Lecture03

# Lecture03 - Electric Field Point charge E= 1 q1 ^ r 4 0 r 2...

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

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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>3 × 10 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 ˆ 4 1 2 0 πε = r r q q F ˆ 4 1 2 2 1 0 πε = v<<c !!! F = q E A Fundamental Rationale

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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.
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