In Practice
In electrical wiring, the maximum current density can vary from 4A∙mm−2 for a wire with no air circulation around it, to 6A∙mm−2 for a wire in free air. Regulations for building wiring list the maximum allowed current of each size of cable in differing conditions. For compact designs, such as windings of SMPS transformers, the value might be as low as 2A∙mm−2. If the wire is carrying high frequency currents, the skin effect may affect the distribution of the current across the section by concentrating the current on the surface of the conductor. In transformers designed for high frequencies, loss is reduced if Litz wire is used for the windings. This is made of multiple isolated wires in parallel with a diameter twice the skin depth. The isolated strands are twisted together to increase the total skin area and to reduce the resistance due to skin effects.
For the top and bottom layers of printed circuit boards, the maximum current density can be as high as 35A∙mm−2 with a copper thickness of 35 µm. Inner layers cannot dissipate as much heat as outer layers; designers of circuit boards avoid putting high-current traces on inner layers.
In semiconductors, the maximum current density is given by the manufacturer. A common average is 1mA∙µm−2 at 25°C for 180 nm technology. Above the maximum current density, apart from the joule effect, some other effects like electromigration appear in the micrometer scale.
In biological organisms, ion channels regulate the flow of ions (for example, sodium, calcium, potassium) across the membrane in all cells. Current density is measured in pA∙pF−1 (picoamperes per picofarad), that is, current divided by capacitance, a de facto measure of membrane area.
In gas discharge lamps, such as flashlamps, current density plays an important role in the output spectrum produced. Low current densities produce spectral line emission and tend to favour longer wavelengths. High current densities produce continuum emission and tend to favour shorter wavelengths. Low current densities for flash lamps are generally around 1000A∙cm−2. High current densities can be more than 4000A∙cm−2.
Read more about this topic: Current Density
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