# How do I calculate the current density? In what measure does the conductance specify?

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According to the definition, the current density would be the intensity of current flowing through the section of a cable for each unit of surface of its section, does that mean in each square millimeter? But if I have a section smaller than 1 mm2, of 0.785 mm2 and circulate 4 A, does not the density calculation run there? And the other, is the conductivity measured in siemens per meter or per square millimeter?

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Let's go by parts:
The current density is usually expressed, as you say, like A /mm^2.
That is, if you have a thread gauge uniform 0.75 mm^2, and you circular 4, ... THE DENSITY would be DIRECTLY:= 4 TO / 0.75 mm^2 = 5.33 A / mm^2
This value is just a value limit for drivers domestic PVC insulated normal where it is usually taken preventively to 5 A/ mm^2 as the density of calculus. For sections greater than the density seen decreasing as the diameter grows.
Obviously if it is a thread-bare copper hung in the air ( or submerged in the water) the density supported may be several times greater.
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The Conductance is , by definition, the inverse of the resistance.Or is C = 1/ resistance = 1/ohm......this is called 1 Siemens.
The conductivity would then the inverse of the Resistivity... As the unit of resistivity = ohm.m ... the unit of Conductivity = 1/ ohm.m. The express S/ m. ( Siemens per meter).