Answer
See work below.
Work Step by Step
Use equation 18-10 to answer these questions.
$$I=neAv_d$$
This leads to
$$v_d=\frac{I}{neA}$$
a. If the length doubles, the wire’s resistance doubles. The current I will be reduced by a factor of 2, and so will the drift velocity.
b. If the radius doubles, the area goes up by a factor of 4, but so does the current, I, so the drift velocity is unchanged. This is just like putting 4 identical wires in parallel – we wouldn’t expect the drift velocity to change.
c. If V doubles while the resistance remains constant, then the current also doubles. Using equation 18-10, we see that the drift velocity also doubles.