Find the magnitude of the current density in a sample of silver for which conductivity equal to 6.17*10^7 S/m and mobility of electron is equal to...

Find the magnitude of the current density in a sample of silver for which conductivity equal to 6.17*10^7 S/m and mobility of electron is equal to 0.0056 m^2/v if 1)The sample is a cube 2.5 mm on a side having a voltage of 0.4mv between opposite faces 2)The sample is a cube 2.5 mm on a side carrying a total current of 0.5A.