Physics Asked on June 13, 2021
I am having trouble reconciling two seemingly contradictory results. Consider two conductors, X and Y, made from the same material with the same length and carrying the same current. If X has twice the cross-sectional area of Y, then from the formula $I=nevA$, we deduce that the drift velocities for X and Y have the ratio
$$v_X=frac{v_Y}{2}.$$
Now, consider the resistance of X and Y. Since resistance is inversely proportional to current, and current is proportional to drift velocity, resistance must be inversely proportional to drift velocity. This gives the result:
$$R_X=2R_Y.$$
However, this would seem to contradict the formula for the resistivity of a wire,
$$R=frac{rho l}{A}$$
which states that resistance is inversely proportional to area. Since X has twice the area of Y (and being made from the same material with equal length, $rho$ and $l$ are constant) we obtain the contradictory result
$$R_X=frac{R_Y}{2}.$$
I have tried to reconcile this by substituting one formula into the other. Rearranging the resistivity formula to make $A$ the subject
$$A=frac{rho l}{R}$$
and substituting this into $I=nevA$ we get
$$R=frac{nrho l e}{I} v.$$
Since $n, rho, l, e$ and $I$ are all constant, we now see that $Rpropto v$. This resolves the contradiction, but I am left confused.
My intuition tells me that $Rpropto 1/v$ seems more reasonable. If the resistance increases, then it should decrease the average velocity of the electrons along the wire. So, my question is, why does drift velocity increase with resistance in this case?
Drift velocity depends on the applied voltage bias, whereas resistance does not. The first ration between the drift velocities is obtained that the two conductors carry the same current, which means that the potential differences applied to the two conductors are different.
You could get a deeper understanding of this by reading about the Drude model.
Correct answer by Roger Vadim on June 13, 2021
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