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Science First Project 12-020 © 1998 Science First Principles ......Project 12-020 Principles of the...
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© 1998 Science First Science First 95 Botsford Place, Buffalo, N.Y. 14216 U.S.A. 716-874-0133 • FAX 716-874-9853 • [email protected] • www.sciencefirst.com ® Introduction: The Van de Graaf generator is basi- cally a combination of two primitive devices, the electrophorus and the Fara- day Cage. The present-day machine has mechanized the hand operations of the electrophorus. One way to understand it is to review the operation of the electro- phorus. The top pulley should be placed at least one entry hole diameter inside the termi- nal. The terminal should have gently rounded edges to minimize corona dis- charges. The terminal should be a con- ductor of electricity - even a poor on will do. Terminals may even be made of pa- per or cardboard, painted with paint that contains graphite or covered with alumi- num foil. Figure 1 - Metal Plate has no charge Figure 2- Insulator induces + charge in plate Figure 3 - Finger removes charge Figure 4 - Plate now has charge (removing + from 0 leaves —) Top: Insulating handle on metal plate Negative ion cloud Belt with + charge Project 12-020 Principles of the Van de Graaf Generator Plus: Building Toy Van de Graaf The Van de Graaf Generator uses a pulley made of insulating material. Fric- tion against a belt generates a charge on it. The belt functions exactly as the elec- trophorus plate. To remove the charge from the belt, the Van de Graaf Generator uses a small cloud of ionized air which gathers around sharp points placed in an electric field. As the pulley turns, the belt and its charge are taken away. The belt is not exactly an insulator but, rather, a partial conductor from face to face due to its large lateral area and small thickness. It is a very poor conductor along its length due to small cross sectional area and long length. When the belt arrives at the top pul- ley, a similar electrophorus operation takes place. The top pulley has the same The electrophorus consists of a metal plate on an insulating handle and a block of insulating material. This material is first rubbed so as to produce a charge on it. Next, the metal plate on the insulating handle is placed against the insulating material and thus acquires a similar charge. By touching the plate with the finger, you can remove this charge and then pull the plate away. The plate now has a charge opposite to the insulating material. By taking the charged plate and putting it inside an insulated container (a hollow ball is ideal) you can transfer the charge to the inside of this container where it is promptly conducted to the out side. Belt with — charge Positive ion cloud Pulley with + charge Negative charge to Faraday Cage Sharp point Pulley with negative charge . . . . m S T 1 Bottom: Insulating disc with positive charge Figure 5 - Action of Van de Graaf One of a dozen practical projects you can build yourself at home or school. charge as the belt. Again a finger of charged ions removes the charge allow- ing the belt to leave with a charge oppo- site to that on arriving. The top terminal of the Van de Graaf Generator may be made in any shape as long as a few limitations are observed. The + charge on the belt and pulley attracts negative ions from the sharp pint. When the belt leaves the pulley it takes some of this negative charge with it, and this negative charge is then carried to the other pulley. As the belt passes over the negative pulley, it passes another sharp point. This point generates positive ions that neutralize the negative charge on the belt. The belt then carries a positive charge back to the first pulley. The first belt-type electrostatic gen- erators were produced unintentionally. Early textile mills used wooden pulleys and leather belts to operate machinery. On cold days when humidity was low, these materials became good enough in- sulators to generate electric charges. Workmen received violent electric shocks near some pulleys, or the sparks might be strong enough to start fires. To solve the problem, pieces of barbed wire were placed near the pulleys and connected to metal stakes driven into the ground. The Van de Graaf known today first appeared around 1933. It was used to accelerate atomic particles used in early atomic studies. Several articles on the machine ap- peared in Review of Scientific Instru- ments about this date.
Transcript of Science First Project 12-020 © 1998 Science First Principles ......Project 12-020 Principles of the...
© 1998 Science First Science First
95 Botsford Place, Buffalo, N.Y. 14216 U.S.A.716-874-0133 • FAX 716-874-9853 • [email protected] • www.sciencefirst.com
®
Introduction:The Van de Graaf generator is basi-
cally a combination of two primitivedevices, the electrophorus and the Fara-day Cage. The present-day machine hasmechanized the hand operations of theelectrophorus. One way to understand itis to review the operation of the electro-phorus.
The top pulley should be placed at leastone entry hole diameter inside the termi-nal. The terminal should have gentlyrounded edges to minimize corona dis-charges. The terminal should be a con-ductor of electricity - even a poor on willdo. Terminals may even be made of pa-per or cardboard, painted with paint thatcontains graphite or covered with alumi-num foil.
Figure 1 - MetalPlate has no charge
Figure 2-Insulator induces +charge in plate
Figure 3 -Finger removescharge
Figure 4 -Plate now hascharge (removing +from 0 leaves —)
Top: Insulatinghandle on metalplate
Negative ion cloud Belt with + charge
Project 12-020
Principles of the Van de Graaf GeneratorPlus: Building Toy Van de Graaf
The Van de Graaf Generator uses apulley made of insulating material. Fric-tion against a belt generates a charge onit. The belt functions exactly as the elec-trophorus plate. To remove the chargefrom the belt, the Van de Graaf Generatoruses a small cloud of ionized air whichgathers around sharp points placed in anelectric field. As the pulley turns, the beltand its charge are taken away. The belt isnot exactly an insulator but, rather, apartial conductor from face to face due toits large lateral area and small thickness.It is a very poor conductor along itslength due to small cross sectional areaand long length.
When the belt arrives at the top pul-ley, a similar electrophorus operationtakes place. The top pulley has the same
The electrophorus consists of a metalplate on an insulating handle and a blockof insulating material. This material isfirst rubbed so as to produce a charge onit. Next, the metal plate on the insulatinghandle is placed against the insulatingmaterial and thus acquires a similarcharge. By touching the plate with thefinger, you can remove this charge andthen pull the plate away. The plate nowhas a charge opposite to the insulatingmaterial. By taking the charged plate andputting it inside an insulated container (ahollow ball is ideal) you can transfer thecharge to the inside of this containerwhere it is promptly conducted to the outside.
Belt with — charge Positive ion cloud
Pulley with +charge
Negative charge toFaraday Cage
Sharp point Pulley with negativecharge
.. .. .
m
S
T
1
Bottom:Insulating disc withpositive charge
Figure 5 - Action ofVan de Graaf
One of a dozen practicalprojects you can build yourselfat home or school.
charge as the belt. Again a finger ofcharged ions removes the charge allow-ing the belt to leave with a charge oppo-site to that on arriving.
The top terminal of the Van de GraafGenerator may be made in any shape aslong as a few limitations are observed.
The + charge on the belt and pulleyattracts negative ions from the sharp pint.When the belt leaves the pulley it takessome of this negative charge with it, andthis negative charge is then carried to theother pulley. As the belt passes over thenegative pulley, it passes another sharp
point. This point generates positive ionsthat neutralize the negative charge on thebelt. The belt then carries a positive chargeback to the first pulley.
The first belt-type electrostatic gen-erators were produced unintentionally.Early textile mills used wooden pulleysand leather belts to operate machinery.On cold days when humidity was low,these materials became good enough in-sulators to generate electric charges.Workmen received violent electric shocksnear some pulleys, or the sparks might bestrong enough to start fires. To solve theproblem, pieces of barbed wire wereplaced near the pulleys and connected tometal stakes driven into the ground.
The Van de Graaf known today firstappeared around 1933. It was used toaccelerate atomic particles used in earlyatomic studies.
Several articles on the machine ap-peared in Review of Scientific Instru-ments about this date.
© 1999 Science FirstScience First
95 Botsford Place, Buffalo, N.Y. 14216 U.S.A.716-874-0133 • FAX 716-874-9853 • [email protected] • www.sciencefirst.com
®
Plastic food containerpint or larger. Cut three holes (in order toreach inside.) Two holes in side are for in-stalling belt. Smaller hole in top gives roomfor pulley. Select size to cover motor.
Vinyl electrical tapearound shaft to make plastic pulley(or use plastic tubing, ball point pen body, orother hollow plastic)
BaseJar cover, cardboard, similar stiff material
BeltCut balloon with scissors to make rubber belts
Tin CanTape to cover sharp edges
Tapeto keep pulley from sliding sideways
For upper pulley -barrel-shaped wood bead
Toy Motortape
About the Author: Franklin B. Leeis founder of Science First, (formerlyMorris & Lee Inc.) designer and manu-facturer of science education equipmentsince 1960. A college teacher and inven-tor of electrostatic devices, he continuesto design new products and build toolsfor Science First Science First Science First Science First Science First products.
You may be interested in the Vande Graaf Generators and Van de Graafparts manufactured by Science First.See page 4 for a complete parts list. Or, ifyou prefer, please phone, fax or e-mailfor complete information on our Van deGraaf kits, parts, and accessories.
Building A ToyVan de Graaf
Suggested Materials• Tin can. For collector sphere.
Cut to size and cover sharp edges withtape to prevent cuts. Taped edge of tincan rests on top of food container.
• Upper pulley - wood. Barrel-shaped wood bead suggested. Insert needthrough hole in bead for a shaft. Applytape to ends of needle to keep 'pulley' inplace and prevent it from sliding side-ways. Shaft goes over top edge of foodcontainer.
• Plastic housing over motor. Pint-size or larger food container recom-mended. Needs to be large enough tocover motor. Use side holes to installbelt. Use top hole for pulley. Wind tapeon shaft.
• Lower pulley. Plastic (vinylelectrical tape, plastic tubing) installedon shaft on motor. Push directly ontoshaft.
• Upper and lower combs. Makefrom stranded copper wire. Upper brush- use screw or tool to fasten to can. Lowerbrush - tape or screw to base.
• Toy motor. Use any 1.5 to 3 vmotor.
Upper comb -use stranded wire, attach to tin can
BalloonbeltLower combStranded wire. Con-nect to motor lead
T
TS
S
US
Motor
Vinylpulley
T
Food con-tainer
Tin Can -tape edge to coversharp edges
UJar cover or cardboard
tapeS
© 1998 Science First Science First
95 Botsford Place, Buffalo, N.Y. 14216 U.S.A.716-874-0133 • FAX 716-874-9853 • [email protected] • www.sciencefirst.com
®
From May 1957 Scientific American's"The Amateur Scientist"
Franklin B. Lee, a chemical engi-neer and faculty member of the ErieCounty Technical Institute in Buffalo,N.Y. has built a number of small Van deGraaff machines and passes along theresults of his experiments to amateurswho prefer to work with generators lessbulky than the earth. "Two importantpieces of information for the designer ofVan de Graaff machines," he writes, "are,first, approximately 50 square inches ofbelt per second passing over the pulleyswill produce one microampere of cur-rent; second, the maximum potential de-veloped by the machine will be equal to70,000 times the smallest radius of cur-vature of the collector in inches. Thus aperfect sphere 12 inches in diameter willhave a theoretical limiting potential of420,000 volts. Holes made in the sphereto admit the belt assembly alter the pat-tern of the field and reduce the theoreticalmaximum. The most effective compro-mise with the ideal shape for a practicalcollector is a spheroid slightly flattenedat the bottom, with minimum radius ofcurvature located at a reasonable dis-tance from the insulator to discouragesparking along the insulator surface.
"The designer's choice of maximumvoltage determines the size of the collec-tor. To realize a large fraction of thetheoretical limiting potential, the collec-tor must be at least two or three diametersremoved from other metallic parts. Thedistance should be greater if sharp-edgedmetal parts are present, and may be some-what less if all parts are covered by arounded metal shield of large radius ofcurvature. The opening which admits thebelt to the collector ought not to be muchlarger than half the diameter of the col-lector and should be smoothly curvedinward, using a generous radius of curva-ture.
"Although a high polish adds to theattractiveness of the collector, it is notessential. Minor surface imperfections,if well rounded, limit the maximum volt-age only slightly. Grind down sharpedges or burrs. Lint and dust particles
will reduce the voltage to 40 percent ofthe theoretical maximum if they protrudefrom the surface as much as 1 per cent ofthe radius. Unpolished commercial alu-minum spinnings, free of lint and dust,will collect about 85 per cent of the theo-retical maximum. A high polish will in-crease the voltage another 1 per cent. Theconcentration of charge around the holethrough which the belt enters accountsfor the remaining 14 per cent, a quantitywhich varies with the size of the hole andits distance from other conductors.
apart at the splices. At high speeds,particularly in the ease of small pulleys,extreme tension must be maintained; thisleads to bearing problems. Lubricationdifficulties limit the shaft speed of sleevebearings to about 5,000 revolutions perminute. The noise level of ball bearingsbecomes annoying above this speed un-less special steps are taken to minimize it.At belt speeds above 100 feet per secondappreciable amounts of power are lostthrough air friction. Finally, part of thecharge appears to be "blown" off the beltat excessive speeds - a phenomenon whichI do not wish to explain.
"Belts may be made of almost anyinsulating material; paper, cloth, rubber,plastic and so on. Rubber, because ofitspoor resistance to ozone, has a limitedlife, but with used inner tubes costing solittle the inducement to improve on it isslight. Rayon, nylon, Dacron and cloth(made into belts with acetone cement)are almost as good. Incidentally, whenthese materials are substituted for rubber,the position of the corona-collectingcombs must be shifted. Cloth belts are
Selection of the desired current out-put determines the size of the belt and thespeed at which it must run. Meeting thisspecification is not so simple as it mightseem offhand, because the properties ofthe materials used for the belt and itsdriving assembly enforce speed limits onboth the belt and the shaft bearings. Formaximum current one should in theoryuse the highest possible belt speed. Butthere are disadvantages in running beltsfaster than about 100 feet per second.Higher speeds aggravate the tendency ofbelts to fray at the edges and to come
by Science First founderFranklin B. Lee
© 1999 Science FirstScience First
95 Botsford Place, Buffalo, N.Y. 14216 U.S.A.716-874-0133 • FAX 716-874-9853 • [email protected] • www.sciencefirst.com
®
The belts may run on either the insideor the outside of the insulating support.Economy, simplicity and high currentsfavor running them on the outside. Ap-pearance and neatness of constructionrequire them to be on the inside. Theformer arrangement permits the use of asmall, relatively inexpensive insulatorwith low current leakage and minimumdeterioration due to corona discharge (thesource of ozone). It also permits use ofthe widest belt possible for a given open-ing in the collector. Problems arisingfrom unequal potentials throughout theinsulator are similarly minimized. Allhygroscopic or fibrous materials shouldbe avoided in the selection of the insulat-ing column because they invite leakagethrough the moisture which forms on the
excited (that is, if the belt is to be ener-gized by friction), the lower pulley shouldeither be coated with or constructed of amaterial of extremely high resistivity. A1/32-inch thickness of polyethylenemakes a splendid covering for smallwooden pulleys. The pulleys should beturned with a slight crown, the edge mak-ing a snug fit with the inside of the poly-ethylene tube. The tub may be made bycutting the ends off a round squeezebottle. The tube is simply pushed overthe wooden core. The choice of pulleyand belt material for self-excited ma-chines determines the polarity of the col-lector charge. A rubber belt running on alower pulley of polyethylene or polysty-rene will usually pump electrons fromthe collector and charge it positively.
The upper pulley must be made of amaterial which is a good electrical con-ductor, such as wood or Bakelite. Sur-prised? At such voltages these materialsare very good conductors for the smallcurrents involved. Scrap plywood maybe glued together to make a highly satis-factory wooden pulley. The lower pulleyshould also be electrically conducting ifa separate 5,000 to 10,000-volt D.C.power supply is used for spraying chargeonto the belt. If the machine is to be self-
more durable than rubber, are quieter andrequire less driving power, but tend tofray at the edges. This is easily remediedby a coat of lacquer.
All things considered, I find that beltsof neoprene joined with a diagonal spliceare a good compromise.
© 1998 Science First Science First
95 Botsford Place, Buffalo, N.Y. 14216 U.S.A.716-874-0133 • FAX 716-874-9853 • [email protected] • www.sciencefirst.com
®
10-060, 10-065 (200,000 v)
surface. The material must also be se-lected with an eye to its mechanical prop-erties: strength, stiffness and toughness.
Finally, it should be readily availableat a reasonable price. Tubing of polyes-ter glass-fiber laminate or polyvinyl chlo-ride meets these requirements and is avail-able in standard pipe sizes. In the three-inch (diameter) size, it costs $2.60 perfoot. Polyvinyl chloride is the less hygro-scopic of the two and may be flanged orformed to other contours by heating it toits softening point in hot paraffin.
"Combs or corona points for apply-ing charge to the belt may be contrived ingreat variety. Often a common pin or asingle phonograph needle can be as ef-fective as the most elaborate comb. A tuftof wire, bound at one end and shearedlike a broom at the other, makes a satis-factory comb, as does a small rectangle ofwire screening. Care must be taken toavoid spraying areas of the belt withunwanted charge. This may happen ifcharges are permitted to mix on the frontand back of the belt near the pulleys.
The problem is met by mounting thecombs on fixtures which provide easyadjustment over a wide range of posi-tions, and by selecting comb sizes whichrestrict the areas that are sprayed withcharge."
Tesla Coil Parts
10-205 Tesla Coil
New! 8-inch ToroidImprove performance and terminal
capacitance of your custom-built TeslaCoil. Radiate charge in all directions foran outstanding demonstration. Spun alu-minum in doughnut, glued, polished.
• 10-209 - 8" diameter, 2" high, raised portion2" wide, 1/4" center hole.
New! Pith Balls• 29-1044 6 Pith Balls on strings.
Balsa wood, 3/8" in diameter, withhole for threading. Great forexperiments with Van de Graafs,electroscopes.
• 29-1049 - Pith balls with attachedsilk thread.
written by Franklin B. Lee12-001 High Vacuum• Creating a Frictionless Environment • Produc-tion of Very Low Temperatures• Construct aDischarge Tube• Estimate the Relative Mass ofthe Electron• Construct a Radiometer
12-008 Make a HomemadeElectrophorusPlus experiments in electrostatics12-010 Electric Field MappingOutdoors12-015 Make a DifferentialBarometerfrom two large glass bottles, rubber stopper,tubing, cardboard box, insulation and newspa-per!12-002 Build a Giant Tesla Coilfrom neon sign transformer & easily obtainedmaterials from building supply stores
Science Fair Kits and Plans
10-209 8" Toroid -
Van de Graaf Parts
As a manufacturer of Van de GraafGenerators, we sell parts that may beuseful to you in building your ownmachine. We regret there is no refundon sales of components as we cannotguarantee our parts will fit machinesnot of our manufacture.
24-1060 Instructions80-0169 Motor, 110 v w/tefl.pulley29-1016 Belt (3/4" wide)29-1070 Hose Clamp36-1062 Motor Mount36-0605 Lower Pulley (Teflon)60-0601 Globe (Oblate) (7")80-0160 Upper Pulley Ass'y80-0161 Upper Comb Ass'y80-0162 Lower Comb Ass'y80-0164 Column Ass'y56-6060 Molded Ring80-0163 Base80-0166 Cord Set80-0167 Housing50-0609 Upper Pulley51-1002 Shaft56-1016 Shaft Collar, pr26-9016 Copper Wire51-1004 Collector Support26-1005 Lower CombTerminal22-6025 Rubber Feet
10-085, 10-086 ( Large)50-0804 Upper Comb Assembly21-9015 Flanged Bearing24-1060 Instructions80-0166 Cord Set, N.America26-2030 Motor, 120 v26-2040 Fan29-1480 Belt50-0805 Lower Comb56-0808 Lower Pulley60-0604 Collector (oblate) (14")51-0800 Chassis81-0801 Aluminum pulley Assembly56-0806 Butyrate Column51-0802 Cover56-0805 Shaft collar26-2032 Motor, 220 volt80-0168 European cord set
26-1008 Neon Bulb26-2010 Generator26-2012 Capacitor, .05 MFD 1600V26-9114 Copper wire, 5 " (primary)29-1020 Lampshell Socket50-0201 Electrode50-0208 Disc electrode, 3"50-0209 Disc electrode, 6"50-1020 3/4" aluminum ball60-0205 Coil AssemblyNew! Plasma TubeAttach to coil, watch streamers move.
