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The drift velocity of an electron is v_d in a conductor of area of cross-section A and carries a current I. Now, the area of cross-section and current flowing through the conductor are double, then new drift velocity of the electron is ________.

Asked in GUJCET 2025 · Drift velocity, mobility and current density

• (1) (v_d)/2
• (2) (v_d)/4
• (3) 4v_d
• (4) v_d

Step-by-step solution

Why the other options are wrong

• (1) (v_d)/2 allows for the doubled area (v_d ∝ 1/A) but forgets that the current is doubled too, which doubles v_d back.
• (2) (v_d)/4 divides by both factors of 2, as if v_d fell with the current as well. More current through the same area means faster drift: v_d ∝ I.
• (3) 4v_d multiplies by both factors of 2, as if v_d grew with the area. A larger area spreads the current out and lowers the drift velocity: v_d ∝ 1/A.

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