### Conductors

When a potential difference of 115 V is applied between the ends of a
9.66-m-long wire, the current density is J = 1.42 A/cm^{2}. Calculate
the conductivity of the wire material.

For a hypothetical electronic device, the potential difference D*V*,
measured across the device, is related to the current *i* by D*V* = (3.55 x
10^{6} V/A^{2})*i*^{2}. (a) Find the resistance
when the current is 2.40 mA. (b) At what value of the
current is the resistance equal to 16.0 W?

Calculate the mean free time *t*
between collisions for conduction electrons in aluminum at 20 C
using tables from the lecture. Each atom of aluminum contributes three
conduction electrons. The density of aluminum is 2.7 g/cm^{3} and it has
a molar mass of 27 g/mol.