REPORT RESUMES

ED 016 401 SE 004 147 BASIC . SCIENCEIN ACTION SERIES, NUMBER14. BY- CASSEL, RICHARD PENNSYLVANIA STATE DEPT.OF PUBLIC INSTRUCTION PUB DATE 67 EDRS PRICE MF -$0.25 HC -$1.04 24P.

DESCRIPTORS- *ELECTRICITY,*ELEMENTARY SCHOOL SCIENCE, *INSTRUCTION, *SCIENCEACTIVITIES, *TEACHING GUIDES, INVESTIGATIONS, *SECONDARYSCHOOL SCIENCE,

THIS TEACHING GUIDE,INVOLVING ACTIVITIES FORDEVELOPING AN UNDERSTANDING OFBASIC ELECTRICITY, EMPHASIZESSTUDENT INVESTIGATIONS RATHER THANFACTS, AND IS BASED ON THEPREMISE THAT THE MAJOR GOAL INSCIENCE TEACHING IS THEDEVELOPMENT OF THE INVESTIGATIVE ATTITUDEIN THE STUDENT. ACTIVITIES SUGGESTED INVOLVE SIMPLEDEMONSTRATIONS AND EXPERIMENTSWHICH THE STUDENT MAY DO.ACTIVITIES INVOLVING STATICELECTRICITY SERVE TO INTRODUCE THE ELECTRICAL NATURE OF THE ATOMAND INCLUDE EXPERIENCES INVOLVING(1) CONDUCTORS, RESISTORS,AND INSULATORS, (2) THEELECTROSCOPE, (3) INDUCED CHARGE,AND (4) ELECTRICAL FIELDS. ELEMENTARYEXPERIMENTS INVOLVING CURRENT ELECTRICITY INVOLVE (1) ASIMPLE VOLTAIC CELL, (2) ELECTROLYSIS, (3) RESISTANCE CIRCUITS, (4) OHM'S LAW, AND(5) SERIES AND PARALLELCIRCUITS. ACTIVITIES SUGGESTEDARE SUITABLE FOR JUNIOR HIGHSCHOOL PUPILS. (DH) .)1880 COMMONWEALTH OF PENNSYLVANIA

U.S. DEPARTMENT OF HEALTH, EDUCATION & WELFARE

OFFICE OF EDUCATION

THIS DOCUMENT HAS BEEN REPRODUCED EXACTLY AS RECEIVED FROM THE

PERSON OR ORGANIZATION ORIGINATING IT.POINTS OF VIEW OR OPINIONS STATED DO NOT NECESSARILY REPRESENT OFFICIAL OFFICE OF EDUCATION

POSITION OR POLICY.

111%11111111111111111111111111111. CT ON

DEPARTMENT OF PUBLIC INSTRUCTION Science in Action Series No. 14

BASIC ELECTRICITY

Prepared by Bureau of General and Academic Education Text material written by Mr. Richard Cassel

Commonwealth of Pennsylvania DEPARTMENT OF PUBLIC INSTRUCTION Harrisburg 1967 rt ,t'ff.031"'!rn -

TABLE OF CONTENTS Page

Introduction iv 1 The 2 Conductors, Resistors, and Insulators 4 The 5 Induced Charge 5 Electric Fields 7 Grounding 9 9 9 Stored Charges 11 Lightning 13 Review 13 Producing Electric Currents 14 Current Electricity 15 Types of Circuits 19 Conclusion 20 INTRODUCTION

This booklet has been written on the premise that the major goal in teaching science is the development of the investigative attitude in the student. It is most important, therefore, that the science teacher trigger the students' curiosity which leads to investigation, and because of this the teacher's approach and meth- ods become even more important than the scientific facts with which he works. By encouraging his stu- dents to draw their own conclusions from theirown observations made during controlled experiments, the teacher can stimulate his students and at the same time develop the correct scientific approach. The material in this booklet is arranged so that the teacher's demonstrations, student experimen ts, drawings, and information allow the student to make logical step by step deductions and in thisway provide the student with the "trial and error" atmosphere so necessary for making him feel personally involved in scientific investigation. Most of the experiments and demonstrations, which were chosento stimulate interest and to develop the reasoningprocess, can be performed safely by the students themselves andre- quire inexpensive or easily constructed equipment. Finally, it should be re-emphasized that thepur- pose of this booklet is not so much to provide facts per se but to help the teacher direct his students in an investigation of electricity. In this way the students will, primarily ask questions and seekanswers, not memorize facts. Implumpompummplopeumusjeippiw

STATIC ELECTRICITY A thorough understanding of the 'behavior of the is necessary to understand electricity because the electron is electricity.It is easiest to study the positively charged electron when it is in a static condition. glass rod Electrified Objects: Experiment 1 Have students run their combs through their hair.(A pen or pencil against a sweater will also .) Then have them try to pick up bits of paper with the comb. Ask them why there is an attraction.(It should also be pointed out to the students that they negatively charged may have done this many times, butprobably rubber rod never really knew why the paper was attached to the comb.)Point out to the students that they have produced electrified objects and that more investigation of such objects may answer the question, "Why is there an attraction?". Rub the suspended rod with a pieceof There are many ways to detect electrified objects, cat's fur or wool cloth.Next rub another but the method of detecting them depends upon the rubber rod with cat's fur. Bring this rod near forces between these objects. the suspended one and point out that some invisible force seems to cause them to repel Electrified Objects: Experiment 2 each other. Have students place a hard rubber rod in Now suspend a glass rod (or large test a stirrup made from a large paper clip(as tube) and rub it with a piece of sheet rubber shown below) and suspend with thread from or silk.Rub another glass rod with sheet a ring stand. This will allow the rod to rotate rubber or silk and bring it near the suspended have freely. one. Observe that these rods also seem to an invisible forcebetween them that causes repulsion. Now rub the rubber rod with cat's fur again and bring it near the suspended glass rod. Now note that this time an invisible force causes the two rods to attracteach other. Try other objects such as fountain pens, or combs, using for all objects the suspended rod as a negativelycharged reference to determine whether they are at- rubberrod tracted to or repelled from it. At this point encourage students to discuss pos- sible explanations for these reactions. Lead them to conclude that objects of the same material which have been "electrified" by the same procedure alwaysrepel, while different "electrified" objects may either attract or repel. negativelycharged rubberrod Electrified Objects: Experiment 3 To investigate this phenomenonfurther, have groups of students suspend pith balls from ring stands. (Puffed rice kernels will substitute for pith balls.)

1 Ask the students to predict what will hap- The terms positive and negative can be used to pen if they rub a rubber rod with cat's fur or designate the type of charges. When the phenomenon wool and bring it near the pith ball. Then ask exhibited by a rubber rod which has been rubbed the students why the ball is first attracted to by cat's fur or wool is referred to, it is said to be and then leaps away from the rubber rod. (The "negatively charged." This use of terms dates back students may conclude that something must to Benjamin Franklin and is a simple way of saying have flowed from the rubber rod into the pith that the rubber rod when rubbed with wool behaves ball and that whatever it was, it must have oppositely tQ the glass rod when itis rubbed with opposed itself.) silk. The glass rod, then is said to be "positively Repeat this prediction and experiment, charged." this time bringing a glass rod rubbed with The conclusions so far can be listed in the follow- rubber or silk near the pith ball. Again it is ing order:

(a) Pith ball attracted (b) Pith ba// (c) Pith bo// repe//ed touches rod by rod

attractedto the rod and thenisrepelled. 1.All charged bodies fall into two electric (Thinking back to the experiment with the states, one similar to that of the rubber rod suspended rods, the students could conclude called negative, and one similar to that of that something different has flowed from the the glass rod called positive. glass rod to the pith ball, than has flowed from 2. Like charges repel each other.(E.g., a posi- the rubber rod to the pith ball.)Give the tively charged glass rod repels a positively students plenty of time to think about this, charged glass rod.) but help them to note that whatever it is that 3.Unlike charges attract each other.(E.g., a flows, it always repels itself. positively charged glass rod attracts a nega- Now while the pith ballisstill being tively charged rubber rod.) repelled by the glass rod, bring the rubber rod near it. The pith ball is attracted to the THE ATOM rubber rod and away from the glass rod. But the students will probably ask, "What are This should lead us to conclude that the these charges and where do they come from?" two different things that flowed must have at- To answer this question the students must ex- tracted each other! (This conclusion will be amine the scientist's concept of the atom in its natural modified by the study of the atom which fol- state. lows.) By diagramming on the blackboard simple , Now attach some labels to the previous observa- such as the hydrogen and oxygen atoms (see below) , tions so that they can be conveniently discussed. the teacher can show the various forces affecting the The term charged can be used to designate the atom.(These diagrams are oversimplified and are process of electrifying an object. only a model to be used for our special purpose.)

2 DIAGRAM OF THE ATOM Electron a'\16- tir Electron orbit (shell) Electrone E lecfron of nov// / neutrons Proton

1 °A. 4 Prot01/ \ CIA. '..1:16 \ lectron .. .CI CIElectron eElecfron , .....0...... ,.. Hydrogen is the :,,,,,;" simplest form of atoms,afre4c5rce; 0.4) Electron with one electron and one proton Oxygen is more comp/ex with 8 neutrons and 8 protons in the nucleus and 8 in two different shells

Point out the following details in discussing the number of electrons and protons together. diagram of the atom: 5. An atom is normally found with an equal number of protons and electrons, and thus 1.The atom is made up of three main particles, is said to be electrically neutral. When elec- the ,neutron with no electrical charge, the trons are "lost", or extra electrons are ac- positively charged proton, and the negatively cepted, the atom develops a charge and then charged electron. is called an ion. 2. The protons and neutrons weigh almost 2000 times as much as the electron and make up Many of our previous observations about electri- the center or nucleus of the atom. fied objects can be explained if we assume that some 3. The lightweight electrons orbit around the of the electrons in the outer shell are rubbed loose nucleus in layers or shells. from the rest of the atom when two objects are rubbed 4. Because the electron and proton have an at- together, leaving an excess of protons on one object traction for each other and are equal in the and an excess of electrons on the other object. The quantity of their charge, the effect of their electrical force or "charge" will exist as long as the forces is neutralized when there is an equal excess of charged particles remains on the two objects. ELECTRON MOTION at a GIVEN MOMENT

Proton 1 /, AofregtrQ4lectr°n

/ Centn Force of rriovinft -'clear**

Electrons move in all orbits N. / within their area (shell) at Electron some time or other 3 Help students to conclude that the electrons must CONDUCTORS, RESISTORS, have flowed back out of the steel rod into the human body, leaving the rod positively charged and the pith AND INSULATORS ball negatively charged.Hence, the pith ball was attracted back to the steel rod. Conductors, Resistors, and Insulators: Experiment 1 Now replace the steel rod with other pieces of material, such as a rubber rod, glass rod, As in the following diagram, have the piece of wood, strip of copper, etc., and repeat. students suspend a pith ball from a ring stand the previous experiment with each item. Have or some other support, so that it just touches the students predict the outcome before trying the end of a steel rod resting on a beaker. the demonstration.

Steel Rod Steel Rod Charged Rubber Rod

Glass Beaker

(a) Ball rests against (b) Ball is repelled steel rod from rod

If the students rub a hard rubber rod Have students discuss their observations and lead with a piece of cat's fur or wool cloth and them to conclude that many materials allow the elec- then touch the rubber rod to the end of the trons to "flow" through them. These materials are steel rod, the pith ball will jump away from called conductors. However, some materials resist the the far end of the steel rod and stay suspended movement of electrons through them.These are away from the steel rod. (The rubber rod may called resistors. If a material has a resistance so high have to be rubbed and discharged into the steel that electrons do not normally flow at all, it is called an . rod several times before it becomes saturated enough to repel the pith ball.) Now ask the students the purpose of the glass beaker under the various rods. Have them Bring the rubber rod near the pith ball try the demonstrations with a tin cup in place and the ball will be repelled. This shows that of the glass beaker. Ask them what the results since like charges repel, the charge that is on indicate. the rubber rod is now on the pith ball. From the previous experiments it can be seen that one of the commonest things we do is to charge Encourage the students to discuss the possible bodies electrically. Every time we rub one kind of ways the charge got from the rubber rod to the pith material across another, as when we comb our hair, ball. Ask students what will happen if they touch or scuff our feet on the carpet, we remove electrons the steel rod. from one body and add them to another. But in some substances this charge is lasting enough and great Touch the end of the steel rod with a enough that we notice it. These substances are non- finger and the pith ball will return to its conductors, like rubber and glass, which cannot readily original position. give up the charge developed upon them.

4 THE ELECTROSCOPE A Gold Leaf Electroscopecan be purchased from any science supply house. The operation of boththe A simple device that isvery useful in observing student-constructed electroscope andthe purchased some of these fundamentals of electricity is the electro- one is the same and quite simple. scope. Students can easily build this piece ofapparatus which is more sensitive than theprevious apparatus used. INDUCED CHARGE

The Electroscope: Because of the sensitivity of the electroscope,more Experiment 1 precise investigationscan now be made. To construct the aluminum leafelectro- Ask students if they observed thatcharged objects scope shown below, use a large jar witha had effects on each other,even though the objects metal lid and a piece of foil froma gum or didn't touch. Again have studentsrun combs through cigarette pack. Drilla hole in the lid of the their hair or rub plasticpens or pencils on their jar to receive thestopper.Pierce stopper to sweaters, and try to pickup bits of paper without make a hole and push thecopper wire through. touching them. They will noticethat the bits of paper seem to jump to the comb. It should beim- Bend the lower end of thewire into a "T" pressed upon the students thataccording to our past shape, as shown. Fasten theupper end of the conclusions only oppositely chargedobjects should wire to an aluminum foil ballby pushing it attract each other. Yet, although thepaper was not halfway into the ball. Useplastic cement to charged, it was attractedanyway. seal the juncture of thestopper and the lid. Induced Charge:Experiment 1 Now removea piece of aluminum foil froma cigarette packor gum wrapper, using alcohol Use the electroscopeto observe this phe- to separate the foil from itspaper base. Cut nomenon more closely.Have the s t u den ts two strips of foil, each aboutan inch long and charge a glass rod with silk.Bring the rod near the foil ball one-half inch wide. Mountthem through the on top of the electroscope. Notice "T"-shaped end of thecopper wire and glue that the leaves spreadapart. Now take the rod with plasticcement. For best results, the jar away and notice the leaves go back together. Repeat this procedure using and lid should be warmedin an oven fora a rubber rod rubbed with wool. Now touchthe ball with a few minutesto drive out all moisture.Be charged rod. Notice that theleaves still stand sure to screw the lid on tightly beforeit cools out after the rod is takenaway. Now touch off.This completes theelectroscope. the electroscope ball witha finger, and notice that the leaves collapse.

Aluminum FoilBall Allow students to discuss theseresults. Help them ,. to reason that electrons probablyflowed into the electroscope ball when itwas touched by the charged rod. Since the ball Lid was connected to both leaves of our electroscope, they would eachhave the same charge and thusoppose each other. The electrostatic Copper force, being muchstronger than that of gravity, pushes wire the leaves apart. Yet therewas no apparent way the electrons couldmove into the electroscope whenthe charged rodwas brought near, but didn't touchthe Aluminum foil ball. There apparentlymust be some way for the electrons tomove from the rod to the electroscope without the two touching.Encourage the studentsto give/possible explanations.

5 The most logical explanation wouldbe the re- Ask students howa charged balloon is attracted arrangement of loosely held electrons within the to a neutral wall. The students should be ableto electroscope due to the force of the chargein a nearby explain that the electronson the balloon pushed some object. Thus, ifa positively charged rod was brought of the free electrons, makingup the wall, back into near the electroscope, the part of the electroscope the wall leaving extraprotons, or a positive charge, nearest the rod (the foil knob) would haveextra on the surface of the wall which thenattracts the electrons attracted into it and in effectcause that part negatively-charged balloon. of the electroscopeto have a negative charge. Of Perhaps the students willmore clearly visualize course, the other end of the electroscope (the the process that brings aboutan induced charge after leaves) one more demonstration. would have to havean opposite positive charge on it, since electrons had been drawnaway from it, leaving induced Charge: behind extraprotons.This reaction is calledan Experiment 3 induced charge, since itwas not caused by mechanical Place two metal rodson glass beakers contact. The electrons returnto their normal posi- (used as insulating stands)so that the rods tions after the charged objectis taken away, leaving touch and makeup one conductor as shown the entire electroscopeneutral again and allowing below.Bring a positively-chargedglass rod leaves to fall back together. (rubbed with silk)near one end of the steel rods. Now while the glassrod isstill held The fact that theleaves went together when the near, separate the two metal rods by pushing electroscope knobwas touched by a fingercan be ex- their beakers apart. plained if the students willthink back to their investi- Ask the students what thecharges on each gations of conductors andinsulators. Remembering metal rod should be, and why. that the human body isa pretty good conductor, the Now let the student check hisconclusions student should concludethat the excess electrons by positively charginga pith ball (puffed rice placed on the electroscope kernel) ona string by touching it with the by the charged rod flow positive glass rod and bringing back off the electroscopethrough our body, making it near each the leaves neutral. of the metal rods. Themetal rod nearest the positive-charged glass rod shouldattract the

knob Rod Rod Rod -touches remove removed meta/

Aluminum Negative Foil chorve remains 0 a (a) Induced charge (b) Conducted charge

induced Charge:Experiment 2 positive-charged pith ball, showingthis metal rod is negative, dueto the attraction of an Another example ofan induced charge excess amount of electrons into it. This leaves can be shown by rubbingan inflated balloon the steel rod farthest from the against a wool charged glass sweater and then placing the rod with anexcess of protons and it should balloon against the wall,where it "sticks?! repel the pith ball. 6 Steel Rods, touching

Beakers

(b)

Rods ore separated withcharged rod nearby (C)

Two pith balls, bothcharged positively, show the chargeson the rods are opposite Induced Charge: Experiment 4 ELECTRIC FIELDS There is finally anotherunusual example With some slightly of more elaborate equipment which will probably than used thus far, thestudents can be shown impress students. Turn the open a water faucet till actual pattern of theelectrostatic fields of force that just a trickle ofwater flows. Bringa charged surround charges. rubber rodor comb near the stream ofwater. Notice how sharplythe stream ofwater is bent Electric Fields:Experiment l by the attractionof the charged rodto the water molecules that havean induced opposite First pour carbontretrachloride (CCL4) charge. into a glass Petri dish.(If the room cannot be well ventilated, filmcleaner should be used Here again theelectroscope can be used,not only instead, for it hasno poisonous fumes.) Then for detectinga charge, but also foridentifying the carefully dropa layer of fine "creeping bent" type of charge. grass seeds on the liquid. Next adda one-fourth By putting inch layer of mineraloil. Now spread the seeds a known type of chargeon our electro- evenly with scope and watching the action a clean dry glass rod. The seeds of the leaves whenan will remain in the unknown charge isbrought near, boundary between the dense we can discover carbon tetrachlorideand the less dense mineral from the reaction,the type of charge.For example, oil as shown below. if we puta positive chargeon the electroscope and Mineral Oil then Wiga negatively-charged rodnear, electrons will be pushed, byinduction, downinto the leaves "bent irass neutralizing the positive .111a seed charge, causing theleaves to Petri Dish--, IMM. MEIN= collapse; whereas a positively-charged rodwould cause the leaves to spreadstill farther. Carbon Tetrachloride

7 Now connect a six-volt cell (with a switch electrons can flow.However, the attraction in the circuit) to an induction coil. (An induc- forces between the charged wires and thegrass tion coil can be purchased from almost any seeds show up because chargesare induced scientific supply house. A small neon sign trans- into the grass seeds causingone end of each former can be used in place of the induction seed to become positively charged and the other coil, but its use can be dangerous due to the end negatively charged.(Thisiscalled a presence of much current at high . To dipole.)Consequently, the negative end of reduce the current flow, therefore, a 40-million each seed will point to the positively charged ohm radio resistor must be used in each of the coat hanger wire. The seeds align themselves leads.) along the lines of force. After connecting the battery to the input or primary terminal of the induction coil, con- Have students notice that the seeds forma pattern nect two right-angled coat hanger wires to the like the spokes of a wheel around eachcoat hanger high voltage or output terminals of the in- wire. duction coil. Now move the two wires closer together Carefully place the two ends of thecoat and see the elliptical pattern formed by the hanger wires into the Petri dish,so that the attractiveforces between the two different wires touch the bottom of the dish.(The in- charges on the wires. This is a dipole electric duction coil will change the six volts of the field pattern. battery into 10,000 volts at the high voltageor Shut off the current and replaceone of out-put terminals. Be careful not to get within the vertical wires witha flat plate. Now the four inches of these terminalsas the current lines of force run from the single pole and will jump several inches.) enter the flat plate at right angles. Again shut Now close the switch. off the current and adda second flat plate in place of the other vertical wire. The lines of Secondary force run from plate to plate, andare uniform. (This is actually a condensor whichwe will discuss later.) Induct/0'n oo Coi /-p Turn off the current and connect two wires to the same terminal. Codt hanger, Ask the students what type ofpattern they will wi cc/itLayerof "bent "gross seeds expect to see. This time the lines of force show that then minera/ oil cover the two charged wiresoppose each other since they would be of thesame charge. Apparatus to see field patterns If a ring is connected toone terminal the pattern shows there is no field inside thering showing that One of the terminals will havea negative charges always reside on the outsideof an object. charge and the other will have a positive charge When a tear-shaped piece of wire isconnected to with a potential difference of from 5,000 to one terminal, it can be seen that the electric fieldcon- 10,000 volts. The carbon tetrachlorideand centrates at the pointed end. (Thereason for pointed mineral oil are good insulatorsso that no lightning rods, which will be discussedlater.) FIELDPATTERNS

(a) 4 single chargedrod (b) Two rods with (c) Two plates with opposite charges 8 opposite charges 300/VVIIVPWrorMalgrI"Aki--

FIELD PATTERNS(continued)

(d ) Two rodswith (e) A meta/ ring ( f ) A wire in some charge teardrop shape GROUNDING (Sealing wax can also beused.) Next attacha flat disk of aluminum toa 4-inch Grounding: Experiment1 or 5-inch piece of broom stick.Now charge thewax surface by rubbing it witha piece of cat's fur. Suggest to the studentsthat they suspend Bring the disk down flatagainst the wax surface. a small metal ball (aluminumfoil) froma silk string and touch Now touch thetop of the metal disk witha finger, the ball to the knobof remove the finger and lift the disk a charged electroscope. Noticethe leaves drop from the wax surface. To test thecharge bring the edge a little showing that the ballabsorbed some of of the the charge. Try metal disknear the knuckle of your finger:a good- a larger ball (recharging the sized spark should jump electroscope each time)and notice that the from the disk toyour finger. leaves dropeven more. Try still larger balls, until finallya very big ball absorbs all ofthe charge on theelectroscope, causing theleaves inch broom stick to collapse completely.The students should handle. estimate and recordthe amount of "drop"of Aluminum cilsk the leaves in eachtrial. The teachercan now relate the foil ballto the huge ballthe earth,and point out thatthe earth can absorb and neutralizeany charge produced. This process of passing charges into the earth is called MEM .110 =ft11 =M.* ImPla Melted m1M, M/ONO =OWN NIMINII MIMI* OM= MNID ,110010MI= Pie POP grounding. Applicationsof this phenomenon breakable will be records orr explained later. sea // taco( Up to this pointthe static chargescreated by fric- tion have been Electrophorus very weak. To detect thesecharges the students havehad to use fairlysensitive pieces of equipment, suchas an electroscopeor a pith ball. It is now timeto investigatestronger charges. To THE VAN DEGRAAFF create these stronger chargesthe teacher andstudents will use slightlymore complex equipmentthan a GENERATOR rubber rod anda piece of cat's fur. Thetwo pieces of equipment usedto produce more powerful The Van de Graaffgenerator can be built from charges plans found in several are the electrophorus and theVan de Graaffgenerator. different source books, butit is probablymore satisfactory to purchaseone of these machines. The Van deGraaff uses a rubber belt ELECTROPHORUS driven by a motorat one end and hasa conducting device at the other endto remove the charge from The electrophoruscan be made by fillinga pie tin with a layer of melted the rubber belt andpass it along to a metal sphere, phonograph records (break- where it is stored. This able type), machine can developup to which can be meltedover a low flame. 200,000 volts.

9 t"g?,.-41PRIPmorworompopimeromripb.,---p.m.,mmwiretpkwv,,,,,,,,,,,tvvr,,moippowoveorposommowillmillalMINIMMINNIIIIIIIIIIMIlli., . ,, ,

Although the voltage is high there is a relatively The Van de Graaff Generator: small current flow, making it a safe device to operate Experiment 2 for anyone with a healthy heart.Nonetheless one should be careful in handling this machine and re- Suspend strips of aluminum foil on the member that although air is a good insulator, 200,000 end of a silk string over the charged metal volts will arc 3 or 4 inches through it. sphere, allow them to touch the sphere, and notice the intense repulsion as the strips stand Metal "straight out." Sphere A more dramatic demonstration of this can roundinsr be done by having a girl, with long fine hair, Ball place her hands on the discharged sphere of the Van de Graaff generator, while she is stand- 111P/astic Cylinder ing on a wood stool or some other insulating platform. Now start the machine and notice that the girl's hair is standing straight out from Motor- her head. Ask the students why this has occurred. They Pulley- should realize that each hair has the same charge and thus they repel each other. Base The main use of the Van de Graaff generator is to provide a charge for charge-storing devices, such as Van de Groaff Generator Leyden Jars or condensors. Ask students why the electrophorus or Van de The Van de Graaff Generator: Graaff generator is able to develop a more powerful charge than the previous equipment we used. The Experiment 1 students may give many answers for this question, but The Van de Graaff generator can be used in it should be pointed out to them that the main dif- many different types of demonstrations. Using ference between the equipment used to produce weak a discharger (a device that can be purchased charges and the equipment used to produce powerful or made and which consists of two conducting charges was the size of the apparatus on which the arms and an insulated handle) , touch the end charge was stored. For instance, the rubber rod had of one arm to the charged metal sphere and very little surface on which to store electrons, com- bring the other arm near the grounded base. pared to the surface area of the electrophorus metal This will produce a larger discharge, looking disk or the Van de Graaff's metal sphere. So it would much like miniature lightning bolts.(A dark- appear that on a larger surface area one can "build ened room enhances the dramatic value.) up" and store a stronger charge. One other major Ask the students why the electricity flows in short difference is that with our first apparatus the charge bursts or bolts, rather than continuously. This might was produced and stored on a non-conducting ma- indicate the insulating value of air. terial, such as rubber or glass; whereas the more powerful machines stored their charges on a conduct- ing material. A stored charge can flow more quickly through the conducting material when it is discharged. Also since the charges are developed on conducting materials they must be kept insulated from an op- positely charged body or neutral body, which might Insulated develop an induced charge. Handle This is the reason for the wood handle on the electrophorus metal disk and the plastic cylinder sep- arating the metal sphere and the grounded metal base Discharger on the Van de Graaff generator.

10

11.47.4.11M STORED CHARGES Stored Charges: Experiment 1 Ask the students to design a device for storing To charge the attach a wire from the sphere of the Van de Graaff generator large charges. From our previous study of the electro- phorus and Van de Graaff generators, the students or electrophorus to the internal wire of the should realize that two plates will be needed and Leyden Jar and a wire from the ground or the base of the generator to the plate on which that they should be made of a conducting material, the jar is resting. Now carefully using in- with something between them as an insulator.In sulated tongs (because the Leyden Jar contains this device, they could put a plus charge onone plate and a negative charge on the other. Their inductive a considerable charge),disconnect the wires. attraction force would pass through the insulator, With the discharging device, touch one end to but the electrons could not. Thus, a store of electrons the external point on the jar and the other could be held on one plate by the attraction of the end to the internal wire. protons on the other. Notice the large spark just before the end An early device for storing charges, used by Ben touches. This shows the Leyden Jar had held Franklin, is called the Leyden Jar. This device is still a charge. More modern devices for storing used today and can be purchased or easily made. To charges are called condensors or , and make a Leyden Jar, the students can coat the inside are usually made up of metal plates separated of a milk bottle with aluminum paint, wiping the by an insulator. paint from inside the top inch of the bottle. Then paint the outside of the bottle with aluminum paint Stored Charges: Experiment 2 up to within one inch of the top. Fit a rubber stopper So far we have discussed the effect of the to the top of the bottle and insert a copper wire size of an object in relation to theamount of through the stopper so that one end (ora chain of charge it can hold. Anotherway of saying this paper clips hung on the end) touches the painted is that the amount of plate surface ofa con- surface inside the jar, and the other end standsan densor is directly proportionalto the amount inch above the stopper. The bottle should siton a of charge it can hold. Studentscan check this copper plate so that the aluminum coating on the by making a small Leyden Jar anda large one; outside of the bottle touches the plate. Now the after charging eachone, they can discharge Leyden Jar is ready to be charged. them and measure the relative length ofeach

Wire tosphere of Wire Van de naafi 41011111 Stopper MEND Generator

ottle

-inside coating Outside coating' Paper clip

copper plate Am, I Wire lo 9eround 4- 4.+ si Homemade Leyden Jar Charged Leyden Jar

11

iaiii104114(04444firo*;rt,.. spark. The spark can be measured by attaching driver and move the metal end slowly towards one end of a wire tothe interior wire of the the wire. When the spark takes place, measure Leyden Jar and the other end to the metal the distance between the end of the wire and shank of a screw driver with a large plastic the end of the screw driver. Each millimeter handle. may under some conditions represent3,000 Next attach one end of the wire to the volts. Students can compute the surface area exterior of the Leyden Jar. Lay the other end of each (the area of aluminum paint of the wire on a paper marked off in milli- on the jar) , and compare this with therelative meters. Grasp the plastic handle of the screw size of the sparks.

Cardboard marked inmillimeters

Wipe Masfic parireSPIRR 4 Ir.. Ai IV handle gina IAMr a II Charged iommaimuEIs crewdriver I -eyden Jar Discharge Wire Device to measure length of spark

the forces of the charges obey the "inverse Stored Charges: Experiment 3 square law" of nature, justlike gravity, mag- The other factor influencing capacitance, netism, and the diffusion of light, that is, the or the ability to store acharge, is the distance effect decreases at the rate of the distance the plates are from each other. squared. To investigate thiseffect, glue a metal coated pith ball to the end of a rod clamp made of plastic or glass; put the other end of the Cardboard damp clamp in a clothespin. Suspend a second pith \-) ball with two silk threads so it moves in only one plane from a ringstand or other support. Ft;ierhreeld Now place a piece of cardboard with one- ir 4cm fourth centimeter squares behind the suspended _. ball.Negatively charge both balls from a -...,i;cluomm,_ ,"450:76:::commli rubber rod rubbed with cat's fur or wool. Place 1117''''''' 14 a light source in front ofeach pith ball so .711.It.; ommii ,....1,,-,,..,g...... sit....._. that its shadow falls on the cardboard. Move 4;;th- 805 Moitimpoommi the pith ball on the clothespin toward the sus- ti "10111111 Clathe.3 pended one until it deflects another square. Pin Again measure the distance between centers. Repeat, until there are at least five readings. Now plot the distance between centers as a Device for measuring electrostatic forces in function of deflection on a graph paper. Notice relation to the distance charges are apart that the effects of the force are approximately inversely proportional (for practical purposes) to the distance the charges are apart. In fact,

12 With this background and with our study sofar, LIGHTNING ask the students how they would safeguard ahouse the against being damaged by lightning. The first answer The biggest of all is will probably be lightning rods. Ask the students to thundercloud.Itis a violent turbulence of moist discuss how lightning rods function. Can they devise precipitation. air rising and being cooled often causing the operation of lightning rods? charge a way to demonstrate The falling rain and hail may cause a negative Have the students consider other problemsof to be produced at thebottom of the cloud and a charges Since the protection from static electricity, such as static positive charge at the top of the cloud. picked up on fuel trucks or the danger of static charges above the bottom top of the cloud may be 40,000 feet in an operating room. In the case of the operating huge insulator of the cloud, the air between acts as a room, cotton clothing is worninstead of silk or nylon; between two huge charges. In effect, we have agiant thus no charge is produced. (Why?) capacitor. Since the base of the cloud is usually only a few thousand feet abovethe ground, it is easy for the electrons on the base of the cloud toinduce a positive charge in objects on the ground and then REVIEW flow to these objects (lightning) , when sufficientvolt- age (many billions ofvolts)is developed. The tre- Following are some review problems and questions: mendous energy of a bolt of lightning produces the 1. Construct an experiment to show that like brilliant, dazzling light; it also causes great heat. The charges repel each other, and unlike charges heated air explodes, much like the hot gases in a attract each other. shotgun shell, and causes the same type of noise. 2.(a) What is an electron?(b) What is a pro- ton?(c) What is a neutron?(d) Compare an electron and a proton as toquantity of charge.(e) Compare an electron and a proton as to mass. 3. Using the electron theory, answer the follow- ing:(a) Why does a piece of glass become positively charged when rubbed with silk? \ (b) What charge does silk acquire? Why? \ (c) What is the functional difference between Zap. neTabveI an insulator and a conductorof electricity? chorye on 4.(a) Explain how to charge an electroscope base of positively by contact.(b) How do the leaves cJoud- of the electroscope indicate that the electro- scope possesses anelectric charge?(c) Has the electroscope lost or gained electrons? 5.(a) Explain how to charge an electroscope Rain and positively by induction.(b) By means of three diagrams show that the electroscope is made to lose electrons during this process. + induced 6.(a) What happens when a positively charged positive body is brought near, but does not touch, the eharTe knob of an uncharged electroscope? (b)What happens when a positively charged body is brought near the knob of a positively charged electroscope?(c) Explain how you would use a negatively chargedelectroscope to determine *around whether a charged piece of wax has apositive or negative charge. Lightning rods conduct electrons 7.(a) What is a condensor used for?(b) How safely to ground is a condensor charged?(c) How is a con- densor discharged?

13 8. Explain how lightning resembles the discharge ions, the copper becomespositively charged, of a condensor. having developed a shortage of electrons.When 9. Explain how the electrophorus is used. the zinc and copper are connectedby a wire 10. State three ways in which electrons are liber- (with a light bulb, voltmeter, etc.) the excess ated from the bodies to which they belong. electrons on the zinc electrode flow tothe positively charged copper electrode. Aslong as the chemical changecontinues to take place between the electrodes and the solution,the PRODUCING ELECTRIC electrons will continue to flow through thewire. CURRENTS Using Electricity: Experiment 2 Currents of electricity can be produced in many Have students place the ends of two wires, different ways; thermoelectrically, photoelectrically, All connected to a fresh 6-volt battery, into a glass magnetically, with piezo crystals, and chemically. sulfuric the of water to which a small amount of the methods of producing electric currents use acid (H2SO4) has been added. (Use caution be- that is,they put the outer same basic principles; cause sulfuric acid is verycaustic to the skin.) electrons of an atom into motion by applying some Point out to them the bubbles that begin to type of energy to the atom.Probably the simplest of climb up the wires. Also notice that one wire method to investigate is the chemical production has about twice as many bubbles as theother. electricity. Ask the students what the bubbles or gases are and what the experiment represents.Sup- Using Electricity: Experiment 1 ply some reference material in which they can Have students place a strip of zinc (an find the answer. electrode) and a strip of copper (an electrode) Call attention to the formula of water, into a container of water to which alittle H2O, noting especially that there are twice the sulfuric acid has been added. Connect a wire number of hydrogen atoms as oxygen atoms in from each electrode to a one and one-halfvolt a water molecule. Whilediscussing the results light bulb. Ask students what caused the bulb thoroughly, impress upon the students that the to light. This is a simple cell.Two or more molecule of water has actually beenseparated cells constitute a battery. into hydrogen and oxygen, and that the atoms To further explore the production of elec- of these elements collected at a certainwire tricity chemically, attach a voltmeter (wrapping because the atoms had developed an electrical the wire around a magnetic compass will serve charge. This illustration should aid in explain- as a crude meter)in the circuit of our cell. ing that all matter is electrical in nature. Now substitute different metals for the zinc and Have the students dissolve copper sulfate copper electrodes such aslead, tin, aluminum, in the water and add a small amount ofsulfuric carbon, iron, etc. acid to the solution. Connect a strip of copper Notice that many combinations of two different to one of the wires going tothe positive ter- metals produce an . What happens minal of the cell and a carbon rod to the nega- is that one metal (called the more active metal)gives tive terminal of the cell.(A carbon rod can up its electrons more easilythan the other. The entire be obtained from a worn-out dry cell.They process in the case of zinc and copperis as follows: are used for the centerelectrode.) Immerse both strips in the solution. After Zinc, the more active metal, dissolves in a few minutesthe students will notice the the solution. Its atoms separate from the strip carbon rod is being coated with copper and in the form of zinc ions. The zinc atoms be- that the copper strip is disappearing. come zinc ions by losingtheir electrons. The electrons lost by the zinc remain on the zinc This is called electroplating, and shows that elec- strip and cause the strip to become negatively tricity can cause atoms to move through aliquid. charged because it has a surplus of electrons. This would also indicate that the atomsthemselves At the same time, hydrogen ions in the acid must have charges. Apossible explanation of this remove electrons from the copper strip inorder would be that the atoms have an extraelectron or a to become molecules of hydrogen gas. Because deficiency of an electron. An atomwhich has de- the copper is losing its electrons to the hydrogen veloped a charge is referred to as an ion.

14 To illustrate resistance pour 100 ml. of CURRENT ELECTRICITY water into the above flask and 100 ml. into a The problem in teaching this area of electricity 250 ml. beaker or wide mouth jar. Invert both is the difficulty students have in visualizing the flow over a sink.Ask students why the beaker of the invisible electron and in seeing that current emptied faster than the flask.This type of electricity is electricity in motion. This problem can analogy for resistance can be carried further be overcome in two ways, however.First compare by doing a quantitative experiment using dif- current electricity to something the student can visual- ferent diameters and lengths of glass tubing ize, such as water. Second, use the one sense that can from the flask. Have the students measure the detect electrons, the sense of touch. Also at this point, volume, flow, and time. the student should be introduced to electrical terms and their meaning. It is helpful, therefore, to draw An analogy can be used to tie these relationships a chart on the board, such as the one below, and give together. Ask the students to imagine that half the the electrical term, its counterpart in a water analogy, class is going to push the other half up the hall in the units, and symbols. baby carriages. If the halls were only one half as wide Electrical units of measurement: it would be much more difficult to get the same num- Analogy ber of baby carriages through in the same time. Terms Units Symbols Water Units Voltage volt V.(E) Pressure lbs./sq. in. Now ask students what each group of students Amperageampere A.(I) Amount ofgal./hr. in the above analogy represents in electrical terms. flow They should see that the students in the baby carriages Resistanceohm f).(R) Area of sq./in. pipe represent electrons and that their movement or flow represents amperage. The students pushing the baby These characteristics can be demonstrated by use carriages represent voltage, or the pressure that moves of a flask equipped as shown in the following diagram: the electrons, and the students passing in the halls ??43ber 140. R066ep.fiose between classes represent resistance or the force that opposes the flow of electrons (or baby carriages) . akiss7;;647ge 2 Hole. Nozzle Many variations of this analogy can be made. For Stopper example, what would happen if two people pushed Gasslrbin one baby carriage, (in effect doubling voltage) ?

.250m /.F /ask Current Electricity: Experiment 2 41:p. ;,4;!.*.zr Wate.r- Have the students connect different lengths, thicknesses, and kinds of wire to a dry cell and Current Electricity: Experiment 1 an ammeter as shown in the following dia- gram: To illustrate voltage, have the student ob- serve that when the flask is inverted the water just trickles out. Now blow into the other tube (for added attention direct the stream toward students) and ask the students why the water had more force when air was blown into the flask. The pressure would represent the voltage DRYBatters, and is the push or difference of potential. LYI*--"" Did more water lcave the flask in the same amount of time when the flask was blown into than 11 when it was simply inverted? This flow of water represents the electrical characteristics of amperage. --Wire Amperage is the number of electrons that flow in a given unit of time.

15 (Ready made boards with different lengths, in diameter than #14 wire. Remember also thicknesses, and kinds of wire on mounted that an amp meter is hooked in series and a spools can be purchased.) Students should then voltmeter is hooked in parallel.) Each student make a comparison of resistance in relation should construct a chart as shown below: to length, thickness, and typeof wire. Have Now carefully read the voltmeter and enter them run several trials and record their find- the reading in the column under volts. Dothe ings. They should conclude that the longer same thing with the ammeter. wire, and/or the thinner wire, allow less current Ask the students if they can see anynumerical to flow, indicating that there is moreresistance. relationship between resistance, which is unknown, They should also discover that steel wire has and amps and volts which they have measured.The more resistance than copperwire. students should be reminded that previousinvestiga- Now see if students can verbally relate tion showed that as resistance increased amperagede- voltage, resistance, and amperage in terms of creased. Suggest that the students try multiplying the quantity. number of volts times the number of amps.(It will Have students connect carefully measured then be seen that this does not give a correct corre- wires of a known diameter to a fresh dry cell sponding proportional resistance reading.) Thenhave with an ammeter and volt meter in the circuit the students divide the number of amps intothe as shown.(Remember that the diameter of number of volts. This does give a correct correspond- a wire decreases as thenumber of the wire ing proportional increase in resistance. An ohm meter increases. For instance, #28 wire is smaller can then be used to verify thesecalculations.

wire making up circuit

battery

T. measure amps connect To measure volts connect ammeter so that all the voltmeter in portal/ to the current must flow through the rest of the circuit

TYPE OF MATERIAL AMPERES OHMS OF WHICH WIRE IS WIRE SIZE LENGTH of WIRE VOLTS MADE AI& German Silver 30 80 cm German Silver 30 160 cm German Silver 30 240 cm Copper 14 200 cm Copper 28 200 cm Copper 30 200 cm 1 Copper 14 2000 cm.

16 The preceding observations,calculations, and con- Here are some typical problems: clusions were made by George Ohm in 1827. Hefound that a current flowing in a circuit isdirectly pro- 1. A circuit has a total resistance of 12 a. If a battery is wired to it, howmuch volt- portional to the applied voltage and inversely propor- .5 amps tional to the resistance of the circuit. This is known age would be required to push through it? as Ohm's Law. Inmathematical form it appears that: (c) step 3 voltage (a) step 1 (b) step 2 Current (amps) = resistance E = .5A X 12 Q E = 6V (answer) or using symbols R E = IR I_ R 2. A television set is wired to a portable 110 Thus if any two factors of a circuit are known, the volt power supply at a hunting camp. If the internal resistance of the television set is third can be found. 12 0, how many amps would it draw at The following diagram will help the students 110 V.? find the relationships between these three factors: (a) step 1(b) step 2 (c) step 3 R 110 V. = I = I = 9.2 Amps (answer) E 12 SZ

3.If 75 amps at:. flowing through a wire at 12 volts, what is the internal resistance of the wire? (a) step 1(b) step 2 (c) step 3 12 V. , , R = R = R= .160 (answer) I 75 Amps If the unknown symbol is covered,the proper The students should work problems such as these be mathematical relationship of the two knowns can until they are thoroughly familiar with the process. seen and used to findthe unknown. Thus we can find voltage (E) by multiplying amperage (I)times re- The following diagrams placed on theblackboard understand the analogies on the sistance (R).Or to find resistance, we divide voltage may help students to by amperage. following pap:

17 The diagram below if placed on the blackboard may help students to understand the analogies on the following pages

water already in pipes

191050 provides A// the water must f/ow through pressure the one pipe that makes up the or system. energy input" pipes

Electrons a/ready in conductor Battery or ---11111 all the electrons must flow Venerator provides through the one wire that pressure Electrical Circuit makes up the circuit. This or Electron Flow energy is called a series circuit

Conductor or wire.

Water pipes have . Electrical Circuit Diagram Electrical conductors Pottery have resistance

Electron 1 /ow

Now the water has two pipes to flow through, Some goes through each pipe.

The electrons have more than one path in which to flow. Electron FloW This is ca /led a parallel circuit. short circuit is one in which a shorter, more directpath TYPES OF CIRCUITS is provided from the source back to the source. This of the Both water and electricity are sent through dif- is usually undesirable since it leaves the rest circuit without current flowing through it. ferent kinds of piping arrangements or circuits. There are two main types ofelectrical circuits called series circuits and parallel circuits. In the series circuit the Types of Circuits: Experiment 1 current flows in an undivided,consecutive, and con- The different types of circuits can best be tinuous path from its source through the circuit and demonstrated by use of the students' sense of back to its source. This is like connecting a water touch. Have the students volunteer to form pipe from the output of a water pump and running "circuits" by linking hands around the room. it through the house and back to the intake of the A hand cranked magneto, like those found in water pump. All the water would have to flowthrough the old hand cranked telephone, serves best each part of the pipe. to supply the current. Attach short leads(wires) In a parallel circuit the current divides into a to each pole of the magneto, and havethe number of separate, independent branches in going students on each end of the "chain" grasp from its source, through the circuit, and back to its these wires.(Assure the students they will source. This is like connectingseveral pipes from a feel only a tickle, but this tickle will also let pump's output, then running them through the house, them know when electrons are flowing through and back to the intake of the pump. In this arrange- the circuit.) ment the water would divide, somegoing through Call attention to the definition of a com- one pipe and some through another,and so on. plete electrical circuit and give the magneto It follows then that all the resistances in a series crank about one half of a turn. (Warning--un- circuit would simply be added to find the total re- comfortable amounts of electricity can be pro- sistance, since all the current would have to flow duced by excessive cranking.)Each student through each resistance. But in a parallel circuit, will feel a tickle. To illustrate an open circuit each resistance represents another avenue for the cur- break the student chain at one point. Now rent to flow along, so that in effect eachtime we add there is no tickle felt by anyone. When hands resistances in parallel circuits the total resistance be- are reclasped the studentswill immediately comes less. This is sometimesdifficult for students to detect the flow of electrons if the magneto is understand. being cranked. Point out that the complete Other situations that are involved in series and circuit also fits our definition of a series circuit, parallel circuits are open circuits, complete circuits, since all students in the circuit felt current and short circuits. The open circuit is that in which passing through them. there is no complete path from the source of electricity A parallel circuit can be demonstrated by back to the source. Whereas a complete circuit does arranging students according to the following have a path from the source back to the source. A pattern:

-1Series portion of circuit Hand crankMagneto

'''% Series portion °far-alit- The students making upthe parallel por- These types of demonstrations canbe carried tion of the circuit will feelnothing when the further on a quantitative basis by placingvolt, ohm, crank is turned because currentis divided be- and ammeter in different portions ofthe student "cir- tween them, but studentsmaking up the series cuit." portion of the circuit will feel thenormal tickle. It should be rememberedthat at this point the To show the advantage of aparallel circuit, student is trying to assemble a largenumber of fairly have the students in (A) circuitunclasp their difficult concepts into one completepicture and it may feel a hands; the students in series will still be helpful to make other analogiesin order to bring still flows tickle. This shows that the current the picture into sharper focus. through the circuit even though part ofparallel circuit is broken. To demonstrate a short circuit arrange diagram: students according to the following CONCLUSION If the student has taken partin the preceding experiments, followed theexplanations, worked the problems and answered thereview questions, he Generator should begin to understand thebasic facts about the nature of electricity.More important, however,he will have gained this information notthrough the memorization of facts but throughsmall, controlled In this way he will have boys make prototypic experiments. Let three of the more aggressive learned additionally the correctscientific approach. up the "shorted"portion of the circuit. The in the circuit will feel Finally, it is hoped that theinvestigation of electricity rest of the students curiosity and will nothing because the three boys will makethe will have whetted the student's investigations on his own. easiest path for the current to follow. stimulate him to make