(Ap 8210) millikans oil drop manual

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(Ap 8210) millikans oil drop manual

  1. 1. 012-06123E Includes Teachers Notes Instruction Manual and and TypicalExperiment Results Experiment Guide for the PASCO scientific Model AP-8210 MILLIKAN OIL DROP APPARATUS
  2. 2. The exclamation point within an equilateraltriangle is intended to alert the user of thepresence of important operating and mainte-nance (servicing) instructions in the literatureaccompanying the appliance.
  3. 3. 012-06123E Millikan Oil Drop Experiment Table of Contents Section Page Copyright, Warranty, and Equipment Return ........................................................... ii Introduction .............................................................................................................. 1–2 Equipment ............................................................................................................... 3 – 4 Equipment Setup ........................................................................................................ 5 Aligning the Optical System ...................................................................................... 6 Functions of Controls ................................................................................................. 6 Adjusting and Measuring the Voltage ....................................................................... 7 Determining the Temperature of the Droplet Viewing Chamber ............................. 7 Experimental Procedure ........................................................................................... 7– 9 Computation of the Charge of an Electron ................................................................ 9 Using a Projection Microscope with the Millikan Oil Drop Apparatus ................... 10 Historical ................................................................................................................ 11–15 Maintenance Notes Cleaning .................................................................................................................... 16 Replacing the Halogen Bulb ..................................................................................... 16 Adjusting the Vertical Reticle and Viewing Scope Alignments .............................. 16 Adjusting the Horizontal Reticle Alignment ............................................................ 17 Touching Up the Black Painted Surface on the Plastic Spacer ................................ 17 Appendix A: Viscosity of Dry Air as a Function of Temperature ......................................... 19 B: Thermistor Resistance at Various Temperatures .............................................. 20 Teacher’s Guide .................................................................................................... 21–24 Technical Support ............................................................................................ Back Cover i
  4. 4. Millikan Oil Drop Experiment 012-06123E Copyright, Warranty and Equipment Return Please—Feel free to duplicate this manual subject to the copyright restrictions below.Copyright Notice Equipment ReturnThe PASCO scientific 012-06123C manual is Should the product have to be returned to PASCOcopyrighted and all rights reserved. However, scientific for any reason, notify PASCO scientific bypermission is granted to non-profit educational letter, phone, or fax BEFORE returning the product.institutions for reproduction of any part of the Upon notification, the return authorization andMillikan Oil Drop Experiment manual providing the shipping instructions will be promptly issued.reproductions are used only for their laboratories andare not sold for profit. Reproduction under any othercircumstances, without the written consent of PASCO ➤ NOTE: NO EQUIPMENT WILL BEscientific, is prohibited. ACCEPTED FOR RETURN WITHOUT AN AUTHORIZATION FROM PASCO.Limited Warranty When returning equipment for repair, the units mustPASCO scientific warrants the product to be free from be packed properly. Carriers will not accept responsi-defects in materials and workmanship for a period of one bility for damage caused by improper packing. To beyear from the date of shipment to the customer. PASCO certain the unit will not be damaged in shipment,will repair or replace, at its option, any part of the product observe the following rules:which is deemed to be defective in material orworkmanship. The warranty does not cover damage to the ➀ The packing carton must be strong enough for theproduct caused by abuse or improper use. Determination item shipped.of whether a product failure is the result of amanufacturing defect or improper use by the customer ➁ Make certain there are at least two inches ofshall be made solely by PASCO scientific. Responsibility packing material between any point on thefor the return of equipment for warranty repair belongs to apparatus and the inside walls of the carton.the customer. Equipment must be properly packed toprevent damage and shipped postage or freight prepaid. ➂ Make certain that the packing material cannot shift(Damage caused by improper packing of the equipment in the box or become compressed, allowing thefor return shipment will not be covered by the warranty.) instrument come in contact with the packingShipping costs for returning the equipment, after repair, carton.will be paid by PASCO scientific. Address: PASCO scientific 10101 Foothills Blvd. Roseville, CA 95747-7100 Phone: (916) 786-3800 FAX: (916) 786-3292Credits email: techsupp@pasco.comEditor: Sunny Bishop web: www.pasco.com ii
  5. 5. 012-06123E Millikan Oil Drop Experiment INTRODUCTIONThe electric charge carried by a particle may be calculated equal to the charge of the electron multiplied by theby measuring the force experienced by the particle in an number of molecules in a mole. Through electrolysiselectric field of known strength. Although it is relatively experiments, the faraday has been found to be 2.895 x l014easy to produce a known electric field, the force exerted electrostatic units per gram equivalent weight (moreby such a field on a particle carrying only one or several commonly expressed in the mks system as 9.625 x l07excess electrons is very small. For example, a field of coulombs per kilogram equivalent weight). Dividing the1000 volts per cm would exert a force of only 1.6 l0-9 faraday by the charge of the electron,dyne on a particle bearing one excess electron. This is a 2.895 x l014 e.s.u./gm equivalent weightforce comparable to the gravitational force on a particlewith a mass of l0-l2 (one million millionth) gram. 4.803 x l0-l0 e.s.u., 23 yields 6.025 x l0 molecules per gram equivalent weight,The success of the Millikan Oil Drop experiment depends or Avogadro’s number.on the ability to measure forces this small. The behaviorof small charged droplets of oil, having masses of onlyl0-12 gram or less, is observed in a gravitational and an EQUATION FOR CALCULATING THEelectric field. Measuring the velocity of fall of the drop in CHARGE ON A DROPair enables, with the use of Stokes’ Law, the calculation of An analysis of the forces acting on an oil droplet willthe mass of the drop. The observation of the velocity of yield the equation for the determination of the chargethe drop rising in an electric field then permits a carried by the droplet.calculation of the force on, and hence, the charge carriedby the oil drop. Figure 1 shows the forces acting on the drop when it is falling in air and has reached its terminal velocity.Although this experiment will allow one to measure the (Terminal velocity is reached in a few milliseconds for thetotal charge on a drop, it is only through an analysis of the droplets used in this experiment.) In Figure 1, vf is thedata obtained and a certain degree of experimental skill velocity of fall, k is the coefficient of friction between thethat the charge of a single electron can be determined. By air and the drop, m is the mass of the drop, and g is theselecting droplets which rise and fall slowly, one can be acceleration of gravity. Since the forces are equal andcertain that the drop has a small number of excess opposite:electrons. A number of such drops should be observedand their respective charges calculated. If the charges on mg = kvf (1)these drops are integral multiples of a certain smallestcharge, then this is a good indication of the atomic natureof electricity. However, since a different droplet has beenused for measuring each charge, there remains the kvf Eqquestion as to the effect of the drop itself on the charge.This uncertainty can be eliminated by changing the chargeon a single drop while the drop is under observation. An mg mg kvrionization source placed near the drop will accomplishthis. In fact, it is possible to change the charge on thesame drop several times. If the results of measurementson the same drop then yield charges which are integral Figure 1 Figure 2multiples of some smallest charge, then this is proof of theatomic nature of electricity. Figure 2 shows the forces acting on the drop when it is rising under the influence of an electric field. In Figure 2,The measurement of the charge of the electron also E is the electric intensity, q is the charge carried by thepermits the calculation of Avogadro’s number. The drop, and vr is the velocity of rise. Adding the forcesamount of current required to electrodeposit one gram vectorially yields:equivalent of an element on an electrode (the faraday) is 1
  6. 6. Millikan Oil Drop Experiment 012-06123E Eq = mg + kvr (2) The electric intensity is given by E = V/d, where V is the potential difference across the parallel plates separated byIn both cases there is also a small buoyant force exerted a distance d. E, V, and d are all expressed in the sameby the air on the droplet. Since the density of air is only system of units. If E is in electrostatic units, V in volts,about one-thousandth that of oil, this force may be and d in centimeters, the relationship is:neglected.Eliminating k from equations ( 1 ) and ( 2 ) and solving V (volts) E (e.s.u.) =for q yields: 300d (cm) mg (v f + vr) (9 ) q= Ev f ( 3) Substituting equations ( 7 ) and ( 8 ) into equation ( 6 ) and rearranging the terms yields:To eliminate m from equation ( 3 ), one uses theexpression for the volume of a sphere: 12 1 9η 3 32 1 v f + vr v f q = 400πd gρ x x e.s.u. 2 b V m = 4 πa 3ρ ( 4) 1 + pa 3where a is the radius of the droplet, and ρ is the density of (10 )the oil. The terms in the first set of brackets need only beTo calculate a, one employs Stokes’ Law, relating the determined once for any particular apparatus. The secondradius of a spherical body to its velocity of fall in a term is determined for each droplet, while the term in theviscous medium (with the coefficient of viscosity, η). third set of brackets is calculated for each change of charge that the drop experiences. 9η v f * a= The definitions of the symbols used, together with their 2gρ (5) proper units for use in equation ( 9 ) are***:Stokes’ Law, however, becomes incorrect when the q – charge, in e.s.u. , carried by the dropletvelocity of fall of the droplets is less than 0.1 cm/s. d – separation of the plates in the condenser in cm(Droplets having this and smaller velocities have radii, on ρ – density of oil in gm/cm3the order of 2 microns, comparable to the mean free pathof air molecules, a condition which violates one of the g – acceleration of gravity in cm/s2assumptions made in deriving Stokes’ Law.) Since the η – viscosity of air in poise ( dyne s/cm2)velocities of the droplets used in this experiment will be in b – constant, equal to 6. 17 x l0-4 (cm of Hg) (cm)the range of 0.01 to 0.001 cm/s, the viscosity must be p – barometric pressure in cm of mercury.multiplied by a correction factor. The resulting effective a – radius of the drop in cm as calculated by equationviscosity is: (5) η eff = η 1 ** vf – velocity of fall in cm/s 1 + pab (6) vr – velocity of rise in cm/s V – potential difference across the plates in voltswhere b is a constant, p is the atmospheric pressure, and ais the radius of the drop as calculated by the uncorrected Note: The accepted value for e is 4.803 x l0-l0 e.s.u.,form of Stokes’ Law, equation ( 5 ). or 1.60 x 10-19 coulombs.Substituting ηeff in equation (6) into equation (5), andthen solving for the radius a gives: *For additional information about Stokes’ Law, the student is referred to Introduction to Theoretical Physics, by L. Page (New York, Van Nostrand), Chapter 6. (7) ** A derivation may be found in The Electron by R. A. Millikan (Chicago, The University of Chicago Press),Substituting equations (4), (5), and (6) into equation (3) Chapter 5.yields: 9η 3 *** Modern calculations of q are usually conducted in SI q = 6π 3 v f + vr v f units. (See Experimental Procedure, Computation of the 2 gρ 1 + pa b ( 8) Charge of an Electron, page 7.) 2
  7. 7. 012-06123E Millikan Oil Drop Experiment EQUIPMENTIncluded equipment: plate charging switch • apparatus platform and plate charging switch (see detailed description below and on page 4) atomizer AT P E PL TO + ND ES ED OU AT GR PL A E E LT T AT P E G O A PL TO V PL – • 12 volt DC transformer for the halogen lamp • non-volatile oil (Squibb #5597 Mineral Oil, density = oil 886 kg/m3)* • atomizer E LTAG 5W 52 LB T V, N BU EN AX E VO M EM 7 03 DC O GEN C AT 6- LA 0V 50 EP PL PA HAL R 12 P/ SC O M MPE – RE SE U R U TO RAT ST MP AL JU LA TIC T EN ER TE IS + AD R M VE TH R E BL 60 Ω BE AM R E G E TA°C X1 CH SC IR IN 1. 73 4 1. 77 N W S E 1. 70 6 TO NC AG 1. U 1. 66 0 C LT EW TA Ω FO 30 1. 63 6 VO SIS 6 Ω W P 31 ER M 1. 60 4 Y RE X10 PO LA PL 32 1. 57 3 AP U 33 1. 547 4 2. 23 0 OR °C T 2. 30 34 NO 2. 16 3 MIST 2. 35 1. 521 DO 2. 11 9 36 6 20 0 49 ER 6 Ω 37 E S VE OP 21 1. 00 3 2. 05 38 BU TO O TH X10 M M L 22 1. 95 0 ST LA TA LA REWEM O LB 39 EN P 23 1. 90 0 JU ON 3. 11 9 R T DR S 3. 23 24 85 2 AD RIZ 3. 00 8 °C 3. 25 81 7 C R SC 2. 89 4 26 5 H 10 2. 79 7 EP 27 CE ON 11 2. 70 5 1. OILATU 28 UR TI 12 2. 61 0 SO ZA 29 13 2. 52 0 NI IO 14 2. 44 6 AN A 15 37 6 R 16 1 ON 17 18 LIK PP 19 OP Y * Note: We measured the density of the Squibb Mineral DR SPRA T MIL A LE ON TI SI PO CE ON 0 UR TI 21 SO ZA NI -8 IO P 0. RIU CE: N O R IO A TH U AT i 2 SO IZ 8 23 N 00 M IO μC Oil and found it to be 886kg/m3. However, the CU ET FO OPL S DR platform 12 V DC CU E FO TICL S RE densities of different lots of mineral oil may vary power adaptor slightly; therefore, for greatest precision, you should determine the density of the mineral oil you are using. Figure 3. Included equipment plate charging switch plate voltage connectors thermistor connectors halogen lamp housing convex lens filament adjustment knob droplet viewing chamber housing (vertical) lamp power jack filament adjustment knob (horizontal) ionization source lever bubble level focusing wire support rod clamping screw support rod clamping screw support rod mount support rod mount droplet focusing ring thermistor resistance table viewing scope reticle focusing ringFigure 4. Apparatus platform 3
  8. 8. Millikan Oil Drop Experiment 012-06123EComponents of platform: It is recommended that you store the • droplet viewing chamber (see details below) equipment in the original packing material. • viewing scope (30X, bright-field, erect image) with After unpacking, remove the foam insert reticle (line separation: 0.5 mm major divisions, 0.1 from the droplet viewing chamber. Store mm minor divisions), reticle focusing ring, and the plate charging switch on the velcro tabs droplet focusing ring located on the platform. • halogen lamp (12 V, 5 W halogen bulb and dichroic, infrared heat-absorbing window, horizontal and Required equipment, not included: vertical filament adjustment knobs) • high voltage, well regulated power supply that • focusing wire (for adjusting viewing scope) delivers up to 500 V DC, 10 mA minimum (for example, the PASCO SF-9585 High Voltage Power • plate voltage connectors Supply) • thermistor connectors (thermistor is mounted in the • digital multimeter (to measure voltage and resistance) bottom plate) (for example, the PASCO SB-9599A Universal Digital Multimeter) WARNING: Do not apply voltage to the • patch cords with banana plug connectors (4) (for thermistor connectors. example, the PASCO SE-9415 Banana Plug Patch Cord) • thermistor table (resistance versus temperature) • stopwatch (for example, the PASCO SE-8702A • ionization source lever (with three positions: Digital Stopwatch) Ionization ON, Ionization OFF, and Spray Droplet Additional recommended equipment: Position) • PASCO ME-8735 Large Rod Stand • bubble level • PASCO ME-8736 Steel Rods, 45 cm (2) • support rod mounts and screws (to permit mounting of platform on a PASCO ME-8735 Large Rod Stand, so lid viewing scope can be raised to a comfortable eye level) • 3 leveling feet • plate charging switch (on a 1 meter cord to prevent vibration of platform during switching activity) housingComponents of droplet viewing chamber (Figure 5) • lid • housing droplet hole cover upper capacitor • droplet hole cover • upper capacitor plate (brass) electrical spacer connector • plastic spacer (approximately 7.6 mm thick) • lower capacitor plate (brass) - thorium-232 alpha source (0.00185 microcurie) base of apparatus - electrical connection to upper capacitor plate lower capacitor • convex lens thorium-232 Note: Thorium-232 is a naturally occurring, low housing level alpha-particle emitter with a half-life of 1.41 x pins 1010 years. It is not regulated in its use and poses no hazard to the user of the PASCO Millikan Oil Drop Apparatus. Figure 5. Droplet viewing chamber 4
  9. 9. 012-06123E Millikan Oil Drop ExperimentEquipment Setup Measuring plate separation 1. Disassemble the droplet viewing chamber by liftingAdjusting the environment of the experiment room the housing straight up and then removing the upper1. Make the room as dark as possible, while allowing for capacitor plate and spacer plate. (See Figure 5.) adequate light to read the multimeter and stopwatch, Measure the thickness of the plastic spacer (which is and to record data. equal to the plate separation distance) with a micrometer. Be sure that you are not including the2. Insure that the background behind the apparatus is raised rim of the spacer in your measurement. The dark. accuracy of this measurement is important to the3. Select a location that is free of drafts and vibrations. degree of accuracy of your experimental results. Record the measurement.Adjusting the height of the platform and leveling it1. Place the apparatus on a level, solid table with the viewing scope at a height which permits the Use care when handling the brass plates experimenter to sit erect while observing the drops. If and plastic spacer to avoid scratching them. necessary to achieve the proper height, mount the apparatus on two support rods (ME-8736) on the large rod stand (ME-8735) (Figure 6).2. Using attached bubble level as a reference, level the apparatus with the leveling screws on the rod stand or All surfaces involved in the measurement the leveling feet of the platform, as is appropriate for should be clean to prevent inaccurate your setup. readings. Adjust to comfortable eye level.high voltage DC power supply support rods rod stand sturdy, non-vibrating table Figure 6. Equipment setup 5
  10. 10. Millikan Oil Drop Experiment 012-06123EAligning the Optical System brightest (in highest contrast compared to the center of the wire).Focusing the viewing scope 2. While viewing the focusing wire through the viewing1. Reassemble the plastic spacer and the top capacitor scope, turn the vertical filament adjustment knob until plate onto the lower capacitor plate. Replace the the light is brightest on the wire in the area of the housing, aligning the holes in its base with the housing reticle. pins. (See Figure 5.) 3. Return the focusing wire to its storage location on the Note: The thorium source and the platform. electrical connection on the lower capacitor plate fit into appropriately sized holes on the plastic spacer. Functions of Controls Ionization source lever2. Unscrew the focusing wire from its storage place on 1. When the lever is at the ionization OFF position, the the platform and carefully insert it into the hole in the ionization source is shielded on all sides by plastic, so center of the top capacitor plate (Figure 7). that virtually no alpha particles enter the area of the drops. 2. At the ON position, the plastic shielding is removed focusing wire and the drop area is exposed to the ionizing alpha upper capacitor particles emitted from the thorium-232. 3. At the Spray Droplet Position, the chamber is vented spacer by a small air hole that allows air to escape when oil lower capacitor drops are being introduced to the chamber. IONIZATION SOURCEFigure 7. Insertion of the focusing wire into the top ON ionization ON positioncapacitor plate SPRAY3. Connect the 12 V DC transformer to the lamp power spray droplet position DROPLET droplet viewing jack in the the halogen lamp housing and plug it into a POSITION chamber housing wall socket. ionization lever IONIZATION SOURCE Check to be sure that the transformer is the ionization OFF position IONIZATION correct voltage: 100, 117, 220, or 240 V). SOURCE: THORIUM 232 0.008 μCi4. Bring the reticle into focus by turning the reticle focusing ring. Figure 8. Ionization source lever settings5. View the focusing wire through the viewing scope, and bring the wire into sharp focus by turning the droplet focusing ring. Plate charging switch The plate charging switch has three positions: Note: Viewing will be easier for 1. TOP PLATE –: negative binding post is connected experimenters who wear glasses if the viewing to the top plate. scope is focused without using the glasses. 2. TOP PLATE +: negative binding post is connected to the bottom plate.Focusing the halogen filament 3. PLATES GROUNDED: plates are disconnected1. Adjust the horizontal filament adjustment knob. The from the high voltage supply and are electrically light is best focused when the right edge of the wire is connected. 6
  11. 11. 012-06123E Millikan Oil Drop ExperimentAdjusting and Measuring the Experimental ProcedureVoltage 1. Complete the reassembly of the droplet viewing1. Connect the high voltage DC power supply to the chamber by placing the droplet hole cover on the plate voltage connectors using banana plug patch top capacitor plate and then placing the lid on the cords and adjust to deliver about 500 V. housing. (See Figure 5.)2. Use the digital multimeter to measure the voltage delivered to the capacitor plates. Note: The droplet hole cover prevents additional droplets from entering the chamber once the experiment has started. Measure the voltage at the plate voltage connectors, not across the capacitor plates. There is a 10 megohm resistor in series with 2. Measure and record the plate voltage and the each plate to prevent electric shock. thermistor resistance (temperature). Introducing the droplets into the chamberDetermining the Temperature of the 1. Put non-volatile oil of known density into the atomizerDroplet Viewing Chamber (for example, Squibb #5597 Mineral Oil, density: 886 kg/m3).1. Connect the multimeter to the thermistor connectors and measure the resistance of the thermistor. Refer to 2. Prepare the atomizer by rapidly squeezing the bulb the Thermistor Resistance Table located on the until oil is spraying out. Insure that the tip of the platform to find the temperature of the lower brass atomizer is pointed down (90° to the shaft; see Figure plate. The measured temperature should correspond to 9). the temperature within the droplet viewing chamber. tip Although the dichroic window reflects much of the heat generated by the halogen bulb, the temperature inside the droplet shaft viewing chamber may rise after prolonged exposure to the light. Therefore, the temperature inside the droplet viewing Figure 9. Correct position of the atomizer tip chamber should be determined periodically (about every 15 minutes). 3. Move the ionization source lever to the Spray Droplet Position to allow air to escape from the chamber during the introduction of droplets into the chamber. 4. Place the nozzle of the atomizer into the hole on the lid of the droplet viewing chamber. 5. While observing through viewing scope, squeeze the atomizer bulb with one quick squeeze. Then squeeze it slowly to force the droplets through the hole in the droplet hole cover, through the droplet entry hole in the top capacitor plate, and into the space between the two capacitor plates. 6. When you see a shower of drops through the viewing scope, move the ionization source lever to the OFF position. 7

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