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If the matter field is taken so as to describe the interaction of electromagnetic fields with the Dirac electron given by the four-component Dirac spinor field ψ, the current and charge densities have form: [2] = † = †, where α are the first three Dirac matrices. Using this, we can re-write Maxwell's equations as:
The conventional "hole" current is in the negative direction of the electron current and the negative of the electrical charge which gives I x = ntw(−v x)(−e) where n is charge carrier density, tw is the cross-sectional area, and −e is the charge of each electron.
The above equations are the microscopic version of Maxwell's equations, expressing the electric and the magnetic fields in terms of the (possibly atomic-level) charges and currents present. This is sometimes called the "general" form, but the macroscopic version below is equally general, the difference being one of bookkeeping.
This is an essential feature of Dirac's theory, which ties the spinor formalism to the geometry of physical space. A manner of regarding a spinor as acting upon a vector, by an expression such as ψv ψ. In physical terms, this represents an electric current of Maxwell's electromagnetic theory, or more generally a probability current.
The difference between copper, steel, and rubber is related to their microscopic structure and electron configuration, and is quantified by a property called resistivity. In addition to geometry and material, there are various other factors that influence resistance and conductance, such as temperature; see below .
Classical electromagnetism or classical electrodynamics is a branch of physics that studies the interactions between electric charges and currents using an extension of the classical Newtonian model. It is, therefore, a classical field theory .
If the geometry is more complicated, or if the resistivity varies from point to point within the material, the current and electric field will be functions of position. Then it is necessary to use a more general expression in which the resistivity at a particular point is defined as the ratio of the electric field to the density of the current ...
In such conditions, Ohm's law states that the current is directly proportional to the potential difference between two ends (across) of that metal (ideal) resistor (or other ohmic device): =, where I {\displaystyle I} is the current, measured in amperes; V {\displaystyle V} is the potential difference , measured in volts ; and R {\displaystyle ...