipole neaby conducting object ing negative chage close. Electons on sphee move away fom od. 7 Some fish geneate chage sepaation electic dipole. Dipole is induced in neaby (conducting) fish Small changes detected by fish. 8 The idea of electic fields EM wave made up of oscillating electic and magnetic fields. ut what is an electic field? Electic field is a way to descibe the foce on a chaged paticle due to othe chages aound it. Foce = chage electic field The diection of the foce is the diection of the electic field. 9 Electic field of a point chage Foce on this chage Q Q due to this chage F = k Q Q ˆ E = F ( ) /Q = Foce/chage Units? N/C 0 Calculating the electic field = 0 cm Q =µc ( )( 0 $6 C) E = kq = 9 "0 9 N # m /C ( 0.m) = 9 "0 5 N /C Electic field Electic field vecto defined at evey point in space. Gives magnitude and diection of foce on test paticle e.g. wind velocity (speed and diection) in diffeent pats of the county.

Supeposition with Electic Fields t any point P, the total electic field due to a goup of souce chages equals the vecto sum of electic fields of all the chages Find the electic field due to q, E Find

3 Supeposition with Electic Fields t any point P, the total electic field due to a goup of souce chages equals the vecto sum of electic fields of all the chages Find the electic field due to q, E Find the electic field due to q, E E = E E Remembe, the fields add as vectos Question Which vecto best epesents the electic field at the ed dot? C D E 4 Electic Field Diection a) q is positive, F is diected away fom q b) The diection of E is also away fom the positive souce chage c) q is negative, F is diected towad q d) E is also towad the negative souce chage 5 Relationship etween F and E F e = qe Valid fo a test chage that does not distub the souce chage distibution If q is positive, F and E ae in the same diection Electic field Å away fom poton E = 9 "09 N # M /C Q p =.6x0 9 C = x0 0 m ( )(.6 "0 $9 C) =.9 "0 N /C ( 0 $0 m) (to the ight) E 6 Which is the diection of the electic field at dot?. Left. Right way fom positive chage (ight) C. Up D. Down E. Zeo y x In this electic dipole, what is the diection of the electic field at point? ) Up ) Down C) Left D) Right E) Zeo x=a Q Q x=a Net E field is to ight. 7 8

4 Electic field: summay Electic field > will be a foce on a chaged paticle. This foce ( and electic field) can aise fom electic chages (via Coulomb s law) ut once electic field is known, don t need to know the chages that poduce it. 9 Can all be appoximated by electic dipole. Two opposite chages magnitude q sepaated by distance s Dipole moment Vecto p The electic dipole Points fom chage to chage Has magnitude qs 0 Calculating dipole electic field On the yaxis E = E y y ˆ E y = E E " = kq ( y " s/) ("kq) ( y s/) ys = kq ( y " s/) ( y s/) " k( qs) y Fo y >> s Since p points fom chage to chage E = k p on yaxis of dipole only s q q y s/ "s/ x Question: electic dipole and ae the same (lage) distance fom dipole. How do the magnitude of the electic fields at and compae? ) ) C) D) E E E = < > = 0 Pictoial epesentation of E: Electic Field Lines Electic field lines Local electic field tangent to field line Density of lines popotional to electic field stength Fields lines can only stat on chage Can only end on chage. Electic field lines can neve coss 4 4

5 Electic field of a dipole Electic field of two chages 5 6 Question How ae the chages and elated? Point paticles ),, < ),, < C),, > D),, < E),, = 7 8 5

The force between electric charges. Comparing gravity and the interaction between charges. Coulomb s Law. Forces between two charges

The force between electric charges. Comparing gravity and the interaction between charges. Coulomb s Law. Forces between two charges The foce between electic chages Coulomb s Law Two chaged objects, of chage q and Q, sepaated by a distance, exet a foce on one anothe. The magnitude of this foce is given by: kqq Coulomb s Law: F whee