# Lec03-42009 - ture 3-1 Electric Field Define electric...

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cture 3-1 Electric Field Define electric field , which is independent of the test charge, q, and depends only on position in space: F E q = ur ur dipole One is > 0, the other < 0 -q q electric dipole of dipole moment : p qd = ur ur d ur - +

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cture 3-2 Dipole in uniform electric fields No net force. The electrostatic forces on the constituent point charges are of the same magnitude but along opposite directions. So, there is no net force on the dipole and thus its center of mass should not accelerate. Net torque! There is clearly a net torque acting on the dipole with respect to its center of mass, since the forces are not aligned. http://qbx6.ltu.edu/s_schneider/physlets/main/dipole_torque.shtml E p r r r × = τ
cture 3-3 Electric Field from Coulomb’s Law 2 0 1 ˆ 4 i i i i q E r r πε = r + + + + + - - - + - - Bunch of Charges dV dq dA dL ρ σ λ = (volume charge) (surface charge) (line charge) Continuous Charge Distribution dq 2 0 1 ˆ 4 dq E r d E r = = ∫ ∫ ur r P r r Summation over discrete charges Integral over continuous charge distribution P i r r i q k http://www.falstad.com/vector3de/

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cture 3-4 Reading Quiz 1 Which one of the following statements is incorrect ? A) Electric fields leave positive charges and end on negative charges B) Electric field lines can intersect at some points in space. C) Electric field field lines from a dipole fall off faster than 1/r 2 . D) Electric fields describe a conservative force field.
cture 3-5 SUMMARY: FIND THE ELECTRIC FIELD GIVEN THE CHARGES 1) GEOMETRY FOR q i or dq i 2) DISCRETE CHARGES q i 3) CONTINUOUS CHARGES dq i line charge density λ (x) surface charge density σ (x.y) volume charge density ρ (x,y,z) Geometry may suggest other coordinate systems, R,θ,Φ or R,θ,Z

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## This note was uploaded on 12/07/2011 for the course PHYS 241 taught by Professor Wei during the Fall '08 term at Purdue University-West Lafayette.

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Lec03-42009 - ture 3-1 Electric Field Define electric...

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