Find the magnitude of the current density in a sample of silver for which σ=6.17x10^7 S/m if:
a) The electric field intensity is 1mV/meter.
b) The same is a cube 2.5mm on a side having a voltage of 0.4mV between opposite faces.
c) The sample is a cube 2.5mm on a side carrying a total current of 0.5A.