A metal wire is subjected to a constant potential difference. When the temperature of the metal wire increases, the drift velocity of the electron in it ______________.
Fill in the blank with the correct answer from the options given below.
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → decreases, thermal velocity of the electrons increases
The drift velocity depends on the relaxation time \( \tau \), the average time between collisions:
\( \displaystyle v_d = \frac{eE}{m}\,\tau \)
With a constant potential difference, the field \( E \) in the wire is unchanged. When the temperature rises, the ions vibrate more strongly and the electrons collide more often, so \( \tau \) decreases and the drift velocity decreases (this is why the resistance of a metal rises with temperature).
The thermal velocity of the electrons is the speed of their random motion, which grows with temperature, so it increases.
NCERT (2026-27):
Temperature Dependence of Resistivity – “… As we increase temperature, average speed of the electrons, which act as the carriers of current, increases resulting in more frequent collisions. The average time of collisions τ, thus decreases with temperature.”