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Unformatted text preview: 1 Chapter 25: Current, resistance, and electromotive force 2 e e e e e e e e A metal wire. Assume electrons flow to the right. Current is a measure of the amount of charge that passes though an area perpendicular to the flow of charge. dt dq I = : Current Current units: 1C/sec = 1 amp = 1 A 25.1 Current 3 Charge will flow as long as there is a potential difference. The direction of current flow in a wire is opposite the flow of the electrons. (In the previous drawing the current is to the left.) 4 Example: If a current of 80.0 mA exists in a metal wire, how many electrons flow past a given crosssection of the wire in 10.0 minutes? ( ) ( ) C . 48 sec 600 A 10 . 80 3 = = = = t I q t q dt dq I electrons 10 00 . 3 C/electron 10 1.60 C . 48 electron per charge q electrons of # 20 19 = = = 5 Electrons in a metal might have a speed of ~10 6 m/s, but since the direction of travel is random, an electron has v drift = 0. 6 It is only when the ends of a wire are at different potentials (E 0) will there be a net flow of electrons along the wire (v drift 0). Typically, v drift < 1 mm/sec. 7 Calculate the number of charges (N e ) that pass through the shaded region in a time t: ) ( ) ( t v nA Al n N d e = = The current in the wire is: ( ) d d nqAv dt dt nAv q dt dq I = = = l 8 Define the current density d nqv A I J = = In vector form d nq v J = A circuit has direct current if the direction of the current does not change. 9...
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 Fall '07
 Fuchs
 Charge, Current, Resistance, Force

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