lecture13 - Current & Resistance Current...

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Unformatted text preview: Current & Resistance Current & Resistance - Current and current density- Ohms 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 shaded disk of area A in time t : 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 Example...
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lecture13 - Current & Resistance Current...

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