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Homework Help
Introductory Physics Homework Help
Current density and drift velocity
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[QUOTE="Parad0x88, post: 4108255, member: 441610"] [h2]Homework Statement [/h2] 1. A copper wire 2 mm in diameter carries a 4 A current. Determine the [B]current density[/B] in the wire, [B]drift velocity[/B] of the electrons and the [B]mean time between collisions[/B]. Assume that each copper atom contributes one free electron. Hint: you may need some additional parameters of copper to solve this problem. [h2]Homework Equations[/h2] A) For current density: J = I/A We know: A = ∏r[SUP]2[/SUP] m[SUP]2[/SUP] B) Drift velocity: I = nqV[SUB]d[/SUB]A . thus V[SUB]d[/SUB] = I / (nqA) We know: n = density/mass atom, or (8.95g/cm[SUP]3[/SUP] X 10[SUP]6[/SUP] cm[SUP]3[/SUP]/m[SUP]3[/SUP]) / 1.05565 X 10[SUP]-22[/SUP]g/atom = 8.478 X 10[SUP]28[/SUP] atoms/m[SUP]3[/SUP] q = 1.602 X 10[SUP]-19[/SUP] C C) σ = (nqτ[SUP]2[/SUP])/m[SUP]e[/SUP], thus τ = σm[SUB]e[/SUB] / nq[SUP]2[/SUP] and σ = 1/p, p being resistivity, and resistivity for copper is 1.7 X 10[SUP]-8[/SUP] ohm*m [h2]The Attempt at a Solution[/h2] A) ∏*0.002[SUP]2[/SUP]m[SUP]2[/SUP] = 1.2265 X 10[SUP]-5[/SUP] m[SUP]2[/SUP] 4 / 1.2265 X 10[SUP]-5[/SUP] = 318,309.89 A B) Simply plug in the numbers: V[SUB]d[/SUB] = 4A / (8.478 X 10[SUP]28[/SUP]/m[SUP]3[/SUP] * 1.602 X 10[SUP]-19[/SUP] C * 1.2265 X 10[SUP]-5[/SUP] m[SUP]2[/SUP] = 2.4012 X 10 [SUP]-5[/SUP] m/s C) For C, I have no clue in which direction to go, I've browsed through my book but can't find any relevant formula Any help for letter C would be appreciated, and also if you can check if what I did in A and B makes sense? Thank you! Edit: I found something in the book, would the formula on C make sense? [/QUOTE]
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Introductory Physics Homework Help
Current density and drift velocity
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