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Current Density
Name: Unknown
Status: N/A
Age: N/A
Location: N/A
Country: N/A
Date: Around 1993
Question:
Why does the current density have a unit of J sub c?
Replies:
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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Update: June 2012
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