Complete Question
Two substances are identical except that the electron mean free time for substance A is twice the electron mean free time for substance B. If the same electric field exists in both substances the electron drift speed in A is:
Select one:
a. half that in B
b. four times that in B
c. twice that in B
d. the same as in B
e. one fourth that in B
Answer:
Option c is correct
Explanation:
From the question given
The electron mean free time substance for substance is [tex]\tau_A = 2 \tau_B[/tex]
The electron mean free time substance for substance B is [tex]\tau_B[/tex]
Generally the drift velocity for substance A is mathematically represented as
[tex]_A\vec v_{d} =-(\frac{e \tau_A}{m}) \vec E[/tex]
Generally the drift velocity for substance B is mathematically represented as
[tex]_B\vec v_{d} =-(\frac{e \tau_B}{m}) \vec E[/tex]
So
[tex]_A\vec v_{d} =-(\frac{e 2\tau_B}{m}) \vec E[/tex]
=> [tex]_A\vec v_{d} =2 _B\vec v_{d}[/tex]
