Where " ? " is the charge density, which can ( and often does ) depend on time and position, " ? " 0 is the electric constant, " ? " 0 is the magnetic constant, and "'J "'is the current per unit area, also a function of time and position.
12.
Where d \ mathbf { l } is a vector whose magnitude is the length of the differential element of the wire in the direction of " conventional current ", \ mathbf { r'} = \ mathbf { r }-\ mathbf { l }, the full displacement vector from the wire element ( \ mathbf { l } ) to the point at which the field is being computed ( \ mathbf { r } ), and ? 0 is the magnetic constant.
13.
The physical constant " & mu; " 0, commonly called the "'vacuum permeability "', "'permeability of free space "', "'permeability of vacuum "'or "'magnetic constant "'is an ideal, ( baseline ) physical constant, which is the value of magnetic permeability in a classical vacuum . " Vacuum permeability " is derived from production of a magnetic field by an electric current or by a moving electric charge and in all other formulas for magnetic-field production in a vacuum.
