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# Ch17 - Chapter 17 PHY213 Lecture 6 Electric Current Chapter...

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Chapter 17 PHY213, Lecture 6 1 Chapter 17 Current and Resistance (skip 17.5: temperature variation) 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 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

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Chapter 17 PHY213, Lecture 6 2 Current and Drift Speed, cont The total charge is the number of carriers times the charge per carrier, q Δ Q = (n A Δx) q The drift speed, v d , is the speed at which the carriers move v d = Δx/ Δt Rewritten: Δ Q = (n A v d Δt) q Finally, current, I = ΔQ/Δt = nqv d A 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
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