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# Lecture15 - Announcement: .5/15: OceHours:TR10:3011:30am...

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Announcement: Test graded by Thursday Average for mul7ple choice 7.5/15: exam will be graded by Thursday Oﬃce Hours: TR 10:30‐11:30 am Physical Science Building Enter through the garage door behind the Harris engineering building

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Current: charge per unit 7me I = dQ dt 1 Amp = 1 C/sec
What is the speed of electrons? 1 volt 0 volt Poten7al energy: 1 electron volt U = 1 2 mv 2 1.6 × 10 19 J = 1 2 × 9.109 × 10 31 kg × v 2 v = 3.2 × 10 19 9.109 × 10 31 m / s = 0.35 × 10 6 m / s But in reality, charge carriers diffuse at mm/sec, why?

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Carrier velocity small because of scaRering v d : “DriU velocity”
A L n: volume density of electrons Q = neAL I = dQ dt = neA dL dt = neAv d Current

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A L Current Density J = I A J = σ E Ohm’s Law σ: conduc7vity
A L Ohm’s Law J = σ E Ohm’s Law 1 σ J = E L σ J = LE L σ I A = LE = V L σ A I = V L σ A = R RI = V

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R = ρ L A ρ = 1 σ R = L σ A Conduc7vity, Resistance and Resis7vity Resistance : Ω Resis7vity : Ω m Resis7vity Silver: 1.59×10 ‐8 Ωm Copper: 1.7×10 ‐8 Ωm Gold: 2.44×10 ‐8 Ωm Glass: 10 10 ~10 14 Ωm
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