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Unformatted text preview: 1 Physics 6C: Fields Lines and Flux 2 Housekeeping Due date for HW1 is extended to Weds 4/6 at 8pm Discussion sections: ! Mon 10-11:30am ISB 235 (R. Dewey) ! Mon 4-5pm ISB 235 (G. Kaminsky) ! Wed 12-1:30pm ISB235 (O. Moreno & E. Santos) ! Thurs1-2:30pm ISB 165 (J. Martinez) ! Fri 12-1:30pm ISB 235 (G. Bowers) There will be 3 MSI section times ! Mon 12:30-1:40 Oaks Learning Center ! Mon 3:30-4:40 Oaks Learning Center ! Fri 11-12:10 ARC 221 I will also have office hours Fridays 9:30 - 10:30 3 Midterms Midterms will stay at the original dates ! Friday 4/22 (material on HWs 1-3) ! Friday 5/13 (material on HWs 4-6) 4 Field far from a dipole Far from a dipole the field of the two charges tend to cancel each other ! but the field is not completely zero Far from a dipole the field falls off as 1/r 3 compared to 1/r 2 for a point charge 5 Field of an infinite line of charge An (infinitely) long line of charge has a cylindrically symmetric electric field ! and has the form ! where ! is the linear charge density (in C/m) ++++++++++++++++++++++++++ 6 Dipoles, torque Torque " = r # F = p # E ! direction of rotation vector given by right hand rule into the paper/screen in this example ! physically, torque produces a rotation that tend to align the two vectors [See next page for expanded discussion of work] $ $ $ $ p = q d d $ $ $ 7 Dipole -- work and potential energy Left to itself a dipole in an electric field will align with the field If the dipole starts aligned with field, a torque is needed turn it to angle % ! external torque does work on dipole ! potential energy of dipole increases If dipole starts at angle % and rotates freely ! field does work on dipole ! dipole potential energy transformed to kinetic energy This is consistent with discussion of eqs. 20.9 and 20.10 in Wolfson....
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- Fall '09