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 Gauss’ 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 1” 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

http://www.dollaritem.com/Dollar_Store/productid.asp?SessionID={7157EBED-6A57-428D-915B-6999DB15C2EE}&ID=44296

$0.69

$0.69

1" Clear Extruded Acrylic Rod Ī.030 - sold by the foot

United States Plastic Corp.

http://www.usplastic.com/catalog/product.asp?catalog%5Fname=USPlastic&category%5Fname=21315&product%5Fid=18413

$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

¾” Square Acrylic Rod

 

http://www.usplastic.com/catalog/product.asp?catalog%5Fname=USPlastic&category%5Fname=21315&product%5Fid=21156

$ 15.68

 

$2.63

 

Two 4” Diameter 1/16” Aluminum Plates

 

 

 

      ~$2.00

 

3/8” aluminum rod 6061

 

http://www.mcmaster.com/itm/find.ASP?tab=find&context=psrchDtlLink&fasttrack=False&searchstring=8974K32

     $10.10

 

        $0.60

 

Misc:  8-32 Set Screw, Short Length Fine Copper Wire, Small Piece of Copper Tape

 

 

 

     ~$0.50

 

Plastic Spring Clamp 2”

 

McMaster Carr Part Number 50065A22

 

       $2.51

 

 

 

 

Total:

$52.24