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Name: Unknown
Status: N/A
Age: N/A
Location: N/A
Country: N/A
Date: Around 1993

Why does the current density have a unit of J sub c?

I think you are misusing the word "unit" here. The units of current density (i.e. current per unit area) are amps per square meter. The reason for this is simply that current is a flow of charge per unit time, and it has a direction (the direction the charge is moving in) and so if you consider a plane perpendicular to the direction of flow, the current is exactly the charge going through that plane per unit time. If you have a uniform material that extends out over an area A in this plane, through which the current is flowing, then the "intensive" quantity (like density, charge density, etc) associated with the current in this material is just the current divided by the area A. I assume the J sub c you are referring to is a symbol used for current density. The most common symbol is simply a J. I guess to differentiate from total current usually denoted I. The "sub c" is probably to distinguish one kind of current from another. The flow of heat can also be measured by a heat current density, and usually has quite a different symbol (something involving Q, for example). But symbols really do not matter that much, although there are conventions in physics that help people communicate more quickly, you can usually be understood even if you use nonstandard symbols.

A. Smith

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