Feb. 13, 2008Physics 208 Lecture 71 From last time… Electric field lines Electric fields A B

Feb. 13, 2008Physics 208 Lecture 72 Exam 1, Wed. Feb. 20, 5:30-7 pm Covers Chap , 22-23, lecture, lab, discussion, HW Review Group/Quiz (solutions on website). Review lab question sheets. Review sample exams on website. Week5 HW (avail. this Thurs, due Fri. next week) covers exam material. Alternate exam time for students with class conflicts: 8 1/2 x 11 handwritten note sheet (both sides) allowed Wed. Feb. 20, 2:30-4 Wed. Feb. 20, 6-7:30 To attend alternate exam, you must me with 1) your conflict, 2) which exam

Feb. 13, 2008Physics 208 Lecture 73 Friday Guest Lecture Prof. Thad Walker UW Physics “Holography”

Feb. 13, 2008Physics 208 Lecture 74 Superposition of electric fields Electric field from a point charge: From a set of point charges: Superpose E-fields from individual charges

Feb. 13, 2008Physics 208 Lecture 75 Charge Densities Volume charge density: when a charge is distributed evenly throughout a volume  = Q / V dq =  dV Surface charge density: when a charge is distributed evenly over a surface area  = Q / A dq =  dA Linear charge density: when a charge is distributed along a line = Q / dq = d

Feb. 13, 2008Physics 208 Lecture 76 Infinite line of charge An infinite line of charge has a uniform charge density Coulombs / meter. What direction is the electric field at point x? x A. B. C. D. E.

Feb. 13, 2008Physics 208 Lecture 77 E-field: infinite line of charge Charge density  Coulombs/meter y r x

Feb. 13, 2008Physics 208 Lecture 78 Add all these up Units?

Feb. 13, 2008Physics 208 Lecture 79 Finite line of charge What direction is the E-field above the end of the line charge? A B C D E

Feb. 13, 2008Physics 208 Lecture 710 Ring of uniform positive charge Which is the graph of the field on the axis? y x z A) B) C) D) E)

Feb. 13, 2008Physics 208 Lecture 711 Quick quiz We have an infinite sheet of charge of uniform charge density . The electric field A.increases with distance from plane B.decreases with distance from plane C.is independent of distance from plane D.changes direction with distance from plane

Feb. 13, 2008Physics 208 Lecture 712 E-field of plane of charge Text does this as adding up rings of charge. Surface charge density

Feb. 13, 2008Physics 208 Lecture 713 Electric fields and forces Original definite of E-field was (Coulomb Force) / charge E-field produced force on charged particle.

Feb. 13, 2008Physics 208 Lecture 714 Uniform E   A) Moves right at constant speed B) Moves left at constant speed C) Remains stationary D) Accelerates to the left E) None of the above Quick Quiz FeFe A negative charge in outer space is in a uniform E-field that points to the right. What is the motion of the particle?

Feb. 13, 2008Physics 208 Lecture 715 Force on charged particle Electric field produces force qE on charged particle Force produces an acceleration a = F E / m Uniform E-field (direction&magnitude) produces constant acceleration Positive charge accelerates in direction of the field Negative charge accelerates in direction opposite the electric field

Feb. 13, 2008Physics 208 Lecture 716 Electron in a Uniform Field The electron is projected horizontally into a uniform electric field The electron undergoes a downward acceleration It is negative, so the acceleration is opposite E Motion is parabolic while between the plates

Feb. 13, 2008Physics 208 Lecture 717 A. Accel left continuously B. Accel left, then moves at const. speed C. Accel left, then slows to stop D. Accel left, then turns around E. None of the above Quick Quiz A negative charge is released in a fluid with a uniform E- field that points to the right. The fluid imparts a force proportional to and opposite to the velocity What is the motion of the particle? Uniform E   FeFe Fluid

Feb. 13, 2008Physics 208 Lecture 718 Motion of charged particle If no other forces, positive charge accelerates in direction of E-field. But many systems have drag forces (e.g. molecules in a liquid, etc) Drag force is complex, but usually depends on velocity. Particle reaches terminal velocity, determined by force balance

Feb. 13, 2008Physics 208 Lecture 719 Application: Gel Electrophoresis Charged macromolecules in ‘gel’ with applied E-field Electric force: F E = qE Drag force: F D ~ -cv v = qE / c Speed depends on charge and drag (molecule size) Sometimes too complex to interpret Protein electrophoresis: soak in detergent to give proteins all the same charge density. Result, small proteins move faster Steady state: F E +F D =0

Feb. 13, 2008Physics 208 Lecture 720 Isoelectric focusing Separate macromolecules by charge, not size Macromolecules put in solution of linearly varying pH Electric field applied Charge on molecule depends on local pH Molecules move until no more force - at their isoelectric point

Feb. 13, 2008Physics 208 Lecture 721 Quick Quiz An electric dipole is in a uniform electric field as shown. The dipole Dipole A.accelerates left B.accelerates right C.stays fixed D.accelerates up E.none of the above

Feb. 13, 2008Physics 208 Lecture 722 Electric torque on dipoles Total torque has magnitude Dipole moment has magnitude Torque on dipole in uniform field

Feb. 13, 2008Physics 208 Lecture 723 Dipole in non-uniform field A permanent dipole is near a positive point charge in a viscous fluid. The dipole will A.rotate CW & move toward charge B.rotate CW & move away C.rotate CCW & move toward D.rotate CCW & move away E.none of the above +