Current Density

(Created prior to 1993)

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