From Last Time…PowerPoint PresentationSlide 3Quick QuizThe electric dipoleForce on an electric dipoleInduced dipoles (charge redistribution)Unusual dipoles: Electrogenic fishThe idea of electric fieldsElectric field of a point chargeCalculating the electric fieldElectric fieldSuperposition with Electric FieldsQuestionElectric Field DirectionRelationship Between F and ESlide 17Slide 18Electric field: summarySlide 20Calculating dipole electric fieldQuestion: electric dipolePictorial representation of E: Electric Field LinesElectric field linesElectric field of a dipoleElectric field of two + chargesSlide 27Point particles1From Last Time…Magnitude of the electric forceDirection of the electric force++2Exam 1 Mon. Sep. 29, 5:30-7 pmCovers Chap. 21.5-7, 22, 23.1-4, 23.7, 24.1-5, 26-27 + lecture, lab, discussion, HWReview Group/Quiz (solutions on website).Review lab question sheets. Review sample exams on website.Students with scheduled class conflicts: stay after lecture Tuesday Sep 23 to arrange time8 1/2 x 11 handwritten note sheet (both sides) allowed3From Last Time…Magnitude of the electric forceDirection of the electric force++4+--Equal but opposite charges connected by a rigid insulating rod, placed near a negative charge as shown. What direction is the net force on the two connected charges?A) LeftB) RightC) UpD) DownE) Zero221rqkqF =Quick Quiz5The electric dipoleDipole momentVector Points from - charge to + chargeHas magnitude qs € r p •Can all be approximated by electric dipole.•Two opposite charges magnitude q separated by distance s6Force on an electric dipole•What is the direction of the force on the electric dipole from the positive point charge? € r p +A. UpB. DownC. Left D. RightE. Force is zeroHow does the magnitude of the force depend on ? € r p7Induced dipoles (charge redistribution)Bring negative charge close. Electrons on sphere move away from rod.charged rubber rod8Unusual dipoles:Electrogenic fish•Dipole + nearby conducting objectSome fish generate charge separation - electric dipole.Dipole is induced in nearby (conducting) fishSmall changes detected by fish.9The idea of electric fields•EM wave made up of oscillating electric and magnetic fields.•But what is an electric field?•Electric field is a way to describe the force on a charged particle due to other charges around it.•Force = charge electric field•The direction of the force is the direction of the electric field.10Electric field of a point charge++Force on this charge……due to this charge+++ € r F = kQ1Q2r2ˆ r Q1Q2 € r E =r F r r ( )/Q2= Force/chargeUnits?N/C11Calculating the electric field+Q1=1µCr = 10 cm € r E =kQ1r2=9 ×109N ⋅m2/C2( )10−6C( )0.1m( )2= 9 ×105N /C12Electric field•Electric field vector defined at every point in space.•Gives magnitude and direction of force on test particlee.g. wind velocity (speed and direction) in different parts of the country.13Superposition with Electric Fields•At any point P, the total electric field due to a group of source charges equals the vector sum of electric fields of all the chargesFind the electric field due to q1, E1Find the electric field due to q2, E2E = E1 + E2Remember, the fields add as vectors14QuestionWhich vector best represents the electric field at the red dot? -ABC-DE15Electric Field Direction•a) q is positive, F is directed away from q•b) The direction of E is also away from the positive source charge•c) q is negative, F is directed toward q•d) E is also toward the negative source charge16Relationship Between F and E•Fe = qEValid for a test charge that does not disturb the source charge distribution•If q is positive, F and E are in the same direction+r = 1x10-10 m Qp=1.6x10-19 CE(to the right)QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.€ E =9 ×109N ⋅ M /C2( )1.6 ×10−19C( )10−10m( )2= 2.9 ×1011N /CElectric field 1Å away from proton17Quick QuizWhich is the direction of the electric field at dot?A. Left B. Right C. UpD. Down E. Zero+-xyAway from positive charge (right)Net E field is to right.18Quick QuizIn this electric dipole, what is the direction of the electric field at point A?A) Up B) Down C) Left D) Right E) Zero+Q -Qx=+ax=-aA19Electric field: summary•Electric field -> will be a force on a charged particle.•This force ( and electric field) can arise from electric charges (via Coulomb’s law)•But once electric field is known, don’t need to know the charges that produce it.20The electric dipoleDipole momentVector Points from - charge to + chargeHas magnitude qs € r p •Can all be approximated by electric dipole.•Two opposite charges magnitude q separated by distance s21Calculating dipole electric fieldOn the y-axis€ s/2€ −s/2xy€ E = Eyˆ y Ey= E++ E−€ ≈k 2qs( )1y3For € y >> sSince points from - charge to + charge € r p € r E = k2r p r3on y-axis of dipole onlysq-q€ =kq1y − s /2( )2+ −kq( )1y + s /2( )2€ =kq2ysy − s/2( )2y + s/2( )222Question: electric dipoleA and B are the same (large) distance from dipole. How do the magnitude of the electric fields at A and B compare?ABA) B) C) D) € r E A=r E B € r E A<r E B € r E A>r E B € r E B= 023Pictorial representation of E: Electric Field Lines24Electric field lines•Local electric field tangent to field line •Density of lines proportional to electric field strength•Fields lines can only start on + charge•Can only end on - charge.•Electric field lines can never cross25Electric field of a dipole+-26Electric field of two + charges27QuestionHow are the charges A and B related?A) A+, B-, B) A-, B+,C) A+, B-, D) A-, B+,E) A+, B-, € A < B€ A < B€ A > B€ A < B€ A = BAB28Point
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