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Re: lightning dangers



At 11:19 4/30/99 -0400, Herb wrote:

What do you mean by "current density?" Current refers to the number of
electrons that pass a given point in a given amount of time.
But CURRENT DENSITY
?????

Herb Gottlieb from New York City
(Where our population density is greatest in the inner city)


I find it helpful (like Herb evidently does) to think in terms
of science keywords.
So density starts out as a particular ratio:
e.g the ratio of mass per unit volume

The idea is generalized to
energy density - the energy per unit volume
charge density - the charge per unit volume
and (incorrectly) to
current density - the current per unit volume
when really, the current per unit cross-sectional area, is meant.

But this usage is by now enshrined and won't be shamed
from existance.

Compare the -density suffix with the specific- prefix.

Specific volume is volume per unit mass, for instance.
Then there's specific activity (activity per unit
mass of pure radioisotope), specific heat capacity
(heat capacity per unit mass), specific resistance (or resistivity)
and even specific impulse: impulse per unit fuel mass,
derived in a slightly tortured way from force or thrust per
unit fuel mass flow rate.
This too enshrined in custom as a 'time' by dividing units of
force by units of mass - not always wise.

Brian

brian whatcott <inet@intellisys.net>
Altus OK