## Microscopic View of Ohm's Law
The current density (electric current per unit area, J=I/A) can be expressed in terms of the free electron density as The number of atoms per unit volume (and the number free electrons for atoms like copper that have one free electron per atom) is From the standard form of Ohm's law and resistance in terms of resistivity: The next step is to relate the drift velocity to the electron speed, which can be approximated by the Fermi speed:
The drift speed can be expressed in terms of the accelerating electric field E, the electron mass, and the characteristic time between collisions. The conductivity of the material can be expressed in terms of the Fermi speed and the mean free path of an electron in the metal.
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## Microscopic View of Copper WireAs an example of the microscopic view of Ohm's law, the parameters for copper will be examined. With one free electron per atom in its metallic state, the electron density of copper can be calculated from its bulk density and its atomic mass. The Fermi energy for copper is about 7 eV, so the Fermi speed is The measured conductivity of copper at 20°C is The mean free path of an electron in copper under these conditions can be calculated from The drift speed depends upon the electric field applied. For example, a copper wire of diameter 1mm and length 1 meter which has one volt applied to it yields the following results. For 1 volt applied this gives a current of 46.3 Amperes and a current density This corresponds to a drift speed of only millimeters per second, in contrast to the high Fermi speed of the electrons.
(If you scale down the voltage applied so that the current is just 3 Amperes, current density 382 A/cm | Index | ||

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## Free Electron Density in a MetalThe free electron density in a metal is a factor in determining its electrical conductivity. It is involved in the Ohm's law behavior of metals on a microscopic scale. Because electrons are fermions and obey the Pauli exclusion principle, then at 0 K temperature the electrons fill all available energy levels up to the Fermi level. Therefore the free electron density of a metal is related to the Fermi level and can be calculated from
The number of atoms per unit volume multiplied by the number of free electrons per atom should agree with the free electron density above. While these two approaches should be in agreement, it may be instructive to examine both for self-consistency. Consider the element zinc with a tabulated Fermi energy of 9.47 eV. This leads to a free electron density of From the Periodic Table, the density of zinc is 7140 kg/m The number of free electrons per zinc atom to make these consistent is This number is what we would expect from the electron configuration of zinc, (Ar)3d | Index | |||

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