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# lecture13 - Current Resistance OhmsLaw Resistivity...

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Current & Resistance Current & Resistance - Current and current density -  Ohm’s Law -  Resistivity -  Resistance

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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

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Charge  Δ Q in length L of wire passes through the  shaded disk of area A in time  Δ t  :    Δ Q  = (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

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Example Example 3 m electrons/    29 10 n 3 C/m    .
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