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# lecture14 - Current Resistance(Conductors NOT in...

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Current & Resistance Current & Resistance (Conductors NOT in equilibrium; E 0) Text sections 27.1, 27.2 Current and current density Ohm’s Law, resistivity, and resistance Practice problems: 27-5, 27-9, 27-15, 27-17 Electrical Current Electrical Current CURRENT I is the charge per unit time flowing along a wire: if charge dQ flows in time dt Units : 1 ampere (A) = 1 C/s Direction : by convention , the direction of movement of positive charge dt dQ I + - + + + + + - - - - - I I

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+ + + + + + + + + + + + A L = v d t Charge passing through the shaded circle in time t : Q = (number of charges/volume) x (charge on each one) x volume Current: I = Q/ t = nqAv d t / t So, Q = n ·q ·(AL) = nqAv d t I = nqAv d v d = average (“drift”) velocity of each charge Current Density Current Density (a vector vector ): A I J flow current area is A where Units: Amps/m 2 (Note that the “current through a surface” is the flux of the current density through that surface.) So, J = nq v d (a vector equation) In normal conductors, J is caused by an electric field in the conductor—which is not in equilibrium .
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lecture14 - Current Resistance(Conductors NOT in...

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