Chabay/Sherwood

[Ludwik Kowalski <kowalskiL@MAIL.MONTCLAIR.EDU>]

Last night I wrote:

> 2) I did collect data on the geometry mentioned by Joseph.
> The diameter of my ring was 220 mm while the diameter of
> the central dot was 6 mm. I tabulated V(r) along four radii:
> left, right, up and down. Numbers are consistent. The average
> values of V(r), from four columns, are shown below.
r(cm)      2    3    4    5    6    7    8    9   10  11
V(vols) 76.0 92.9  105  115  123  129  135  140  145 150
> I calculated |E|= dV/dx and plotted them versus 1/r. The
> resulting straight line confirms what Joseph was saying ....

This is supposed to be the same as in electrostatics for a coaxial
cable. Theoretically (using Gauss's Law), the distribution of E
between the central wire and the shield is E=Z/r, where Z is
lambda (linear charge density divided over 2*PI*esp_o. The
slope of my E versus 1/r line turned out to be close to 40 SI
units. I calculated lambda from Z=40 and found it to be
2.3*e-9 C/m. The thickness of my silver circle is probably
close 0.005 mm. Multiplying this thickness by lambda I find
the charge of 1.15*e-14 C. This is 72,000 elementary charges.

Is it reasonable to think that this is the static charge constantly
residing on the central circle when the steady current is flowing
through the Pasco sheet? If not then what else can one learn
from the experimental value of the slope Z? For the time being I
am ignoring the suspected deviation from the 1/r law at r<2 cm.
Ludwik Kowalski