"John E. Davis" wrote in message
...
On Sat, 10 Mar 2007 13:08:39 GMT, Dave
wrote:
no, i'm afraid you can't just put a 't' on each side and have it make
sense
in the general case. time varying charge implies a current, a current
implies a magnetic field, then you have to include Ampere's law and add
curl(E)=-dB/dt to the mix. while you may be able to constrain the changes
in rho(t) to some short time or constant current and eliminate the dB/dt
part of the problem, that would only apply in specific conditions, not to
the general case.

I encourage you to review the Maxwell equations in a book on
electrodynamics. I personally like the book by Jackson, which is
oriented more towards physicists. In any case, the equation that I
wrote is one of the 4 Maxwell equations. It is valid for arbitrary
time-dependent electric fields. All it says is that the divergence of
the electric field at a point is proportional to the charge density at
that point:

div E(x,t) = 4\pi\rho(x,t) (Gaussian Units)

If you integrate this over a closed surface, and then use the
divergence theorem you get

\integral dA.E(x,t) = 4\pi \integral dV \rho(x,t)

The integral on the right-hand side is 4\pi times the total
(time-varying) charge enclosed by the surface. The other equations
are also valid, including the one you wrote.

Coincidently earlier this morning I was reviewing the derivation of
the energy loss of a heavy charged particle as it passes through
matter. The derivation made use of a very long cylinder with the
charged particle traveling along the axis of the cylinder. One point
in the calculation required the integral of the normal component
electric field (dA.E) produced by the charged particle over the
surface of the cylinder. That is, the left hand side of the above
equation. The answer is given by the right hand side of the above
equation. In this case, the charge density \rho(x,t) was created by
the moving charged particle.

--John

Gauss's law in Jackson's 'Classical Electrodynamics' 2nd edition, ppg
30-32,33 has NO 't'. nor does it in Ramo-Whinnery-VanDuzer 'Fields and
Waves in Communications Electronics' ppg 70-72(differential form),
75-76(integral form)

your final statement means that you are obviously outside the applicability
of Gauss's law since you have a moving charged particle, which can not be
described by a static field. i would guess that whatever derivation you are
looking at placed some other restrictions on the conditions such that you
could approximate the field by that type of equation. possibly a small
velocity or short distance or very short time period.