# lecture 17-1 - Chapter 17 Current and Resistance Electric...

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Chapter 17 Current and Resistance

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Electric Current Whenever electric charges of like signs move,  an  electric current  is said to exist The current is the  rate at which the charge  flows through this surface Look at the charges flowing perpendicularly to a  surface of area A The SI unit of current is Ampere (A) 1 A = 1 C/s Q I t
Electric Current, cont The direction of the current is the direction  positive charge would flow This is known as  conventional current direction In a common conductor, such as copper, the current is  due to the motion of the negatively charged electrons It is common to refer to a moving charge as a  mobile  charge carrier A charge carrier can be positive or negative

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Current and Drift Speed Charged particles move  through a conductor of  cross-sectional area A n is the number of  charge carriers per unit  volume n A Δx is the total  number of charge  carriers
Current and Drift Speed, cont The total charge is the number of carriers  times the charge per carrier, q ΔQ = (n A Δx) q The drift speed, v d , is the speed at which the  carriers move v d  = Δx/ Δt Rewritten: ΔQ = (n A v d  Δt) q Finally, current, I = ΔQ/Δt = nqv d A

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Current and Drift Speed, final If the conductor is isolated, the  electrons undergo random motion When an electric field is set up in the  conductor, it creates an electric force on  the electrons and hence a current
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## This note was uploaded on 09/06/2009 for the course PHYSICS Physics 2A taught by Professor Sydney.sukuta during the Fall '09 term at San Jose City College.

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lecture 17-1 - Chapter 17 Current and Resistance Electric...

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