| Chronology | Current Month | Current Thread | Current Date |
| [Year List] [Month List (current year)] | [Date Index] [Thread Index] | [Thread Prev] [Thread Next] | [Date Prev] [Date Next] |
Brian Whatcott wrote
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.
Brian, this tread has strayed a bit from lightning (and that was very
interesting) but I think your comments about Density are worth
thinking about.
I would think the original idea of density was a measure of how
thickly or thinly something existed in a space, so the more crowded
something is the more dense it is.
That means that the something is any appropriate quantity and the
space is 3-dimensional or two-dimensional or one-dimensional (or
n-dimensional, if for example we want to refer to the density of
states in some kind of phase space).
Now, in physics, if we just say density, the quantity we are probably
referring to is mass and the space is 3-dimensional, so its a (volume)
mass density, the ratio of mass to density with SI units kg/m^3. (By
the way, I find the term" per volume" rather meaningless, but that is
another story.)
Beside this mass density, there are many other densities that fit the
general concept:
surface mass density (how thickly mass is distributed across a
surface; I guess that is a superficial density);
linear mass densities (how thickly mass is distributed along a line; a
linear density;
volume, surface and linear charge densities;
volume, surface and linear number densities;
cross-sectional current densities (nothing anomalous, just one of many
densities in this family);
volume and cross-sectional energy densities;
etc, etc
Brian also comments on "specific" as ratios of quantities to mass.
One of these is unfamiliar to me and I can't get the hang of it:
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.
I agree that unless one is looking for a quantity with dimensions the
same as acceleration, it is not wise to do this but you lost me along
the way.
Brian McInnes