lecture13

# lecture13 - Cur r ent& Resistance Cur r ent&...

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Unformatted text preview: Cur r ent & Resistance Cur r ent & Resistance - Current and current density- Ohm’s Law- Resistivity- Resistance Electrical Current Electrical Current CURREN T 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 Deter mining the cur r ent 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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lecture13 - Cur r ent& Resistance Cur r ent&...

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