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Unformatted text preview: Current & Resistance Current & Resistance  Current and current density Ohm’s Law Resistivity Resistance Electrical Current Electrical Current CURRENT I is the amount of positive charge flowing past a fixed point in the wire per unit time : if charge dQ flows in time dt Units : 1 ampere (A) = 1 C/s Direction : by convention, current is the direction of movement of positive charge + + + + + + I I dQ I dt = Electron Velocities Electron Velocities • Random velocities of electrons are large (several km/s) • Drift velocity is a slow, average motion parallel to E start end start end net displacement + ( ) F e E = + r E no field Charge Q in length L of wire passes through the Δ shaded disk of area A in time Δ t : Q Δ = (number of charge carriers/volume) x (charge on each one) x volume + + + + + + + + + + + + L = v d ∆ t E Determining the current Current: I = ∆ Q/ ∆ t = nqAv d ∆ t / ∆ t So, Charge: ∆ Q = n q V = n q (AL) = n q A v d ∆ t (since L=vt) I = nqAv d v d = average (“drift”) velocity of each charge q = charge on each particle n = number of charge carriers per unit volume A = cross section area L = length AL = volume Example Example 3 m electrons/...
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This note was uploaded on 03/16/2011 for the course PHYSIC 1e03 taught by Professor Dr.oak during the Spring '10 term at McMaster University.
 Spring '10
 dr.oak
 Charge, Current, Resistance

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