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A current I flows through a uniform wire of diameter d when the mean drift velocity of the electrons is v. If the same current is passed through a wire of the same material but diameter d/2, what is the new mean drift velocity of the electrons?

1. v/4
2. v/2
3. 2v
4. 4v

Correct answer: 4v

Solution

Since I = n e A v_d is fixed and the area falls by a factor of 4 when d -> d/2, the drift velocity rises by a factor of 4 to 4v.

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