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A wire of non-uniform cross-section carries a steady current. If the cross-section at P is larger than at Q, how do the electron drift speeds vₚ (at P) and v_q (at Q) compare?

1. vₚ < v_q
2. vₚ = v_q
3. vₚ > v_q
4. data insufficient

Correct answer: vₚ < v_q

Solution

Since I = neAv is constant and the area at P is larger, the drift speed at P is smaller; thus vₚ < v_q.

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