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Lecture 03

# Lecture 03 - Electric Field Point charge E= 1 q1 r 4 0 r 2...

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Electric Field ! E = 1 4 !" 0 q 1 r 2 ˆ r Point charge: Dipole: for r>>s : ! E = 1 4 0 2 qs r 3 ,0,0 at <r,0,0> ! E = ! 1 4 "# 0 qs r 3 ,0,0 at <0,r,0> +q -q s x y z at <0,0,r> ! E = ! 1 4 0 qs r 3 ,0,0 Dipole moment: p = qs Uniformly charged sphere: r r Q E sphere ˆ 4 1 2 0 = ! for r>R (outside) 0 = sphere E ! for r<R (inside)

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Chapter 14 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

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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. Conservation of Charge The net electric charge is conserved in any physical process. Charge can be transferred from one object to another. Annihilation: e + + e - = γ +
How can we decide if a piece of invisible tape is electrically charged? Charge creates E field; detected by another charged object: • the magnitude of E is proportional to amount of charge • the magnitude of E decreases with distance • the direction of E points directly away or to the charge E q F ! ! = force has the same properties as E Observing Electric Interactions

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1. Interaction of two U tapes Repel, attract or no interaction? Is it an electric interaction? direction
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Lecture 03 - Electric Field Point charge E= 1 q1 r 4 0 r 2...

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