Lecture05 - 1 January 14, 2008 Physics 132-Winter 2008...

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Unformatted text preview: 1 January 14, 2008 Physics 132-Winter 2008 Prof. Jim Beatty-Ohio State 1 Physics 132 Introductory Physics: Electricity and Magnetism Winter Quarter 2008 Lecture 5 January 14, 2008 Physics 132-Winter 2008 Prof. Jim Beatty-Ohio State 2 Field Lines of an Electric Dipole Electric dipole: two point charges +q and -q separated by distance d No net charge (neutral), but creates E field anyway Many diatomic molecules act like dipoles (e.g. CO,NO,HCl) Axial symmetry +- +- Two adjacent equal dipoles: “quadrupole”; … 2 January 14, 2008 Physics 132-Winter 2008 Prof. Jim Beatty-Ohio State 3 Electric Field on the Axis of a Dipole +q-q d E + E- P What is E field at point P? Add fields due to two charges: superposition z r E = r E + + r E − E + > E − at P r E + = + q 4 πε z − d 2 2 ) z r E − = − q 4 πε z + d 2 2 ) z r E = q 4 πε 1 z − d 2 2 − 1 z + d 2 2 ) z = q 4 πε z 2 + zd + d 2 4 − z 2 − zd + d 2 4 z − d 2 2 z + d 2 2 ) z = q 4 πε 2 zd z − d 2 z + d 2 2 ) z = q 4 πε 2 zd z 2 − d 2 4 2 ) z January 14, 2008 Physics 132-Winter 2008 Prof. Jim Beatty-Ohio State 4 “Far Field” of an Electric Dipole (on axis) r E = q 4 πε 2 zd z 2 − d 2 4 2 ) z ≅ q 4 πε 2 zd z 4 ) z r E ≅ qd 2 πε z 3 ) z +q-q d E + E- P z Far away, z >> d. How does E behave? r p = q r d r E = r p 2 πε z 3 Vectors p and d point from- to + charge The combination qd is the dipole moment 3 January 14, 2008 Physics 132-Winter 2008 Prof. Jim Beatty-Ohio State 5 Continuous Charge Distributions Q Q Q Q λ =Q/L σ =Q/A ρ =Q/V So far: Point Charges Now consider charge Q “smeared out” Line of charge λ : charge per length Surface/sheet of charge σ : charge per area Volume of charge ρ : charge per volume January 14, 2008 Physics 132-Winter 2008 Prof. Jim Beatty-Ohio State 6 Calculating E for a Charge Distribution Charge is really discrete (quantized)… But in real materials there are so many charges we can regard them as continuous....
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This note was uploaded on 04/18/2008 for the course PHYS 132 taught by Professor Beatty during the Winter '08 term at Ohio State.

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Lecture05 - 1 January 14, 2008 Physics 132-Winter 2008...

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