Lecture_11_(Chap.18) - Mean S. D. N 37.67 16.68 43 14.58...

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Mean 37.67 14.58 S. D. 16.68 7.64 N4 3 4 3 1
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Electron Current n electrons m 3 A m 2 v m s t s  nA v t electrons mobile electron ensity wire Cross sectional ea Average drift eed electrons density area speed Electron current: # electrons in A v t 2
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Typical Mobile Electron Drift Speed Typical electron current in a circuit is ~ 10 18 electrons/s. What is the drift speed of an electron in a 1 mm thick copper wire of circular cross section? 3 28 0 # electrons Av 3 28 10 4 . 8 m n 10 4 . 8 m n nAv t 2 3 2 A D 4 3.14 1 10 m 4 8 10 7 m 2 v 10 18 s -1 nA 10 18 s 8.4 10 28 m 3  8 10 7 m 2 1.5 10 5 m/s 3
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Typical Mobile Electron Drift Speed Typical electron current in a circuit is 10 18 electrons/s. What is the drift speed of an electron in a 1 mm thick copper wire? /s 5 0 3 28 10 4 . 8 m n m/s 10 5 . 1 v How much time would it take for a particular electron to move through a piece of wire 30 cm long? hours! s m/s m 5 . 5 10 2 10 5 . 1 3 0 4 5 . v s t How can a lamp light up as soon as you turn it on? 4
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Conventional Current In some materials current moving charges are positive: Ionic solution b i ti fi ld d i “Holes” in some materials (same charge as electron but +) Observing magnetic field around copper wire: Can we tell whether the current consists of electrons or positive ‘holes’? 2 0 ˆ 4 r r v q B 2 0 ˆ 4 r r v e B    0 2 ˆ 4 ev r r  The prediction of the Biot-Savart law is exactly the same in either case. 5
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Conventional Current 2 0 ˆ 4 r r v e B    0 2 ˆ 4 ev r r  Metals: current consists of electrons Semiconductors: - pe ectrons n type electrons p -type – positive holes Most effects are insensitive to the sign of mobile charges: v nA q i q I introduce conventional current : nits: /s (Ampere) Units: C/s A (Ampere) André Marie Ampère (1775 - 1836) 6
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Exercise A typical electron current in a circuit is 10 18 electrons/s.
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This note was uploaded on 04/23/2011 for the course PHYS 272h taught by Professor Staff during the Spring '08 term at Purdue University-West Lafayette.

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Lecture_11_(Chap.18) - Mean S. D. N 37.67 16.68 43 14.58...

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