Lect_10_note

Lect_10_note - Chapter 27 - 1 Current and Resistance Free...

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Chapter 27 - 1 Current and Resistance
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Free Charge in E field ± Free charges move from high electrical potential to low electrical potential.
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Charge Flow - Electric Current ± Assume charges are moving perpendicular to a surface of area A ± If Δ Q is the amount of charge that passes through A in time Δ t , then the average current is av Q t = I
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Electric Current ± Electric current is the rate of flow of charge through some region of space ± The SI unit of current is the ampere (A) ± 1 A = 1 C / s ± The symbol for electric current is I
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Current in a conductor ± Charged particles move through a conductor of cross- sectional area A ± n is the number of charge carriers per unit volume ± nA Δ x is the total number of charge carriers
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Current ± The total charge is the number of carriers times the charge per carrier, q ± Δ Q = ( nA Δ x ) q ± v d is the speed at which the carriers move ± v d = Δ x / Δ t, Δ x = v d Δ t, ± Rewritten: Δ Q = ( nAv d Δ t ) q ± Finally, current, I av = Δ Q / Δ t = nqv d A av Q t = I
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Current Density ± Current per unit area ± J = I / A = nq v d
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Relation between Current and E field E J = ? ( E )
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Georg Simon Ohm (1789 - 1854) Ohm’s Law ± J = σ E , = const. ± Linear relation between the current density and the electric field ± The ratio is a constant that is independent of the electric field producing the current – conductivity ± ρ =1/ : resistivity E
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This note was uploaded on 01/28/2011 for the course PHYS 011 taught by Professor Nianlin during the Fall '08 term at HKUST.

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Lect_10_note - Chapter 27 - 1 Current and Resistance Free...

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