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Suppose current, $I=nevA$ with the electron density $n$, the cross section of the conductor $A$ and the drift velocity $v = \frac{V}{RneA}$ where $R$ is the resistance of the wire. This equation becomes $v/dlne$ where $d$ is the resistivity of the wire if we break the $R$ into the formula $R=\frac{dl} A$. So drift velocity should not depend on $A$ or area of cross section. Which way am I wrong?

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  • $\begingroup$ have a look here, hyperphysics.phy-astr.gsu.edu/hbase/electric/miccur.html , drift velocity does not depend on area. The current does. $\endgroup$ – anna v Apr 19 '18 at 9:14
  • $\begingroup$ @annav is my derivation right. And I didn't get it in your link $\endgroup$ – TIME RUB Apr 19 '18 at 10:10
  • $\begingroup$ in the same site , hyperphysics.phy-astr.gsu.edu/hbase/electric/ohmmic.html . It has a calculation of resistance. I was refering to your title. $\endgroup$ – anna v Apr 19 '18 at 10:46
  • $\begingroup$ Where does your second equation come from? $\endgroup$ – BowlOfRed May 9 '18 at 17:29
  • $\begingroup$ @BowlOfRed sorry to reply that later. Put I=V/R in my first equation. Cross multiplying you'll get the amount of v or drift velocity as my second equation. $\endgroup$ – TIME RUB May 10 '18 at 1:04
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$V$ in your equation is the voltage drop over the restive element. It's not the voltage in the circuit.

It depends on the resistance of the element and cannot be presumed to be constant.

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