Apparatus Competition
2007 AAPT Summer Meeting
Greensburo, NC
A Simple Apparatus for the Study of
Electrostatic Forces Between Charged Parallel Plates
A.
Tomasch, A. Chang, D. Gerdes and M. Love
University of Michigan
Department of Physics
Abstract
We describe a simple apparatus to quantitatively
demonstrate the forces produced by the electrostatic attraction of charged
parallel plates. The apparatus is
based on an inexpensive digital scale with a least count of 0.01 gram,
sufficiently sensitive to measure the force produced on one of two circular
parallel plates with a ~5 cm radius.
The apparatus can be used to explicitly demonstrate the dependence of
the electrostatic force on the potential between the plates (
)
and the distance of separation between the plates (
). Students can derive the force acting on
a plate as a function of the potential difference between the plates and the
plate separation by using GaussÕ Law to relate the plate surface charge density
to the electric field between the plates and then applying the definitions of
the electric field and electrostatic potential. By measuring the electrostatic force on a plate for a known
potential difference and plate separation, 
can be measured to within ~10% of the
accepted value.
Description
This is an
exceptionally simple apparatus to construct. A small table is constructed from a commercial plastic
cutting board and 1Ó Lucite rod support legs. Two banana binding posts are mounted to the side of this
small support table. One plate
rests on the pan of the digital scale supported by a lightweight foam
pedestal. The second plate is
supported above the first with a simple Lucite plastic scaffold assembly. The upper plate can slide vertically to
set the inter-plate gap and is secured with a simple set screw. A fine copper wire lead, attached with
conductive copper tape, connects the bottom plate to the positive binding post. The upper plate is connected to the negative
binding post with a standard banana jack and alligator clip. A small plastic ruler supported by a
piece of scrap aluminum angle stock can be set next to the plates to estimate
the inter-plate gap.
Alternatively, an inexpensive digital caliper can be used to measure/set
the inter-plate gap. A special
low-current high-voltage power supply maintains a fixed potential
(0-6 kV)
between the plates. A photo of the
apparatus appears below:
Parts and Materials for
Electrostatic
Force Demonstration Apparatus
|
Equipment |
Supplier |
Website |
Bulk Price |
Price Per Unit |
|
Jennings
JS-50x Precision Digital Pocket Scale |
SaveOnScales |
http://www.saveonscales.com/product_jennings_js50x_digital_pocket_scale.html |
$35.90 |
$35.90 |
|
Plastic
Cutting Board (21 x 31 cm) |
Concord
Enterprises |
$0.69 |
$0.69 |
|
|
1"
Clear Extruded Acrylic Rod ±.030 - sold by the foot |
United
States Plastic Corp. |
$1.86 |
$1.86 |
|
|
Banana
Jack Dual Binding Post - 274-718 |
Radio
Shack |
http://www.radioshack.com/product/index.jsp?productId=2102838&cp |
$4.99 |
$4.99 |
|
Alloy Steel Button Head Socket Cap Screw 1/4"-20 Thread, 3/4"
Length - 100 per package |
McMaster
- Carr |
http://www.mcmaster.com/nav/framework.asp?reqtyp=catalog&CtlgPgNbr=2979&sesnextrep=616853790379949&CtlgEdition=112&k1=91255A540&t1=PN |
$13.91 |
$0.56 |
|
¾Ó
Square Acrylic Rod |
|
$ 15.68 |
$2.63 |
|
|
Two 4Ó
Diameter 1/16Ó Aluminum Plates |
|
|
|
~$2.00 |
|
3/8Ó
aluminum rod 6061 |
|
$10.10 |
$0.60 |
|
|
Misc: 8-32 Set Screw, Short Length Fine
Copper Wire, Small Piece of Copper Tape |
|
|
|
~$0.50 |
|
Plastic
Spring Clamp 2Ó |
|
McMaster
Carr Part Number 50065A22 |
|
$2.51 |
|
|
|
|
Total: |
$52.24 |