PASCO ES-9060 Charge, Equipotential and Field Mapper - Manual (Page 17)

Charge, Equipotential, and Field Mapper Model No. ES-9060
16 ®
Charge Mapping Experiments
The purpose of a charge mapping experiment is to investigate the way
charge is distributed over a surface by measuring variations in charge
density. A charged surface will be sampled with a proof plane. The
proof plane will be inserted inside the Faraday Ice Pail to measure the
charge. By sampling different sections of the surface, the relative
charge density can be observed. For example, you may find that the
amount of charge on two equal sized regions on the surface of a
conductor may differ in magnitude or even in sign. This occurs for
non-uniform charge distribution. Alternately, you may observe that
everywhere on the surface the charge has the same magnitude and
sign. This occurs for uniform charge distribution.
An important aspect of measuring charge distributions is charge
conservation. The proof plane removes some charge from the surface
it samples. If the proof plane is grounded after each measurement, the
surface will be depleted of charge with consecutive measurements.
However, by not grounding the proof plane (and by not letting it touch
the ice pail), the charge on the surface is not depleted. That charge
which the proof plane removed for one measurement is always
returned to the surface when the next sampling is made.
The Proof Plane
As shown in Figure 10, the
proof plane is an aluminum-
covered conductive disk
attached to an insulated
handle. The conductive disk
material is carbon-filled black
polycarbonate (about 10
3 Ω)
with an aluminum disk. The
nonconducting neck is white
polycarbonate (about 10
14 Ω).
The proof plane is used to sample the charge density on charged
conductive surfaces. A Faraday Ice Pail can then be used to measure
the charge density on the proof plane.
Figure 10: Proof Plane
aluminum surface
handle
non-conductive neck
(white)
conductive disk
(black)