Lecture 14 - Current Resistance. Ohm s law

Lecture 14 - Current Resistance. Ohm s law - Electric...

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1 Lecture 14 Current. Resistance. Ohm’s law. Electric current = charges in motion. Units: A (Ampere) 1 A = 1 C/s dQ I dt Current for uniform current A dI J dA     Current density (current per cross- sectional area) A = cross-sectional area (Example: cross-section of a wire of radius r is π 2 ) In terms of microscopic quantities… current Current density = charge that goes though a cross-section of area in a time interval dt v d = drift velocity (average velocity of carriers) = charge in the box n = density of carriers (~ 10 29 m 3 ) carrier d Jq nv charge dA dt (charge density)(volume)  carrier q dt dA Historical parenthesis For historical reasons, even though we now know that the carriers are usually negative (electrons), current I is usually defined as positive from higher to lower potential . This means, opposite to the actual motion of the electrons or as if the positive charges were the the electrons, or as if the positive charges were the ones that are really moving.
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Lecture 14 - Current Resistance. Ohm s law - Electric...

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