Lecture_03

Lecture_03 - Electric Field ! E = 1 4 !" q...

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Unformatted text preview: Electric Field ! E = 1 4 !" q 1 r 2 r Point charge: Dipole: for r>>s : ! E = 1 4 !" 2 qs r 3 ,0,0 at <r,0,0> ! E = ! 1 4 "# qs r 3 ,0,0 at <0,r,0> +q -q s x y z at <0,0,r> ! E = ! 1 4 "# qs r 3 ,0,0 Dipole moment: p = qs Uniformly charged sphere: r r Q E sphere 4 1 2 !" = ! for r>R (outside) = sphere E ! for r<R (inside) Choice of System Multiparticle systems: Split into objects to include into system and objects to be considered as external. To use field concept instead of Coulomb ` s law we split the Universe into two parts: the charges that are the sources of the field the charge that is affected by that field Example: Oscilloscope Charges on metal plates are the sources of an uniform E field Convenience: know E at some location know the electric force on any charge: Example : if E > 3x10 6 N/C air becomes conductor Retardation Nothing can move faster than light c c = 300,000 km/s = 30 cm/ns Can describe the electric properties of matter in terms of electric field independent of how this field was produced. Coulomb ` s law is not completely correct it does not contain time t nor speed of light c . r r q E 4 1 2 !" = ! r r q q F 4 1 2 2 1 !" = ! v<<c !!! ! F = q ! E A Fundamental Rationale Chapter 15 Matter and Electric Fields Matter is made out of atoms. Atom contains charged particles: electrons (-e ), protons ( +e ) Neutral atom: number of electrons and protons is equal: Example : Hydrogen atom: 1 proton, 1 electron net charge = (+e) + (-e)=0 Sodium atom: 11 protons, 11 electrons Sodium atom (Na) can lose an electron: Sodium ion (Na + ): (+11e) + (-10e) = +e Ordinary matter is electrically neutral. However, can be charged by adding/removing charged particles Net Charge The net charge of a system and its surroundings cannot change If one object gets charged positively, there must be an object which gets charged negatively....
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Lecture_03 - Electric Field ! E = 1 4 !" q...

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