Project Physics Handbook 5, Models of the Atom

DOCUMENT RESUME

ED 071 904 SI 015 Sti

TITLE Project Physics Handbook 5,Models of the Atom. . INSTITUTION Harvard Univ., Caml.ridge, Mass. Harvard Project Physics. SPONS AGENCY Office of Education (DHEW),Washington, D.C. Bureau of Research. BUREAU NO BR-5-1038 PUB DATE 68 CONTRACT OEC-5-10-058 NOTE 42p.; Authorized Interim Version

EDRS PRICE MF-S0.65 HC -$3.29 DESCRIPTORS *Atomic Theory; Instructional Materials; Laboratory Experiments; *Laboratory Manuals; Physics; *Physics Experiments; *Science Activities; Secondary Grades; * Secondary School Science IDENTIFIERS Harvard Project Physics

ABSTRACT Five experiments and 19 activitiesare presented in this Unit 5 handbook. The experimentsare related to electrolysis, charge-to-mass ratio, elementary charge determination, photoelectric effects, and spectroscopic analyses. The activitiesare concerned with Dalton's theory, water electrolysis, periodic tables, single-electron plating, cloud chambers, accelerators,counters, spectrographs, Einstein's work, cathode rays,x-rays, ionization, differences in atomic models, Rutherford atom, and de Brogliewaves. A reprinted chemi-crostic and a specially designed cigar boxatom are also incorporated in the activities. Moreover, three film loopsare introduced in terms of production of sodium electrolysis, Thomson's atomic model, and Rutherford scattering. Demonstrations,construction projects, and Self- directed instructions are stressed in the activities. The four chapters in the handbookare designed to correspond to the text, with complete instructions in each experiment. Some experiments and activitiesare suggested for assignment, and the remaining are used at student discretion. Illustrations are provided for explanation purposes. The work of Harvard Project Physics has been financially supported by: the Carnegie Corporation of New York, the Ford Foundation, the national Science Foundation, the Alfred P. Sloan Foundation, the UnitedStates Office of Education, and Harvard University.(CC) US DEPARTMENT OF Hi ALTH EDUCATION& WEIFARc OFcICE OF FOUCATIOr I., S DOf OW N.':.`, Eq.'...t.', DU( ED EXAl '.....,R.'. .0 'N.N Project Physics Handbook ilE t'ERSU% 'A ..)PA%,"... ,N. P 5 iNATIN., '' POtN', ).t ... P N. tONS SrATED;, ".

CA` 0% P,,, ' ON.ti-i P

An Introduction to Physics Models of the Atom This hcndbook is the authorized interim version of one of the many iAstructional materials being developed by Harvard Project Physics, including text snits, laboratory experiments, and teacher guides.Its development has profited from the help of many of the colleagues listed at the front of the text units.

Copyright is claimed until June 1, 1969. After June 1, 1969, all portions of this work not identified herein as the subject of previous copyright shall be in the public domain. The authorized interim version of the Harvard Project Physics course is being distributed at cost by Holt, Rinehart and Winston, Inc. by arrangement with Project Physics Incorporated, a non-profit educational organization.

All pe-sons making use of any part of these materialsare requested to acknowledge the source and the financial support given to Project Physics by the agencie: named below, and to include a statement that the publication of such material is not necessarily endorsed by Harvard Project Physics or any of the authors of this work.

The work of Harvard Project Physics has been financially supported by the Carnegie Corporation of New York, the Ford Foundation, the National Science Foundation, the Alfred P. Sloan Foundation, the United States Office of Education and Harvard University.

Consult the Project Physics text for a partial list of the current and recent staff, consultants and advisory com- mittee.

03-073540-8

90123 69 9876543 Project Physics Handbook

An Introduction to Physics5Models of the Atom

Authorized Interim Version 1968-69

Distributed by Holt, Rinehart and Winston, Inc.New York Toronto Chapter 17 The Chemical Basis of AtomicTheory

Experiments

41. Electrolysis 1 Activities

Dalton's Puzzle 5 The Electrolysis of Water Periodic Table 5 5 Single-electrode Plating 7 Activities from Scientific American 7 Film Loops

46. Production, of Sodium Electrolysis 9

Chapter18Electrons and Quanta

Experiments

39. The Charge-to-Mass Ratio foran Electron 11 40. The Measurement of Elementary Charge 42. 15 The Photoelectric Effect 19 Activities

Writings By and About Einstein 23 Measuring g/m for the Electron 23 Cathode Rays in a Crookes' Tube 23 X Rays from a Crookes' Tube 24 Lighting a Light Bulb witha Match Photo- electrically 24 Chapter 19 The RutherfordBohr Model of the Atom

Experiments 43. Spectroscopy 27 Activities

Scientists on Stamps 31 Measuring a Quantum Effect: Ionization 31 Modeling Atoms with Magnets 33 Chemi-Crostic 34 Cigar box Atoms 36 Another Simulation of the Rutherford Atom 36

Film Luops

47. Thomson Model of the Atom 37 48. Rutherford Scattering 38

Chapter 20 Some Ideas From Modern Physical Theories

Activities

Standing Waves on a Band-Saw Blade 41 Turntable Oscillator Patterns Resembling 41 de Broglie Waves Sanding Waves in a Wire Ring 41 This Student Handbook is different from laboratory manuals youmay have worked with before. There are far more things described in this handbookthan any one student can possibly do. Only a few of the experiments and activities will be assigned. You are encouraged to pick and choose from the restany of the activities that appear interestingand valuable for you. A number of activities may occur to you that are not described in the handbook ana thatyou would prefer to do instead. You should feel free to pursue these in consultation withyour teacher.

There is a section correspondingto each chapter of the text. Most sections

are composed of two major subsections--- Experiments and Activities.

The Experiments section containscom- plete instructions for the experiments your class will be doing in the school laboratory. The Activities sectioncon- tains suggestions for demonstrations,construction projects and other activities you can do by yourself. (The division between Experiments and Activitiesis not hard and fast; what is done in theschool laboratory and what is done by the student on his own may vary from school to school.)

The Film Loop Notes give instructions for the use of the film loops whichhave been prepared for this cour.,e. Chapter 17 The Chemical Basis of Atomic Theory Experiments

EXPERIMENT 41Electrolysis Electric current is provided by a power supply that converts 110-volt alter- Volta and Davy discovered that electric nating current into low voltage direct currents can create chemical changes never current. The current is controlled by a observed before. They were the first to variable transformer (or a rheostat) and use electricity to break down stable com- measured by an ammeter in series with pounds and to isolate new chemical ele- the electrolytic cell as shown in Fig. 1 ments. or Fig, 2, But that was not all.

Later Faraday and other experimenters compared the amount of electric charge used and the amount of chemical products formed. Their measurements fell into a regular pattefn that hinted at some un- derlying link between electricity and matter.

In this experiment you will use an electric current to decompose a compound, comparing the charge used ith the mass of one of the products. From these results you can compute the mass and volt.me Fig. 2 of an atom of the product. As loig as there is current in the Background cell you can observe the increasing mass A beaker of copper sulfate (CuSO4) of the cathode as copper forms on it. solution in water is supported under one With the help of a watch to measure the arm of a balance (Fig. 1). One copper time the current flows, you can compute electrode, the cathode, is supported in the solution by the balance arm so that you can measure its mass without removing it from the solution, A second copper electrode, the anode, fits against the inside wall of the beaker. -

4g.)104er ,/(/'

Fig. 1 Experiments

the electric charge, q= Ix t, that positive terminal of thepower supply is passed through the cell. connected directly to the anodein any If you know the amount of charge convenient manner

carried by a single electron (1.( x 10-39 Before any measurementsare made, coulombs), you can calculate the number operate the cell long enough (10or 15 of electrons transferred. From number of minutes) to form a preliminarydeposit of electrons transferred, the total mass on the cathode--unless this has already of copper deposited, and the number of been done. In any case, run the current electrons requireC to releaseone copper long enough to set it at thevalue recom- atom from solution, you can calculate mended by your instructor,probably about the mass of a single copper atom. 5 amperes.

Procedure When all is ready, adjustthe balance Either an "equal-arm" or a "triple- and record the reading. Pass the current beam" balance can be used for thisex- for the length of time recommendedby periment. Arrange the cell and the your instructor. Measure and record the balance as shown in the appropriate current, I, and the time duringwhich figure. The cathode cylinder must be the current passes, t. Check the ammeter supported far enough above the bottom occasionally and, ifnecessary, keep the of the beaker so that the balancearm current set to its original valueby ad- can move up and down freely when th' justing the rheostat or variabletrans- cell is full of the copper-sulfatesolu- former. tion. At the end of the run, resetthe bal- Next connect the circuit as illus- ance and record the new reading. Find trated in the figure. Note that the by subtraction the increasein mass of electrical connection from the negative the cathode. Also record the current I. terminal of the power supplyto the Since the cathode is buoyedup by a cathode is made through the balancebeam. liquid, the masses you havemeasured are The knife edge and itsseat must be by- not, unfortunately, the truemasses. passed by a short piece of thinflexible Because of the buoyant force exertedby wire, as shown in Fig.1 or Fig. 3. The the liquid, the mass of thecathode and its increase in ,..ass,Am, will both appear to be less than they wouldbe in air. To find the truemass increase, Am, we must divide the observedmass increase by the factor (1- De/Dc), where D eis the density of theelectrolyte and D cis the density of thecopper.

Your instructor will giveyou the values of these two densitiesif you cannot measure them for yourself. He will also explain how the correctionfactor is derived. The important thing foryou to understand here is why a correctionfactor is necessary. Fig. 3

2 Experiments

Results Q6 The mass of a penny is about 3 grams. Ql How much charge was transferred to If it were made of copper only, how many the cathode? atoms would it contain? In fact modern pennies contain zinc (5%) as well as In the solution this positive charge copper. is carried from anode to cathode by doubly ++ charged copper ions, Cu . At the cathode Q7 The volume of a penny is about 0.3 the copper ions are neutralized by elec- cm3. How much volume does each atom oc- trons and neutral copper atoms are deposi- cupy? ted. Q8 You may have learned the concept of Q2 How many electrons (single negative gram-atomic weight. The atomic weight charge) are required to neutralize each of copper is 63.5. How many atoms are copper ion? there in one gram atomic weight?

Q3 How many electrons were required to you repeated this experiment with neutralize the total charge transferred? a different solution and made a deposit (Each electron carries -1.6 x10-19 cou- of say, silver, you could determine the lomb.) mass and approximate size of a silver atom. But the answer to the last ques- Q4 How many copper atoms were deposited? tion "How many atoms in one gram atomic Q5 What is the mass of each copper atom? weight?" would be the same. It is the same for all the elements.

3 Activities

ACTIVITY Dalton's puzzle If the assumption of simplicity is Once Dalton had his theory to work relaxed, what other atomic masses and with, the job of figuring out relative molecular formulas would be consistent atomic masses and empirical formulas with the datl? boiled down to nothing more than working through a series of puzzles. Here is a ACTIVITY The electrolysis of water very similar kind of puzzle with which The fact that electricity can decom- you can challenge your classmates. pose water was an amazing and exciting

Choose three sets of objects, each discovery, yet the process is one that you can easily demonstrate with materials set having a different mass. Large ball at your disposal. bearings of about 70, 160, and 200 grams Figure-17.2 on page 28 mass work well. Let the smallest one provides all the necessary information. represent an atom of hydrogen, the middle- Set up an electrolysis apparatus and sized one an atom of nitrogen, and the demonstrate tt'e process for your class- large one an atom of oxygen. mates.

From these "atoms," construct "mole- :;n Fig. 17.2, it looks as if twice as cules" by concealing combinations of many buLbles were coming from one elec- atoms in covered styrofoam coffee cups trode (which one?) as from the other. (or other light, opaque containers). Does this happen in your apparatus? For example, NH3 would be represented Would you expect it to? by three small objects and one middle- What are the two gases that bubble sized one,, N20 by two middle-sized ones off the electrodes?Can you prove their and one large, and so forth. Label the identity? Devise a method f'r collecting "compounds" ), B, etc., and mark on each the products of the electrolysis. Is the symbols (with either present-day water really just these two gases "put symbols or Dalton's ideographs) of the together" chemically? If so, can you elements contained in the compound. put the gases together again and get Dalton would have obtained this infor- back the water you started with? dhat mation by qualitative analysis. happens to all the electricity you sent Give the covered cups to other stu- flowing through the water? dents Instruct them Lc) measure the What made this simple process so mas f each compound and to deduce the amazing when it was first observed? Why relative atomic masses and empirical wasn't it observed earlier?After all, formulas from the set of masses, making experiments with electricity had been Dalton's assumption of simplicity. If going on since ancient times. the objects you have used for "atoms" are so light that the mass of the styro- ACTIVITY Periodic table foam cLps must be taken into account, You may have seen one or two forms of you can either supply this information the periodic table in your classroom, but as part of the data or leave it as a many others have been devised to empha- complication in the problem (a compli- size various relationships among the ele- cation that Dalton didn't have to deal ments. Some,su,-..h as the one on the with). next page, are more visually interesting than others. Check various sources

5 Activities

" 3

is.45-49413' U. OUR HERITAGE OF THE ELEMENTS' Adapted from paoh deliverd before the American ooleV for Matls, Mtal-Mtrisil Congress, PMIdelphis, P.. Ootob.r 19, 111151A

Reprinted from Chemistry, July 1966.

6 Activities

in your library and prepare an exhibit of Repeat these experiments and see if the various types. An especially good the effect is true generally. What ex- lead is the article, "Ups and Downs of planation would you give for this effect? the Periodic Table" in Chemistry, July (Adapted from Ideas for Science Investiga- 1966 which shows 12 different forms of tions, N.S.T.A. 1966). the table.

It is also interesting to arrance the ACTIVITY Activities from Scientific American elements in order oc discovery on a linear The following articles from the time chart. Periods of intense activity "Amateur Scientist" section of Scientific caused by breakthroughs in methods or American relate to Unit 5.They range extended work by a certain group of in- widely in difficulty. Each listing has vestigators show up 3n groups of names. title, year, month and page number. A simple way to do this is to use a type- writer, letting each line represent one Accelerator, electron, 1959 Jan. 138. year (from 1600 on). All the elements Beta ray spectrometer, 1958 Sept. 197. then fit on six normal typing pages which Carbon 14 dating, 1957 Feb. 159. can be fastened together for mounting on Cloud chamber, diffusion, 1952 Sept. 179. a wall. A list of dates for all elements Cloud chamber, plumber's friend, 1956 Dec. 169. is on pages 48 and 49 of the Unit 6 text. Cloud chamber, Wilson, 1956 Apr. 156. Cloud chamber, with magnet, 1959 June 173. ACTIVITY Singleelectrode plating Cyclotron, 1953 Sept. 154. Gas discharge tuLes, how to make, 1958 A student asked if copper would plate Feb. 112. out from a solution of copper sulfate if Geiger counter, how to make, 1960 May 189. only a negative electrode were placed in Isotope experiments, 1960 May 189. the solution. It was tried and no copper Magnetic resonance spectrometer, 1959 Apr. 171. was observed even when a high voltage was Scintillation counter, 1953 Mar. 104. applied. Another student suggested that Spectrograph, astronomical, 1956 Sept. 259. only a very small (i.e., invisible) amount Spectrograph, Bunsen's, 1955 June 122. of copper was deposited since copper ions Spinthariscope, 1953 Mar. 104. should be attracted to a negative elec- Spectroheliograph, how to make, 1958 trode. Apr. 126.

A more precise test was devised. A Subatomic particle scattering, simulating, 1965 Aug. 102. nickel sulfate solution was made containing several microcuries of radioactive nickel (no radio-copper was available). A graphite electrode was immersed in the solution, and a high negative voltage applied for five minutes. The electrode was removed, dried, and tested with a Geiger counter. The rod was slightly radioactive. A control test was run using identical test conditions, except that NO voltage was applied to the electrode.The control showed MORE radioactivity.

7 Film Loops

FILM LOOP 46Production of Sodium metallic sodium: by Electrolysis Na e - Na.

Humphrey Davy in 1807 first produced The sodium accumulates in a thin shiny metallic sodium by electrolysis of soda layer floating on the surface of the liq- (sodium hydroxide), The film shows how uid. this is done. Sodium is a dangerous material which First, sodium nydroxide (NaOH) is combines explosively with water. When placed in an iron crucible and heated with the experimenter scoops out a little of a gas flame until it becomes molten at a the metal and places it in water, energy temperature of 318° C. Then, iron rods to is released rapidly. Sore of the sodium serve as electrodes are inserted into the is vaporized and emits the yellow light melt. A rectifier circuit connected to a characteristic of the spectrum of sodium. power transformer supplies a steady cur- The same yellow spectrum is easily seen rent through the liquid NaOH, Sodium ions if common salt (NaC1) or some other sodi- are positive and are therefore attracted um compound is sprinkled into an open to the negative electrode; there they pick flame. up negative charges (electrons) and become

-.Asossinv

9 Chapter18Electrons and Quanta Experiments

EXPERIMENT 39 The Charge-to-Mass Ra.io Take a cardboard tube about 3" in diam- for an Electron eter and 3" long. Cut a slot 11/4" wide in In Sec. 18.2 of the text you can read it. Your electron-beam tube should fit about J. J. Thomson's work on cathode into this slot (Fig. 1). rays. If you did Experiment 37 in Unit 4, Ncw wind a pair of coils, one on each "Electron Beam Tube," you have already side of the slot. Use insulated copper done some work with cathode rays and have wire (magnet wire). Use about 20 turns seen how they can be deflected by elec- of wire for each of the two coils, one tric and magnetic fields. In this experi- coil on each side of the slot, Make the ment you will be able to repeat Thomson's coils as neat as you can and keep them quantitative measurements that convinced close to the slot. Wind both coils in physicists that the rays are charged the same sense (that is, both clockwise) particles, each with the same ratio of and use one piece of wire for both coils. charge to mass. Leave about 10" of wire free at both ends For the version of the experiment de- of the coil. (Don't cut the wire off the scribed here, you will use your own elec- reel until you have found how much you'll tron beam tube. There are other methods need!) not described here that you may wish to try. Ask your instructor for details.

Electron-beam tube You will need a tube that gives an electron beam at least 5 cm long. If you kept the tube you made in Experiment 37, you may be ab1.2 to use that. If your class didn't have much success with t - experiment, it may mean that your vacuum pump is not working well enough, in which Irtl "t- case you will probably decide to use Fig. 1 Sketch of coils another method. Now that you have made your magnet, Thomson's method is summarized on you must calibrate it You can use the page 41 of Unit 5, and you should read current balance, as you did in Experiment that section of the text before beginning 36. Use the shortest of the balance this experiment. "loops" so that it will fit inside the coils as shown in Fig. 2. The magnetic field In this experiment you need to be able

to adjust the strength of the magnetic --1-41i111 field until the magnetic force on the charges just balances the force due to the electric field. To enable you to change the magnetic field, you will use a pair of coils instead of permanent magnets. You can vary the magnetic field by changing the current in the coils.

Fig. 2

11 Experiments

Measure the force F for a current Iin the loop. Use F = BIB tocalculate the magnetic field due to the currentin the coils. Do this for severaldifferent values of current in the coil and plota graph of magneticfield B against coil current I,

Experimental Set up your beam tube as inExperiment 37,

ztejleei, 1(-donoda Plate PI Ate.

6 V I /oo v Fig. 4 The magnetic field 1 is parallel to the axis of the coils; the electric and magnetic fields are perpendicular to Fig. 3Connections of beam tube each other andto the electron beam. Pump it out and adjust the filament Turn off the electric current until you field (by making have an easilyvisible both anode and beam. deflecting platesthe same Connect a wirebetween the pin for potential). Turn on and slowly the deflectingplate and the pin increase for the the currentin the coils until anode plate. the mag- There is nowno field be- netic field isstrong enough to tween the platesand the electron deflect beam the electron beam should go straight noticeably, This de- up the center ofthe flection must beopposite to the tube between thetwo plates; if deflec- it does tion caused by not, it is probably the electricfield. because the filament You may have toreverse the direction and the hole inthe anode are of the not properly current in the coils. aligned. Now turn the electric Now, without field onagain. releasing thevacuum, Which field hasthe greater effect mount the coilsaround the tube on the as shown beam the electric in Fig: 4. or the magnetic?Ad- just the magneticfield by controlling Connect the twoleads from your coils the current inthe coils until to a power supply the ef- capable of givingup fects of the two fields balanceeach other to 5 amps directcurrent. There must be and the electronbeam goes straight a rheostat in the along circuit to controlthe the middle ofthe tube. current and an ammeter to measureit. Record the currentin the coils, Now put a potential and difi once of the potential about 100 volts difference betweenthe de- between the anodeplate flecting plate and and the deflecting anode plate. You can plate, to deflect the find the magneticfield B from beam as in Fig. your cal- 18.4 (c). Observe which ibration graph of coil currentvs. B. way the beam isdeflected. You need to knowone more quantity

12 Experiments

before you can calculate qe/m. This is You can measure x and d, It follows R, the radius of the arc which the elec- from Pythagoras' theorem that R2 = d2 tron beam is bent into by the magnetic (R - x)2, so R d2 x2 field alone. To find R, turn off the 2x electric field again, but leave the mag- Now you can use the formula qe/m = netic field on with the same current in E/B2R, obtained by equating the magnetic the coils. Observe the deflection of and electric forces on the charges, to the beam due to the magnetic field alone. calculate your value of qe/m, The magnetic field is uniforn the re- If possible you should find other gion between the coils and so this curved pairs of values of E and B which leave arc should be circular. You won't be the beam undeflected, measure R for each able to measure the radius directly, but value of B and calculate more experimen- here is one way you can do it from meas- tal values for qe/m. urements that are easy to make:

Fig. 5

13 Experiments

EXPERIMENT 40 The Measurement of Elementary Charge In this experiment you are going to measure very small electric charges to see if there is a limit to how small an electric charge can be. Try to answer these three questions before you begin to do the experiment in the lab.

Q1 What is the electric field between two parallel plates separated by a distance d meters, if the potential difference between them is V volts?

22 What is the electric force on a particle carrying a charge of q coulombs in Fig. 1 Electron microgyaph of latex spheres an electric field of E volts/meter? 1.099i in diameter (- 1.1 x 10-4 cm), silhouetted against diffracting grating of 28,800 lines/inch. Q3 What is the gravitational force on a (Courtesy L. J. Lippie particle of mass m in the earth's gravi- Dow Chemical Company, tational field? Midland, Michigan.) with their diameter recorded on the bot- Background tle. When the suspension is sprayed into This experiment is substantially the the air the water quickly evaporates and same as Millikan's famous oil-drop experi- leaves a cloud of these particles in the ment, described on page 43 of Unit 5. The air. They become charged by friction following instructions assume that you have during the spraying. In the space between read that description. the plates of the Millikan apparatus they We measure electric charges by measur- appear through the 50-power microscope as ing the forces they produce and experi- bright points of light against a dark background. ence. The extremely small charges which interest us require that we measure ex- Since the electric field between the tremely small forces. Objects on which plates can pull them upwards against the such small forces can have a visible ef- force of gravity, we know the spheres in fect must also in turn be very small. the spray are charged electrically.

Millikan used the electrically charged In our experiment we adjust the elec- droplets of oil in a fine spray. Such tric field until a sphere hangs motion- droplets vary in size, which complicated less. Then we know that the upward elec- his measurements. Fortunately we now tric force on it, Eq, is exactly equal have available suitable objects whose to the downward gravitational force, ma sizes are accurately known. We use small We know the electric field strength, E, latex spheres (about 10-4 cm diameter) from the potential difference and the V which are in any given sample almost plate separation: E = a. We also know identical in size. In fact, these spheres, the mass m of the sphere (all the spheres shown magnified in Fig. 1, are used by have the same size and mass). We can electron microscopists to find the mag- then easily solve the equation, equating nification of their instruments. The the electric to the gravitational force, spheres can be bought in a water suspension, Eq = ma for the charge on the sphere, q.

15 Experiments

Using the apparatus Squeeze the bottle of latex suspension If the apparatus is not already in two or three times until five or ten par- working order, consult your instructor. ticles (latex spheres) drift into the Study Fig. 2 until you can identify the view seen through the microscope. You various parts. Then switch on the light will see them as tiny bright spots of source and look through the microscope. light. You may have to adjust the focus You should see a series of lines in clear slightly to see a specific particle clear- focus against a uniform, light gray back- ly. Notice how the particles appear to ground. move upward. The view is inverted by the microscope the particles are actually falling in the earth's gravitational 1151it source cal(kan field.

Now switch on the high voltage across the plates by turning the switch up or 6V For down. If9trt sourt, Notice the effect on the particles of varying the electric field by means of the voltage-control knob.

Notice the effect when you reverse the electric field by reversing the switch position. (When the switch is in its voltmeter 22ov for' bade of vottoor reverim5 mid-position, there is zero field between oil Plates latex spheres svoick. the plates.)

Fig. 2(a) Q4 Do all the particles move in the same direction when the field is on?

Q5 How do you explain this? The lens may mist up as the heat from Q6 Why do some move much more rapidly in the lamp drives moisture out of thelight- the field than others? source tube. If this happens remove the Q7 Do the rapidly moving particles have lens and wipe it on a clean tissue.Wait for the tube to warm up thoroughly be- larger or smaller charges than the slowly moving ones? fore replacing the lens. Sometimes a few particles cling together, making a clump that is easy to see the clump falls more rapidly than single particles when the electric field is off. Do not try to use these for measuring q.

Try to balance a particle by adjusting the field until the particle hangs motionless. Observe it carefully to make sure it isn't slowly drifting up or down. The 7; fixde.). smaller the charge, the greater theelec- 51)pely,//To vottmete, tric field must be to hold up the particle. Fig. 2(b)

16 Experiments

Taking data Treatment of results It is not worth working at voltages On a balanced particle carrying a much below 100 volts. Only highly charged charge q, the upward electric force, Eq, particles can be balanced in these small and the downward gravitational force, fields, and we are interested in obtain- ma are equal, so ing the smallest charge possible. mag = Eq. Set the potential difference between The field E = V/d, where V is the the plates to about 100 volts. Turn the voltage between the plates (the volt- switch up and down a few times so that meter reading) and d is the separation the more quickly moving particles (those of the plates. with greater charge) are swept out of the Hence field of view. Any particles that remain marfd q (1) have low charges. If no particles remain, squeeze in some more and look again for All of the quantities on the right can be some with small charge. measured.

When you have isolated one of these You already know ag and have a record particles carrying a low charge, adjust of V for each balanced particle. the voltage carefully until the particle The separation of the two plates, d, hangs motionless. Observe it for some is given by the manufacturer as 5.0 mm, time to make sure that it isn't moving or 5.0 x 10-3 m. You may want to check up or down very slowly, and that the ad- this. justment of voltage is as precise as pos- The mass of the spheres, m, is worked sible. out from a knowledge of their volume and Read the voltmeter. Then estimate the density. precision of the voltage setting by see- Mass = volume x density,, or ing how little the voltage needs to be 4 3 n changed to cause the particle to move m= r x 3 just perceptibly. This small change in The diameter (careful: 2r) has been pre- voltage is the greatest amount by which viously measured and is given on the sup- your setting of the balancing voltage can ply bottle, and the density, D, is 1077 be uncertain. kg/m3 (found by measuring a large batch A few precise readings of the balancing of latex before it is made into little voltage are more useful than several less spheres). Notice that ma d is a constant for reliable values. When you have balanced g a particle, make sure that the voltage all measurements and need only be found setting is as precise as you can make it once. Each value of q will be this con- before you go on to another particle. stant ma d times 1/V, as Eq.(1) shows. The most useful range to work in is 100- Because balancing voltage required is 150 volts, but try to find particles that greater for smaller charges, q will be can be brought to rest in the 200-250 proportional to 1/V. volt range too, if the meter can be used Tabulate youz values of V and 1/V x in that range. Remember that the higher 10-2 in parallel vertical columns. the balancing field the smaller the charge (Multiplying all the 1/V valueS by 10-2 on the particle. gives numbers that are simpler to handle.)

17 Experiments

Then arrange your values of 1/Vx 10-2 Q9 What is the spacing between theob- in increasing order. Notice that some served values of 1/V, and what isthe values occur several times. Plot a bar difference in charge that correspondsto graph in which the vertical axis is this difference in 1/V? 1/V x 10-2 and the horizontal axis,num- Q10 What is the smallest value of 1/V bered 1, 2, 3,... etc., is the position that you observed? of the value in your ordered column. What is the corresponding value of q? Q8Does your bar graph suggest that all Q11 Do your experimental resultssup- values of q are possible and thatelec- port the idea that electric charge is tric charge .is therefore endlesslydivi- quantized? If so, what is your value sible, or does it come in least-sized for the quantum of charge? chunks?

18 Experiments

EXPERIMENT 42 The Photoelectric Effect larger. This output current is read on a microammeter. This experiment gives you evidence The voltage control knob on the .":_ut the nature of light. Specifically, you will see what happens when light phototube unit allows you to vary the falls on a photoelectric tube; then you voltage between emitter and collector. As you turn the knob clockwise the will compare the abilit-.es of the wave photocurrent drops. You are making model and the particle model of light to tie collector more and more negative expl&in what you see. Before doing the experiment you and fewer and fewer electrons get to should read Sec. 18.4 (Unit 5). and then it. Finally the photocurrent ceases study the apparatus in Fig. 1. The red altogether--all the electrons are wire on the phototube unit goes to the turned back before reaching the col- red (input) terminal of the amplifier. lector. The voltage between emitter and collector that just stops all the electrons is called the "stopping How the apparatus works voltage." Because there is some drift Light that you shine through the of the amplifier output, the most window of the phototube falls on a half- sensitive zeroing method is to alterna- cylinder of metal called the emitter. tely cover and uncover the phototube The light drives electrons from the with black paper. Then turn up the emitter surface. collector voltage until covering and Along the axis of the emitter (the uncovering the tube has no effect on center of the tube) is a wire called the the meter reading--the exact location collector.When the collector is made of the meter pointer, which may drift a few volts positive with respect to the around a little, isn't important. emitter, practically all the emiLIed The position of the knob at the electrons are drawn to it, and a current cutoff gives you a rough measure of appears in the external circuit. the collector voltage. To measure The small photoelectric current it more precisely, connect a volt- input to the amplifier controls an out- meter as shown in Fig. 2, on the put current several thousand times next page. r

Fig. 1

19 Experiments

In the experiment you measurestooping the stopping voltage change? voltages as light of various frequencies (3) To see if there is time delay falls on the phototube. The filters se- be- tween light falling on the emitter and lect frequencies from themercury spec- the emission of photoelectrons,cover trum emitted by a fluorescent lampor mercury lamp. the phototube and than quicklyremove the cover. Useful frequencies of the mercury spectrum lines are: Q4 Can you detect any time delay between the moment that light hits the phototubeand yellow 5.2 x 1014/sec the moment that the detector signalsthe green 5.5 . 10'4/sec passage of photoelectrons through the blue 6.9 . 1014/sec phototube? violet 7,3 x 1014/sec

(ultraviolet 8.2 x 1014/sec) In the second part of the experiment you make more precise measurements of You can use a hand spectroscopeto find stopping voltage. the highest frequency line passedby To do this, adjust each filter. the voltage control kaob to the cutoff (stopping voltage) position and then Doing the experiment measure V with a voltmeter (Fig. 2).

The first part of the experimentis .-- qualitative.

(1) To see how the kineticenergy of photoelectrons depends on the frequency of incident light, measure the stopping Fig. 2

voltage with various filtersover the Connect the voltmeter only afterthe window. cut-off adjustment is madeso that the The Unit 5 text (Sec. 18.4) explains voltmeter leads will not pickup any ac that the maximum kinetic energy of the voltage (induced from otherconducting photoelectrons is Vqe, where V is the wires in the room) and contribute itas hum in the earphone. stopping voltage and qe = 1.60 x 10-" coulombs, the charge on an electron. Measure the stopping voltage, V, for Ql How does the stopping voltage three or four different lightfrequencies,, and plot the data on a graph. change as the light is changedfrom yellow through blue or ultraviolet? Along the vertical axis plot electron energy, Vqe. (2) To see how the current in the (When the stopping voltage V is in volts, and qe is in coulombs,Vqe phototube depends on the intensityof will be energy, in joules.) incident light, vary the distanceof the light source. Along the horizontal axis plotfre- quency cf light f. Q2 Does the number of photoelectrons emitted from the sensitive surfacevary with Discussion light intensityi.e., does theoutput As suggested in the opening paragraph, current of the amplifier vary with the we are interested in comparing the intensity of the light? wave model of light and the particlemodel. Q3 Does the kinetic energy of the photo- Consider, then, how these modelsexplain electrons depend on intensity---,.e.,does your observations.

20 Experiments

05 If light striking your phototube acts In this equation -c is a constant, the as waves value of y at the point where the straight line cuts the vertical axis, a) Can you explain why the stopping and k is another constant, namely the voltage should depend on the frequency slope of the lined Therefore, the slope of light? of a graph of Vqe against f should be h. b) Why would you expect the stopping voltage to depend on the intensity of Oi Vaat is the value of the slope of the light? you: g7aph? Would you expect there to be a delay c) Q8 How well does it compare with the between the time that light first strikes value of Planck's constant, h = 6.6 x the emitter and the emission of photo- 10-34 joule-sec? electrons? Would it matter if the inten- With this equipment, the slope is un- sity of light were very low? likely to agree with the accepted value Q6 If light is acting as a stream of of h (6.6x 10-34 joule-sec) more closely particles, what would be the answer to than an order of magnitude. Perhaps you each of these questions? can give a few reasons why your agree- If you went on to draw the graph in ment cannot be more than approximate. the second part of the experiment, you Q9 On your graph what is the threshold should be prepared to interpret it. frequency, f0? This is the lowest fre- Remember that Einstein predicted its quency at which any electrons are emitted form (in 1905) and by experiments similar from the cathode surface. At this fre- to yours, Millikan verified Einstein's quency Ilmv2max 0 and hfo = W, where W prediction (in 1916). is the "work function." Your experimen- Einstein's photoelectric equation tally obtained value of W is not likely (Unit 5 text, Sec. 18.4) describes the to be the same as that found for very energy of the most energetic photoelec- clean cathode surfaces, more carefully trons (the last ones to be stopped as filtered light, etc. The important thing the voltage is increased), as to notice here is that there is a value

= Vqe = hf - W. of W, indicating that there is a minimum Ilmv2max energy needed to release photoelectrons This equation has the form from the emitter.

y = kx - c. Einstein's equation was derived from the assumption of a particle (photon) A model of light. t 010 If your results do not fully agree 5rot,e - A with Einstein's equation, does this mean that your experiment supports the wave theory? t. C fa

Fig. 3

21 Activities

ACTIVITY Writings by and about ACTIVITY Cathode rays in a Crookes' tube Einstein In addition to hls scientific works, Use a Crookes' tube to demonstrate to Einstein wrote many essays on other the class the deflection of cathoderays areas of life which are easy to read, in magnetic fields. You can also show perceptive, and still very current. The how a magnet focuses cathode rays: bring chapter titles from Out of My Later Years one pole of a strong bar magnet toward (Philosophical Library, N.Y. 1950) indi- the shadow of the cross-shaped obstacle cate the scope of these essays: Convic- on the end of the tube, a:: shown. Watch tions and Beliefs, Science, Public Af- what happens to the shadow as themagnet fairs, Science and Life, Personalities, gets closer and closer to it. What hap- My People. This book includes his writ- pens when you switch the poles of the ings from 1934 to 1950. The World As I magnet? What do you think would happen See It Includes material from 1922 to if you had a stronger magnet? 1934. Albert Einstein: Philosopher- Can you demonstrate deflection byan Scientist, Vol. I (Harper Torchbook, electric field? For any given voltage 1959) contains Einstein's autobiographi- supply, how can you produce the greatest cal notes, left-hand pages in German and possible electric field?One way is to right-hand pages in English, and essays bring your electrodes close together,but by twelve physicist contemporaries of the size of the Crookes tube sets a limit Einstein's about various aspects of his on this procedure. Another way is to use work. See also the three articles, sharply pointed electrodes, but sharp "Einstein," "Outside and Inside the points tend to sparkjust because they Elevator," and "Einstein and Some Civi- produce high electric fields. lized Discontents" in the Unit 5 Project Physics Reader. You can keep sparks from interfering with your observations by reducing the ACTIVITY Measuring (lint forthe electron pressure in the tube until there is too little gas present to provide an ionized With the help of a "tuning eye" tube path for the sparks to follow. This is such as you may have seen in radio sets, how Thomson first solved the problem in you can measure the charge-to-mass ratio 1897. If your vacuum pump is good, try of the electron in a way that is very it. Pump the electron -beam tube you made close to J.J. Thomson's original method. in experiment 37 to a good enoughvacuum Complete instructions are in the PSSC so that you can observe electric deflec- Physics Laboratory Guide, Second Edition, tion without sparking. (This will require D.C. Heath and Company, 1965, Experiment rather high voltages be careful.) Then IV-12, The Mass of the Electron, pp. 79- slowly let in air until the effect isno 81. longer visible. At this higher pressure, can you still obtain magnetic deflections? When Thomson made his earliest measurements of this kind, he did not of If you have an electrostaticgenerator, course obtain the electron mass but such ala small Van de Graaff or a Wims- rather qe/m. To compute the mass sepa- hurst machine, try deflecting therays rately, you need to know qe, which was using parallel plate:, across the generator. not known accurately until Millikan measured it.

23 Activities

ACTIVITY X-rays from a Crookes' tube Equipment

A Crookes' tube havinga metal barrier 1P39 photocell inside it for demonstrating thatcathode 6-volt, 15-watt light bulb ;light source rays travel in straight lines may be from the Milliken apparatus) available in your classroom. In use the 6-volt, 2.5-amp power supply (or dry cell) tube is excited by a Tesla coilor in- amplifier duction coil. transistor switch

The photocell is the sa.neone as is used in Experiment 42, The Photoelectric Effect. The light source from the Milli- ken apparatus is suitable.

Position the photocell and II,' as close to each other as possbic, leav- ing just enough room to insQrt a lighted match between them (3/4 inch or so). As- semble the rest of the equipment anslwire

it as indicated in Fig . 1. Turn the GAIN up all the way.With the

Pig. 1

To demonstrate that x rayspenetrate r' materials which stop visible light,place a sheet of 4 x 5 3000 speed Polaroid

film, still in its protectivepaper C jacket, in, contact with the end ofthe Crookes' tube. (A film pack cannot be tP3,1 used unless you are willingto sacrifice the entire pack. Any other photographic film in a light-tightpaper envelope Fig. 1 could be substituted.) Support the film DC OFFSET near zero and the DC-ACswitch on books or the table so that it doesn't set at DC, turn on the amplifier. Now move during the exposure. Figure 1 was turn the DC OFFSET up until the light a 1-minute exposure using a hand-held bulb goes on, then turn it downvery Tesla coil to excite the Croo}s' tube. carefully until the bulb is justnot quite lit. ACTIVITY Lighting a light bulb with a match photoelectrically When the circuit is adlustedproperly, a lighted match inserted between the bulb Here is a trick that you can challenge and the photocell activatesthe photocell, your friends with. it illustrates one turning on the current throughthe transis- of the many amusing and usefulapplica- tor switch to the bulb. Once the bulb tions of the photoelectric effect in real is lit, it keeps the photocell activated life. by its own light; youcan remove the match and the bulb will stay lit.

24 Activities

When you are demonstrating this effect, tell your audience that the bulb is really a candle and that it shouldn't surprise them that you can light it with a match. And of course one way to put out a candle is to moisten your fingers and pinch out the wick. When your fingers pass between the bulb and the photocell, the bulb turns off, although the filament may glow a little, just as the wick of a freshly snuffed candle does. You can also make a "candle-snuffer" from a little cone of any reasonably opaque material and use this instead of your fingers. Or you can "blow out" the bulb: it will go out obediently if you take care to remove it from in front of the photocell as you blow it out.

25 Chapter 19 The RutherfordBohr Model of the Atom Experiments

EXPERIMENT 43 Spectroscopy together, the grating separates (disperses) the different wavelengths much more, and Spectra are immensely useful. Elements can be used to make accurate measurements and compounds can be identified by exam- of wavelength. ining their spectra. Scientists learn about the structure of atoms and mole- Observing spectra cules and determine the composition of You can observe diffraction when you distant stars and nebulae by studying look at light that is reflected from a their spectra. phonograph record. Hold the record You are about to observe the spectra (diffraction grating) so that light from of a variety of sources to see how they a distance source is almost parallel to differ and to learn how to measure wave- the record's surface (Fig. 1). You will lengths of the light emitted. You will see that the grooved surface disperses be able to measure the wavelengths of the light into a spectrum. spectrum lines of hydrogen and to relate these wavelengths to the structure of the hydrogen atom.

Materials can be made to give off Fig. 1 light (can be "excited") in several dif- Use a diffraction grating to see spec- ferent ways: by heating in a flame, by tra simply by holding the grating close an electric spark between electrodes made to your eye with the lines of the grating of the material, or by an electric cur- parallel to a light source. Better yet, rent through a gas at low pressure. arrange a slit about 25 cm in front of The light emitted can be dispersed the grating, as in a pocket spectroscope. into a spectrum by either a prism or a Look through the pocket spectroscope diffraction grating. at a fluorescent light, at an ordinary In this experiment you will use a dif- (incandescent) light bulb, at mercury- fraction grating to examine light from vapor and sodium vapor street lamps, at various sources. A diffraction grating neon signs, at light from the sky (but consists of many very fine parallel don't look directly at the sun) and at a grooves on a piece of glass or plastic. flame into which various compounds are The grooves can be seen under a 400-power introduced (such as salts of sodium, microscope. potassium, strontium, barium and calcium).

In Experiment 32 (Young's experiment) You should now be able to answer some you saw how two narrow slits spread light questions about spectra. of different wavelengths through different QlWhich colors are bent (diffracted) angles, and you used the double slit to most and which least by the grating? Are make approximate measurements of the the long wavelengths diffracted more than wavelengths of light of different colors. the short, or vice-versa? The distance between the two slits was about 0.2 mm. The astance between the Q2 What different kinds of spectra can lines in a diffraction grating is about you describe? 0.002 mm. And there may be about 10,000 Make a table showing the type of grooves instead of just two. Because spectrum produced by each of the light there are more lines and they are closer sources you observed.

27 Experiments

Photographing the spectrum Then fasten the grating over the camera lens and set up the camera with You can make measurements of thewave- its lens in the same positionyour eye lengths of spectral lines froma photo- was. graph of the spectrum. Remember that the grating linesmust be An advantage of a photograph is that parallel to the source. it reveals a greater range of wavelengths than can be seen by the humaneye. Now take a photograph that shows both the scale on the meter stick and the When you hold the gratingup to your spectral lines. You may be able to take eye, the lens of your eye focusses the a single exposure for both, or you may diffracted rays to form a series of col- have to make a double exposurefirst ored images on the retina. If you put the spectrum, and then, with more light the grating in front of thecamera lens in the room, the scale. It depends on focussed on the source, the lens will the amount of light in the room. Consult produce sharp images on the film. your instructor. The spectrum of hydrogen isa particu- larly interesting one to measure because Analyzing the spectrum hydrogen is the simplest atom and its Count the number of spectral lines on the photograph. spectrum is fairly easily related toa Use a magnifying eyepiece model of its structure. In this experi- to help pick out the faint ones, ment atomic hydrogen gas in a glass tube Q3 Are there more lines than you can is excited by an electric current. see when you hold the grating up to your Set up a meter stick just behind the eye? If so, are the new lines in the tube (Fig. 2). This is a scale against visible part of the spectrum (between which to observe and measure the position red and violet) or in the ultravioletor of the spectrum lines. infrared?

Look through the grating at the glow- The angle e through which light is ing tube to locate the positions of the ben' by a grating depends on the wave- visible spectral lines against themeter length A of the light and the distance d stick. between lines on the grating.The formula is a simple one: X = d sin O. To find 0 you need to find, as shown in Figure 3, tan 0 = x/24 x is the distance of the spectral line along the

meter stick from the source, and R. is the distance from the source to the grating. Use a magnifying glass to read x

imil66OFsooltk

6077/v6 at . , s

/ ..

Fig. 2 Fig. 3

28 Experiments

from your photograph, then calculate emitted photon is the difference in tan 0. Then look up the corresponding energy between the initial and final values of eand sin e in tables. states of the atom.

To find d remember that the grating Make the assumption (which is correct) space is probably given as lines per that for all lines of the series you have inch. You must convert this to the observed (Balmer series) the final energy distance between lines in meters. One state is the same. The energies that inch is 2.54 x10-2 meters, so if there you have calculated represent the energy are 13,400 lines per inch, then d is of various excited states above this final (2.54 x10-2)1(1.34 x 10-4) = level. 1.89 x10-6 meters. Draw an energy level diagram something Calculate the values of Xfor the like the one shown here (Fig. 4). Show various spectral lines you have measured. on it the energy of the photon emitted Q4 How many of them are visible to the in transition from each of the excited eye? states to the final state.

Q5 What would you say is the shortest wavelength to which your eye is sensitive?

Q6 What is the shortest wavelength that you can measure on the photograph?

Compare your values for the wavelengths with those given in the text (Unit 5, Table 19.1 on page 69), or the more complete list given for instance in the Handbook of Chemistry and Physics. The ,63 differences between your values and the published ones should be less than the (2 6/ experimental uncertainty of your measurements. Are they? 61?0a /ID57-147e This is not all that you can do with f(641.1)1612 S AS Cr R nt the results of this experiment.

You could work out a value for the Fig. 4 Rydberg constant for hydrogen. (See Unit 5, text page 69 for the formula Q7 How much energy does an excited that defines RH.) hydrogen atom lose when it emits red light? More interesting perhaps is to calculate some of the energy levels for the excited hydrogen atom. Use Planck's constant (see Unit 5, Sec. 18.5 and Experiment 42 on the photoelectric effect) to calculate the energy of photons of various wavelengths, E = hf = hc/X, emitted when hydrogen atoms change from one state to another. The energy of the

29 Activities

NIELS PEONRS AlOPITEORI ,I9131963

6ANMARK

z 0

1042 1727 POLSKA 155

ACTIVITY Scientists on stamps

As shown above, scientists are pic- shop and assemble a display for your tured on the stamps of many countries, classroom. often being honored by other than their See also "Scientists on Stamps," in homeland. You may want to visit a stamp the Unit 4 Student Handbook.

ACTIVITY Measuring a quantum effect: "jumps" between states. This is the ionization Franck-Hertz effect. With an inexpensive thyratron 885 tube, you can demonstrate an effect that But if you keep increasing the energy, is closely related to the famous Franck- you will finally get on one which is Hertz effect. large enough to separate an electron en- tirely from its atom that is, enough to Theory ionize the atom. This energy is called According to the Rutherford-Bohr model, the ionization energy. an atom can absorb and emit energy only Now imagine a stream of electrons be- in certain amounts. If an atom is ex- ing accelerated by an electric field posed to larger quanta of energy, it will through a region of space filled with at first be very finicky, accepting only argon atoms. This is the situation in those quanta that correspond to permitted a thyratron 884 tube with its grid and

31 Activities

es the air so that it can conduct light- ear01, ,t ning. As argon ions recapture electrons, I they emit photons of ultravioletand of visible violet light. ! Anode This violet glow is evidence that the argonis in an excited state.

For theoretical purposes, theimportant point is that ionization takes !CA-Aodg place in any gas at a particular energy that is characteristic of that gas. This is an easily observed evidence ofone special i case of Bohr's postulated discrete 4,11, energy states. Fig. 1 The thyratron tube anode both connected to a scurce ofvar- Equipment iable voltage, as shown schematically in Fig. 1. As long as the energy is be- thyratron 884 tube low that needed for "jumps", the atoms octal socket to hold the tube (notessen- tial but convenient) will not accept any energy, As you in- voltmeter (0-30 volts dc) crease the accelerating voltage, the ammeter (0-100 milliamperes) electrons become energetic enough toex- potentiometer (10,000 ohms, 2 wattsor cite the atoms, as in the Franck-Hertz larger) (OR variable transformer, 0-120 effect. However, our equipment is not volts ac) sensitive enough to detect the result- power supply, capable of delivering 50- 60 ma at 200 volts dc. ing small energy absorptions. So nothing much seems to happen: the Connect the apparatus as shown in Fig. 2, electron current from cathode to anode If you use a appears to increase quite linearly with the voltage, as you would expect until the electrons get up to the ionization energy of argon. This happens at the ionization potential Vi, which isre- 0 lated to the ionizationenergy El and to the charge qe on the electronas follows: - ( r E..= q V. (1) I oi.o. As soon as electrons beginto ionize argon atoms, the current increases sharply. The arcjon is now in a different state, called ' V an ionized state, in which it conducts electric current much more easilythan before; this sudden decrease inelectrical resistance makes the thyratrontube useful as an "electronic switch" in such devices as stroboscopes. A similar process chang- Fig. 2Schematic diagram

32 Activities variable transformer to run the power sup- Equipment ply (instead of controlling its output with a potentiometer), you will need a separate A dry ripple tank 6.3-volt supply for the filament of the tiny plastic beads to reduce friction on the glass surface of the ripple tank tube. one large, flat, cylindrical magnet about two inches in diameter (to repre- Procedure sent an e.l.pha particle) five or six small flat disc magnets about With the potentiometer set for the low- an inch in diameter (to represent the deflecting charges within the atom) est available anode voltage, turn on the power and wait a few seconds for the fila- To show how alpha particles would be ment to heat. Now increase the voltage expected to behave in collisions with a by small steps. At each new voltage, call Thomson atom, represent the spread-out out to your partner the voltmeter reading. "pudding" of positive charge by a roughly Pause only long enough to permit your part- circular arrangement of the small magnets, ner to read the ammeter and to note both spaced four or five inches apart, under readings in your data table. Take data as the center of the tray, as shown in Fig. 1. rapidly as accuracy permits: your potentiometer will heat up quickly, especially at high currents. If it gets too hot to touch, turn the power off and wait for it to cool before beginning again. t Watch for the onset of the violet glow. Note in your data table the voltage at which you first observe the glow, and then note what happens to it at higher Fig. 1 voltages. The arrangement of the Plot current versus voltage, and mark magnets for a "Thomson the point on your graph where the glow Atom". first appeared, From your graph, deter- Use tape or putty to fasten the magnets mine the first ionization potential of to the underside of the glass. Put the argon. Compare your experimental value large magnet (representing the alpha with published values, such as the one in particle) down on top of the glass so the Handbook of Chemistry and Physics. that it is repelled by the small magnets, Use E. = qeVi to compute the Now fire (push) it from the edge of the energy (in electron volts and in glass toward the "atom."As long as it joules) that an electron must have in has enough momentum to reach the other order to ionize and argon atom. side, its deflection by the small magnets under the tray will be quite small never never more than a few degrees. ACTIVITY Modeling atoms with magnets For the Rutherford model, on the other Here is one easy way to demonstrate hand, gather all the small magnets into a some of the important differences between vertical stack under the center of the the Thomson "pudding" atom model and the tray, as shown in Fig. 2. Turn the stack Rutherf, nuclear model. (continued on page 36) Activities

After each definition is a dash for each letter in the the letters ma\ be transferred to the definitions by word defined11 rite eat h ord on the appropriate using the number and letter of the square in%% Inch the dashes and %%rite each !cue, in the square mai the letter appearsI or instance, if numbers 11-P, 118R, corresponding number in the diagram Vhen the and 119H in the puzzle read A1), the letter N squares are filled, the diagram \sill gii e a quotation, might be supplied for the ru.ssing letterto complete reading ft om left to right, from a \sell kiwis n scientist the is ord AND The letter N is as insquare number Black squares indicate the ends of ss ords and if there is I 3sR Therefore, going back to the definitionnum- no black square at the right side of the diagram, the bered R, the letter N should be filledinto the line ord continues on the next line When all the is ords mark d 138 in that definition The completequota- have been supplied for the definitions, the first letter tion for the crostic ma} be found in the Journal of of each word defined, reading from top to bottom Chennial Education [39, p 288, line 15 (1962) j.This (definitions A to U).Nsill spell the name of thescientist puzzle uas contributed En Marc Ellen Schaff, I'myersity of and the topic of the quotation If a ssord can he corn- Wisconsin, Milicaukee,It is(Solution uill Open, next pleted in the diagram is ithoutusing the definitions, month

1 02R 3 H4 15 E 6 B7 F809 G10 F 11S12 0

13 H 14S15 C16 F17M 18 E19 B20 F21 Q22 023 J24B25 D 26 N27 A 28 G29 N30 R31 E32 U 33 H34 J35 A36 J 37 S 38 E39 D 40 P41 Q42 Q43 E 44 E45 H46 L47 048 M49 F50 T 51 K 52 1i53 T 54 H55 D 56 F57 K 58 U59 T 60 H 61 N62 P 63 M 64 F65 L 66 1167 M68 L 69 U70 1171 E72 H

73 G74 E 75 A76 U 77 Q78 A79 F80I81 1182D83 R84 T85 F

86 0 87 K881 89 R90 H 91 J92 E93 L11 94 C95 P96 A

97 C98I 99 M 100 A101 H102 G 103 A104 I105 U106P 107Q108C

109R11011 III Q112 G113 S 114 11115C 116 D117 0 118 D 119 E

120U121 L122M123 N 124 F125 P 126 B127 G128H129 I130 13131 A

132 1133 K134 0135 F136 F 137 P 138 R139 0 140K141C 11142A143 R

144A145 E 146 F147 D148 M149 U 150 I151 E152R 153 0154 1155D

156 J157E158E

38 CHEMISTRY VOL 39 NO 10

Reprinted from Chemistry, November, 1966.

34 Activities

A.Scattering of light by molecules resulting in a shift in wavelength (two words) 96 35 144'5'8 13110090r142 103

B. Marked by disturbance 158 19 6130 2481 126

C.Polymerized tetrafluoroethylene 141 97 94 11515 108

D. Has subsided (two words) 12 14739 11625 118 155 55 82

E.Optical isomers 145 151 38 31 7418 9271 15743 11944 5

FAn acid important in protein synthesis (two cords) 136 56 146 12416 13549 7 7985 1064 20

G. Object irrationally reverenced 102 289 127 73 112

H. Nobel Prize %inner in chemistry, 1909 (last name first) - -- 72 12833 3 4570 13966 10160 11013 114 54

I.Elevated 80 150 154 129 10498

J.Lack of sympathy 23 15688 132 4 3634 91

K. A river nymph 57 87 140 133 51

L.Emotionally disturbed 934668 121 65

M. The order of marine mammalia containing whales 14863 17 67 12248 99

N. Unexploded (of a shell) . 26 61 29 123

0. A plant which obtains nutrients from air (as spelled here, sixth letter is "1") 22 47 1 134 1538 11786

P.Branch of geometry (Leonhard Euler) 13762 12595 10640

Q. To pay a second call 21 4241 107 111 77 52

R. Series of radioactive elements 143 1092 83 13889 15230

S.The objective case of they 11 37 113 14

T. Heed a call 5384 50 59

U. Error 58 120763269 149 105

OCTOSER 1996 CHEMISTRY 39

Reprinted from October 1966 Chemistry. (Answer appears at end of activities for Chapter 20.)

35 Activities

ACTIVITY Cigar box atoms

Place two or three different objects, such as a battery, a small block of wood, a bar magnet or a bail bearing, in a cigar or shoebox. Seal the box, a- have one of your fellow students try to tell you as much about the contents of your "atom" as possible, without opening the box. For example, sizes might be deter- mined by tilting the box, relative masses Fig. 2. The arrangement of the by balancing the box on a support, or magnets for a "Rutherford atom." whether or not the "atom" is magnetic by so that it repels "alpha particles" as be- checking with a compass. fore. This "nucleus of positive charge" The object of all tnis is to get a now has a much greater effect on the path feeling for what you can or cannot infer of the "alpha particle." about the structure of an atom purely on This magnet analogue is good enough the basis of secondary evidence. It may so that you can do some quantitative help you to write a report on your inves- work with the scattering relationships tigation in the form you may have used that Rutherford Investigated (see Sec. for writing a proof in plane geometry, 19.3 and Film Loop 48, Rutherford Scat- i.e., the property of the atom in one tering). Try varying the sizes of the column and your reason for asserting that magnets. Devise a launcner so that you the property is present in the other col- can control toe velocity of your pro- umn. Evidence: "a compass is deflected jectile magnets and the distance of when brought near. Conclusion: my atom closest approach. is magnetic."

ACTIVITY Another simulation of the Rutherford atom A hard rubber "potential-energy hill" is available from Stark Electronics In- 17 1. Hold the projectile velocity v struments, Ltd., Box 670, Ajax, Ontario, constant and vary the distance b (bee Canada. When you roll steel balls onto Fig. 3); then plot the scattering angle this hill, they are deflected in some- : versus b. what the same way as alpha particles are deflected away from a nucleus. The po- .,old o constant and vary the tential-energy hill is very good for

speed of the projectile, tnen plot : quantitative work such as was suggested versus v. for the magnet analogue above. 3. Try scattering hard, nonmagnetized

discs off each otner. Plot : versus b and; versus v as before. Contrast the two kinds of scattering-angle distribu- tions.

36 Film Loops

FILM LOOP 47 Thomson Model of the Atom

Before the development of the Bohr theory, a popular model for atomic structure was the "raisin pudding" model of J. J. Thomson. According to this model, the atom was supposed to be a uniform sphere of positive charge in which were embedded small negative "corpuscles" (electrons). Under certain conditions the electrons could be detached and observed separately, as in Thomion's historic experiment to measure the charge-mass ratio.

The Thomson mode. did not satisfactori- ly explain the stability of the electrons Fig. 1 and especially their arrangement in "rings," as suggested by the periodic table of the elements. In 1904 Thomson performed experiments which to him showed the possi- tended to cause the magnets to spread bility of a ring structure within the broad apart. The outward repulsion was counter- outline of the raisin-pudding model. Thom- acted by the radial magnetic field direc- son also made mathematical calculations of ted inward toward the center. When the the various arrangements of electrons in floating magnets were placed in the tub his model. of water, they came to equilibrium configurations under the combined action of In the Thomson model of the atom, the all the forces. cloud of positive charge created an elec- Thomson saw in this experiment a partial verification of his cal- tric field directed along radii, strongest culation of how electrons (raisins) might at the surface of the sphere of charge and come to equilibrium in the raisin-pudding decreasing to zero at the center. You are model of the atom. familiar with a gravitational example of such a field. The earth's downward gravi- In the film the floating magnets are tational field is strongest at the surface 3.8 cm long, supported by ping pong balls. and it decreases uniformly toward the cen- Equilibrium configurations are shown for ter of the earth. various numbers of balls, from 1 to 12. Perhaps you can interpret the patterns in For his model-of-a-model Thomson used terms of rings, as did Thomson (Fig. 2 on still another type of field--a magnetic the next page). field caused by a strong electromagnet above a tub of water. Here, too, the Thomson was unable to make an exact corre- field is radial as shown by the pattern lation with the facts of chemistry. For of iron filings sprinkled on the glass example, he knew that the eleventh elec- bottom of the tub. Thomson used vertical tron is easily removed (corresponding to magnetized steel needles to represent the sodium, the eleventh atom of the periodic electrons; these were stuck through corks table), yet his floating magnet model and floated on the surface of the water. failed to show this. Instead, the pat- The needles were oriented with like poles terns for 10, 11 and 12 floating magnets pointing upw,rd; their mutual repulsion are rather similar. Film Loops

.I.0

.40 . .

Fig. 1

cleus are deflectedthrough smaller angles. The interactionbetween the alpha particle andthe nucleus is the , electrostatic force ofrepulsion between two positively chargedbodies,Accord- ing to Coulomb's law,this force varies Fig. 2 inversely as thesquare of the distance between the alpha particleand the nu- Thomson's work with thisapparatus il- cleus. lustrates how physicaltheories may be The program used inthe computer was tested with the aid ofanalogies. He was a slight modification ofthat used in disappointed by the failureof the model the film loop "ProgramOrbit." The only to account for the detailsof atomic structure. difference is that theoperator selected A few years laterthe Rutherford an inverse-square law of model of a nuclearatom made the Thomson repulsion instead of a law ofattraction such as model obsolete, but inits day the Thomson that of gravi'y. The output of the model receivedsome support from experi- com- puter was displayedon a cathode-ray tube. ments such as those shownin the film. Points are shown atequal time intervals. Careful measurementswould show that the law of areas is valid FILM LOOP 48 RutherfordScattering for the motion of the alpha particles. Why would youex- A computerprogram was used to make pect this to be true? this film, whichshows the scattering The scatteringis shown only for parti- of alpha particlesby a nucleus in a cles which are in the near vicinityof a solid-like gold foil(Fig. 1). Notice nucleus. Drawn to scale, the that particles whichare aimed to pass nearest ad- jacent nucleus wouldbe about 500 feet close to the nucleusare scattered above the nucleus shown(if the image through large angles. Other particles from your projectoris 1 foot high) which are not aimedso close to the nu- Any alpha particle4 movingacross this large

38 Film Loops area between nuclei would move toward the but differently than for the inverse- right '4ith no observable deflection. square force law. We use the computer to tell us what the result will be if we shoot particles Working backward from the observed scat- at a nucleus. Remember that the computer tering data, Rutherford deduced that the program doesn't "knew" about Rutherford inverse-square Coulomb force law is cor- scattering. The operator has "built in" rect for all motions taking place at dis- only Newton's laws of motion and the phys- tances greater than about 10-1" m from the ical force law (inverse-square repulsive scattering center, but he found deviations force). It would be easy enough to change from Coulomb's law for closer distances. the program to test the effect of come In this way a new type of force was dis- other force law, for example, F = K/r2'1 covered, called nuclear force. Ruther- instead of F = K/r2. The scattering would ford's scattering experiment showed the be computed and &splayed on the CRT;, the size of the nucleus to be about 10-1" m, angle of each scattering deflection would which is about 1/10,000 the distance be- depend cn the distance of closest approach, tween the nuclei in solid bodies. Chapter 20 Some Ideas From Modern Physical Theories Activams

ACTIVITY Standing waves on a each stationary pattern that you get, band -saw blade check whether the back-and-forth frequency Standing waves on a ring can be shown is an integral multiple of the circular by shaking a band-saw blade with your hand. frequency. To protect your hand wrap tape around the blade for about six inches. Then gently ACTIVITY Standing waves in a wire ring shake the blade up and down until you have a feeling for the lowest vibration With the apparatus described below, you can set up circular standing waves rate wnich produces reinforcement cf the that resemble vaguely the de Broglie vibration. Then double the rate of shaking, and continue increasing the rate of wave models of certain electron orbits. snaking, watching for standing waves at Equipment all times. You should be able to main- tain five or six nodes. wire ring Project Pnysics close up power supply (with transistor switch) ACTIVITY Turntable oscillator pattarns strong permanent magnet resembling de Broglie waves Set the oscillator RANGE switch to 5-50 cyc/sec. Connect the square-wave oscil- If you set up two turntable oscilla- lator output to tne TRANSISTOR SWITCH tors and a Variac as shown in Fig. 1, you input of the power supply. can draw pictures like those show.in The output current of the oscillator is Fig. 20.4 of your Unit 5 text resembling much too small to set up visi!'e standing de Broglie waves. waves in the wiie ring. However, the oscillator current can operate the transistor switch to contra. a much larger I current from the power supply.

Place a paper disc on the turntable. Set both turntables to their lowest speeds. Before starting to draw, check the back- and-forth motion of the second turntable to be sure the pen stays on the paper. Turn both turntables on and use the Variac as a fine speed control on the second turntable. Your goal is .1.2t the pen to follow exactly tile same path eac.time the paper disc goes around. Try higher fre- 1.4A quencies of back-and-forth motion to get more wavelengths around the circle. For Fie. 1

41 Activities

The wire ring must be made of copper You can demonstrate the various modes or any non-magnetic metal. Insulated cop- of vibration to the class by setting up per magnet wire works well: twist the ends the magnet, ring and support on the plat- together and support the ring at the form of an overhead projector. Be careful twisted portion by means of a binding not to break the glass with the magnet, post, Fahnestock clip, or ring-stand especially if the frame of the projector clamp. Remove a little insulation from happens to be made of a magnetic material. each end for electrical connections. Project Physics film loop 47, Vibra- A ring 4 to 6 inches in diameter made tions of a Wire, also shows this. of 22-gauge enameled copper wire has its lowest rate of vibration at about 20 cy- cles/sec. Stiffer wire or a smaller ring will have higher characteristic vibrations which are more difficult to see.

Position the ring as shown, with a section of the wire passing between the poles of the magnet When the signal current passes through the ring, the current interacts with the magnetic field, producing alternating forces which cause the wire to vibrate. In Fig. 1, the 7-ag- netic field is vertical, and the vibrations are in the plane of the ring. You can turn the magnet so that the vibrations are perpendicular to the ring.

Because the ring is clamped at one point, it can support standing waves that have any integral number of half wavelengths. In this respect they are different from waves on a free wire ring (which is more appropriate for comparison toan atom) which are restricted to integral numbers of whole wavelengths.

When you are looking for a certain mode of vibration, position the magnet between expected nodes, The first stationary state that the ring can support in its plane is the first harmonic, having two nodes: the one at the point of support and th,a other opposite it. In the second mode, three nudes are spaced evenly around the loop, and the best position for the magnet is directly opposite the support, as shown in Fig. 1.

42 Activities

Answer to "Chem. Crostic," on page 34 of this handbook.

1r wAls MEE@ UM H Nobel Prize winner in the= r% 1909 tlast E mon IN,cwEpum name first) OSTWALD WILHELM LE Eamon TMAT H 1. Elesated RAISED :S Emu HmmmeE J.Lack of r, mpattn, DYSPATHY D no mn IN mg m K. A river nymph N %IAD L.Emotionally disturbed UPSET um in WEs EmMo M. The orderof marine mammaha containing ST EE EENEEmplu whales CE 'ACEA E ME BO @MU EOM N Unexploded (of a shell) LIVE ED A nuninEmN IN O A plant which obtains nutrients from air (as CH OMOML Mil A Y spelled here sixth letter is "i'') EPIF HITE ILK@ MEI TIOSUE P.Branch of geometry (Leonhard Euler)AFFINE PAYEE] AN!)IT CA To pas a second call REN, MMMUCK AED-ULIT Series of radioactive elements ACTINIDE YOU S. The objective case c: they THEM T.Heed a OBEY A Scattering of light bsmolecules resulting in a U.Error MISTAKE shift to w aclength (two words) RAMAN EFFECT The puzzle quotation. from "The Nuclear Atom" by Sir Ernest Rutherford, appeared in the Journal BMarked by disturbance UNQUIET of Cnenaral Edutatton (June 1962, p 288) At one time, C.Poi% mcri zed tet rafluoroeth lc nc TEFLON atoms were thought to be negative electricity scat- D. Has subsided two words) HAS ABATED tered through a sphere of positive charge However, E.Opticalisomers ENANTIOMORPHS when metals were bombarded with helium nuclei, the F. An acid important inprotein synthesis (two particles bounced back This was not in accord with words) RIBOSE NUCLEIC theory. and Rutherford described the phenomenon as " quite the most incredible event " G. Object irrationally reverenced FErISH Reprinted from Chemistry, November, 1966, p. 42.

risfAkE5 OF 711e AVM' IANICif t5e-toteE uSerut. WHICH GtfE 15 THE Atoll FEALLy LIKE 9