Physics73.1 Activity Manual

7/30/2019 Physics73.1 Activity Manual

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The Lab Manual Authors 2004Percival AlmoroAlvin BacligJohnrob BantangRoland CaballarMarilou CadatalAlberto Francia, Jr.Ma. Adoracion ManuelSheila MarcosMaricor SorianoJunie Jhon VequizoEdited by: Maricor SorianoCover Design : CorleonTorralba

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Copynght @ Lab Manual Authors 2004All rights reserved. No part of this publication maybe reproduced or transmitted in any form or by anymeans, including photocopy, without written penrrissionfrom the Lab Manual Authors


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Prepared byPercival AlmoroSheila MarcosJvlaricor Soriano

Juni'e John Yequizo(Physics 73.lLab Manual Authors 2004)

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Tempetaflrre Measurement ........ 1Ltneat Expansion .. ..1,3Specific Heat and Heat of Fusion....... .........r.. :.ZlHeat Engine ........ .........41Spectral Fingeqprinting .......77


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Name Section Date Received by.

Prelab Quiz : Temperature MeasurementRead the manual and answer the following questions:1. What is temperature?

2. Define thermal equilibrium.

3. r07hat ate the quantities that will be calculated in the datasheet? What will be measured?

4. If the time constant of a certain thermometet is found to be 10 seconds, how long must you waitin otder to get a reliable temperatue reading after it is put in contact with a bodyi

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Physics 73.{

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73,1

ObjectivesAt the end of this activity, you shotrld be able to:

o Identi& the physical conditions needed fot temperaturemeasufement.. Opetate diffetent temperature seflsors.

Measure the thetrnal time eonstant of different temperature sensors.Know the minimum waiting time before reading measutements takenwith temperature sensors.

aa

lntroductionThe clinical thernometer is no doubt the first temperatufe sensor most of ushave experienced using. After applying it under the tongue or the armpit weare asked to v/ait a few minutes before reading the tempe.rature. We associatethe word tefipemtaft with the measure of warrnth or coldness of an object ora systern. But unlike mass, length and ".ne which can be measued bycomparing with a standard mass, length and time unig temperature cannot bemea$uted as ditectly. Instead; we measlue some obsetvable reaction ofmatedals to heat or cold. These teactions ate called tbetrnometric pnpcrtieg, Thrsactivity will introduce you to different temperatrre sensors iong with thetherrnometric ptoperties they exploit and will show that thete is a mioimumwaiting time before the reading of a temperature sensor becomes teliable.TheoWhen two objectS, olle warm and one cold, are placed in contact with oneanotler, the warmer-one cools while the coolet one warms up. Specificallnwe say they are tn themal nnua. Eventually, there comes a poini wheo nomore changes occur and they will feel the same. The two rr. th* said to bern tbcrrnal eq*ihbitaz.We 9an define tenprat*n thetefore as the quantity whichis the same for both systems h themal cortect when they are in therm2l


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Figure 1. When two objects areplaced in thermal contact andachieve thermal equilibrium,the two objects are said to havethe same temperature.

Temperature Measurement Physics 73.{equilibrium.Say now we have three types of objects, A, Band C. If A and B are put in contact andachieve thermal equilibdum, afld tf B and Care also put in contact and also setde tothermal equilibdum then A and C are inthermal equilibrium. This observation is calledthe ryrotb law of therruodlnamics. It follows thatA, B and C have the same temperature.It takes time for thermal equiJibrium to beachieved. Heat fiom the hotter obiect willtransfer to the cooler one such that their cofirmofl temperature is differentftom their original temperatures. If a thermometer is placed in thermalcofltact with a hot body what we actually read is the temperature of thethermometet itself'! By the act of putting a thermometer in touch with thebody we have changed its temperature. It is thetefore necessary that heat

dtawn or withdrawn by or from the thermal sensor is minimal such that itdoes not change the temperature of the obiect significantlyWe have to wait until thermal equilibrium is established befote we carlreliably read the temperature. An indicator of how fast a thermal sensor canachieve thermal equilibdum with an object being measured is given by thethermal time constant of the seflsor, T.Consider a temperature sensor initially at temperaflre T, which is placed inthermal contact at time t=0 rvith an object that is maintained at a constaflttemperature. Aftet sufficient time, the temperature seflsor will have a finalteading Tr. At any time d the sensor has a reading T(t). The differencebetween TlandT(t) is gqvenby AT,r.e

47=Tf-T(t).As time progresses, the diffetence between the sensor reading and its finalreading vanishes. If the seflsor is a first order, linear device, the rate ofchange of the difference can be assumed to be proporilonal Io the difference

(1)

Q)

of initial and final temperatute, i.e.d(Ln : _kLTdt

There is a negative sign because AT decreases -in time. The left side ofequation (2) has dimensions tenperaturef tima. To keep the dimensions thesame as on the dght side, the constant , must have dimensions of / /time.Let k = / f t, where rhas units of time. Thus,

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Physlcs 73.'l Temperature MeaSurementd(^r)

Solving fot T(t) we obt2in,T(t) : Ti + AT,(l- exp(a/Q)

where ATo is the difference between the final and initialreadings. petails of the derivation are given in the Appendix.)A plot of Fquation (4) is shown in Figure 2 below.

= -1lrtt (3)(4)

temperatute

Temperature

T(t) = Ti+ (0.632)ATo

t:0 timeFigure 2. tf we set time t=0 as the time when the temperature sensor was placed in thermalontact wit}l an object kept at constant temperature, then the ptot shows the temperaturereading of the sensor in time. Ti is the initial temperature reading of the sensor, Tr is its finalreading, T(t) is the temperature'at time t, ATo is the difference between the initiat and flnatreading, AT is the differenee between the final temperature and the temperature reading attime t; and r is the thermal time constant, or the time it takes for the lensor to reach 63.2% ofits final temperature.As / approaches infinity, the exponential term vanishes and we are left with

T(*) - Ti + AT" = Ti* Tr-7, = T,which is the final temperature.!7hen t = t, Equation (4) becomes

T(r):Ti+ AT,(0.63). (6)we now have a physical intelpretation for the thermal time constant. AfteroJle tlTe coflstant, the sensor will have a reading equal to its initial readingplus 0.632 times the difference between irs inital and final readin{

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Temperature Measurementwe get temperature values as shown in Table 1'

Tabte 1, Temperature reading after every t (thermal time conttant) interval'Time Temperature

T Tr+ (0.63)ATo2r T,+ (0.86)ATo3t T, + (0.95)AT.4t, T, + (0.98)AT.5r, Tr+ (0.99)ATo

Physics 73.{

Figure 3. Glassthermometer

The time constant of a particular sensof, thetefore, can be used to specifyhow long one must *aitio get a reliable tempefatwe reading. Dep^ending onth" ,..rrir.y required, it is tmmon engineering_practice to wait ftom three(3) to n"" (s; time constants before recording the output of a temperatureSCNSOI.

Temperature SensorsTemperature can be felt but canflot be measuted directly. Instead we obsetvethe tlaction of matedals to heat and cold and measure the degree of reacd'on'Fot example, metals expand when heated' We can then'measrre how muchthe expan^sion is ,nd "qoat it to a cettain tempelatufe. These and othermeasurable reactions to Leat are,called thermometic pruperties and they are."pf"l .a - the tempetature sensors we will use in this activity'Glass ThermometersMost substaflces expafld when heated. Merc.ury or cslored alcohol in thecapillary of glass thermometers - expand lileady with increasing,"irp"rr*r". yo* can make your ownthermometer by evacuating a thin glasstube and fil1ng half of it rviih colored alcohol (mercury is toxic and hard toprocure). Calibration is done using the zetoth law of thermodynamics' Seal'trr" tou" and dip the end in ice. Due to thermal expansion, the liquid d d'.'Matk the tevel Lf fiquid and label it 0"C. Nex! dip it in b9fug watet, wait forthe liquid to stop expandiog and mark the level of liquid as 100'c. Since the.rp#ioo of &e [d,id is lt.rr with temperatue,_livi{e the.space betweenthe two matks irrto too equalty-spaced divisions. You have )ust made youro-1r c.tri* (centigrade)^r.rt".^In accotdance with the zetoth law oftfr".-"ay"r-i.r, if tf," thermometer is placed in therrnal contact with a bodyand the il"gtlr of the liqurd inside the capillary teaches the same level as thatmatked fot 100"C, then that obiect is at 100"C'

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Physics 73.{ Temperature MeasurementThermocouplesWhen two wires made from different metalsare welded together and exposed to a hot otcold teg'ion, electrical cuffent will flowthrough the wire. This is knorvn as the Seebeckffcct. Sensors exploiting this phenomenon areknown as thermocouples. When connectedto a calibrated read-out device, the systembecomes 2 .ligjtal thermometer.Thermistors

. A semiconductor is a material with propertiesi b"t*""r, that of a conductor and an insulatotI but when it is heated, it becomes rnofe, conducting. They. can be formed into various' shapes such as beads or rods. Their resistance' varies nonlineady with tbmperature such thatwhen a constant current is passed through them,voltage across the sensor will vary. If used in thismanner, the semiconductor device is known as athermistor. The stainless steel temperature probe to be used in thisexperiment has a thermistor inside it.

MaterialsThetmometer, digi12| thetmometer (thermocouple probe), dmer, stainlesssteel temperature probe and PC with interface> stove, large beaker or caldero,cold and tap water.

ProcedureCAUTION : Steam from boiling water reaches ,lO0"G. Toprotect your hand from scalding use mittens when holdingsensors near steam sources .

1. Fill a beaker or caldero with water 3/o f,;ll and boil. I(eep the waterboiling throughout the activity.

2- Take the glass thermometer (take note if it has mercury or coloredalcohol inSide it) and write down its curent temperature reading, T, .3. \We assume that the water is at 100'C and that the final temperarwereading will be apptoximately 100'C. Compute the following valuesand tabulate in the worksheet.

Figure 4. Digital thermometerwith a thermocouple as sensor.

Figure 5. stainless steeltemperature probe



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Temperature Measuremert PhyclCs 73.:{. AT" = 100"C -Tio (0.632) AT.o T(t) = Ti + (0.632) AT"

Dip the glass thetmornetef iust above the surface of the boiling watef(the steatn itself is around 100"C!) and measute the time it takes fotthe reading to teach T(t). If the mouth of the beaker is small, it isenough to place the sensor neat the water sutface.Cool down the thetmometer by dipping it in tap water and then incold watet. Repeat steps 2 to 4 sevetal times.Repeat the same procedute usihg: (1) a ghss thetmometet made witha different liquid, and @ a stainless steel temPetature ptobeintedaced to a computer.

7. Obserye how fast a digital thetmometet tegistets a teadingbdngirig a thetmocouple neat the steam sorrtce. Is it possiblerneasure its time cotstant with a timet?

Will the thennal dme constant rcmaio the same if the initiattempetatute varies? Design an experiment to check your hypothesis.The dedvation of the thermal time constant assumes that the seflsofteacts tineady with temperature. Howwet, some matedals do notOne way to check if the matetial has a linrcar ot nonlineatthermometdc proPerty is to compare its tesporse to heating andcooling. Get the time constant of t thermometer ftom a hottemperahue (e.g. boilin$ to ambient temperatLue. Dip the sensotinto boiling watef for a few minutes then lift it out of tle watet. Howdo the "ooliog and heating time constants compare? If they are thesame, then the sersor behaves lineatly. If not, then the system isnonlineat.Perfonn the above experiment using the stainless steel temperatueprobe interfaced to a comPutet, fit an exponentid function and getthe thermal time constant ftom the plot' Does the plot fot heatingand cooliog agee with an exponential function? Ate the thetmal timecoflst nts the same fot heating and cooliog?

4.

5.

6.

byto

Suggested Extension ExPeriments

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Plrysics 73.{ Temperature Measurement

Get

where C is a.coflstaflt. Taking the exponential of both sides of the equationwe obtain

AI = exp(-t I t + C) = e-"' e' (42)The conotant eQ .*nbe foirnd from initial conditions. If we talie I = 0 to bethe time iust before the sensor was placed in contact with the object, thetemperature difference is equal to the iflitial temperatute diffetence betweenthe body and the thennometet,

AppendixSolution to the,differential equation given by

d(Lr) =*kLTdtd(LD : _dtLTr$.(L\ - 1 dtJ Lr -J-;

Ih(Afl = -t-+ CT

Af@=Tf-T(0):\-7,=AToThus,

ATP)= AT"={sc - scand ,: AT = AT"expe/ r)Substituting Equation (1) for AT,we get

fr- f(0 : AToexp(t/r).E:ptessing Tlts ATu + fl'ftom Equation (5) we get

ATo + n- T(g : ATuexp(-t/r).Solving forT(t) we obtzir,

:'Tr * AT,(1- exp(-t/

(41)

(43)

(44)

(As)

(47)

(46)

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Tem,perature Measurement Physics 73.'l



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Name Section Date Score

Groupmates

Worksheeil Temperatu re Measu rementsData Table 1. Thermal sensor:Ti 'aTo =100oC - Ti (0.632) ATo T(c) Time Constant,r

Best value of T :Data Tabt 2. Thermal sensot:

Best value of r :Data Tabt 3. Thermal sensor

Ti ATo =100oC . Ti (0.632) aTo T(") Time Constant,r

Ti aTo =1000c -Ti (0.632) aTo rk) Time Constant,t

Best valie of T :

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Worksheetl Ternperature Measurement Pftysics 73.{Sample calculations:

1. Based on yoru obserwations, which amoflg the themrat sbnsors provide the fastest reading?Which is the slowest?i*Ir4 - W-'*(r"/,n,/uow+it -

2. Is the final temperature reading of a sensot egual to the actual temperature of the obiect?w- AM ^,w&O', vhilbil ,*b ,Wy @ ry

3. If the tepperatute of a system changes in time (e.g oven, engine, body, foom, atmosphete) howvzould you know which is the best temperatute scllsot to usc?L*,r,5n 1> 37'sv

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Name Section Date Received by

Prelab Quiz : Linear ExpansionRead the manual and answer the following questions:1. Give the definition of the linear expansion.coefficient.

2. Why is an ohmmeter needed to be connected actoss the linear expansion sehrp?

3. nflhere will the temperature sensor be inseted?

4. What is the pqpose of the steam generator in the setup?



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FrEn QuI= UiiG[ altloll Iffirtl

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Physics 73.1

ObjectivesAt the end of this activity, you should be able to:o Determine. the coefficient of expansion of various rnetalsmicrometer screw type lineat expansion apparatus

o Compare the coefficieot of linear expansion of metals.lntroductionMost solids especially metals expand when heated. The stuck lid of a iat ofmayonnaise will open easily if the lid is soaked in hot water for severalseconds. Bddges are usually constructed such that they have gaps to allow fotexpansion during hot weather. If the material is fonned into a rod or bar, itwill noticeably expand ot contmct along its tength proportional to the changein temperatue. If we are building structures or parts that will be exposed toheat it will certainly expand, so it is important to know the coefficient ofIinear expansion of the materials used to determine how much allowanceshould be given for expansion.TheoryDifferent metals expand at different rates. The fractionzl rate of lengthchange, Al/L"where /L is the change in length and L"is the initial length ofthe object, is proportional to the change io temperatute AT = Tr-T,where7] is the initial temperature when the length was Loand [is the femperpturewhen the rod has expanded to the length L.+.AI-. That is,

M-u=aLT (1)The constant a is called the coeficient of linear expansionand is different for different solids. Table 1 showsvalues of a for different metals.

Table 1. Coefficient of [inear expansion of some metals.

usmg

Metal/r\_t['c./

l0 -sdx

Aluminum 2.30Brass 1.90

Copper 1.70Lead 2.80



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Figure 1. Linear Expansion SetuP.

Linear Expansion Physics 73.1

Materials and Setup

Shown in Figure 1 ate the labeled pafts of the Linear Expansion setup. (1)Linear expansion appatatus; (2) Stove; (3) Stearn generator; (4) Labthermometer or drgrtal thermometer; (5) Copper and aluminum tods' Theend of the appafatus labeled B has a mictometet scfew for lengthmeasurement.In addition, you will need : Metetstick, ohmmetel to check fot electricalconnectivity, Rags and PotholdetsProcedure

GAUTION Io STEAM REAGHES {OO"C. BE CAREFUL NOT TO SGALDYOUR HANDS. USE THE POTHOLDERS PROVIDED,o AVOID GETTING ELEGTRONIG EQUIPMENT WET'Measure the length of the metal rod up to the flearest 0.1 mm andrecord in yout worksheet.Insert a cork through one end of the rod' Avoid bending the todby rotating the cork instead of pushing it in' Insert the rod in thefackGt and plasg the othet cork on the othet end of the jacket'Ensute that the rod ends protrude out of the corks as shown inFigure 2.

1.

2.

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Physics 73.{ Linear Expansion

Figure 2. Rod must protrude out of the cork stopper.Notice that there ate three perpendicular pipes along the length ofthe jacket. Place the jacket in its frame holder with the middle,bigger pipe in the upright position. Insert the thermometer.intothe middle pipe (glass tube if lab thermometer, thermocoupleprobe if digrtal thermometer) through a cotk stopper until thetemperature probe tip is close to the rod but not touching it.Tighten the suppott screril's on each end of the frame just enoughto hold the jacket in place.Fill the steam generator t'wo-thirds firll of water. On one end ofthe jacket is a smaller tube pointing up. Connect a rubbei tubefrom this end to the steam generator. On the other end is anothertube pointing down. This is for &aining water out of the jacket.Place a beaket or cup below this pipe.Attach an ohrnmeter across the screw ends A and B of the jacket.Screw B is the one with the mictometer scale. Turn and fix sctewA and tum screw B until electrical contact is established. This isseen as a deflection in the needle of the ohrnmeter or a beep if it isset to check for connectivity. Record the initial reading of themicrometer scfew. Then turn screw B a few turns back to allowfor explrnsion. Take an initial readirtg of the temperature.Place the steam generator ofl top of the stove. Turn on the stoveand allow the watet to boil. If steam escapes from the steamgeneratot pl"g th" leaky regions with damp clothe. Wait around 5to 10 minutes fot the steam to evenly heat the rod inside thejacket. Lightly feel the jacket to determine if it is heated up to itsend.When the tod is sufficiently heated e90"C) turn the mictometerscrew forward until electtical cortact is established and tecord themicrometer rcading. At the same time tecord the temperature.Allow the setup to cool and repeat the procedure for anothermetql rod. Compute fot the linear expansion coefficient andznswef the questions in yout worksheet

3.

4.

5.

6.

7.

8.

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Linear Expansion Physlcs 73.1

Sug gestedr Extension ProjectsDoes the linear expansion coefficient depend on the initialtempetature? Find out by tepeating the expedment while the todis initially hottet than the ambient temPerature.What happens when a tod shaped into a dng is heated? Will thehole in the middle shdnk or Srow latge? Find out by heating adng and measudng the innet and outet diametet.r Will the a. change u,ith different tod lengths? Find out, byheating tods of diffetent lengths.

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Name Section Date Score

Groupmates

Worksheet: Linear ExpansionData Table 1

Sample calculations: (o

BeSt estimate of a I0Petcent Esot

Type of Rod !nitiallength lnit.Microme-terReading

FlnatMlcrome.terReadlng

AL lnit.Temp.

FinalTemp.

AT (Imeasured caccepted

trt

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Worksheet: Linear ExPansion Dhyslcs 7!.1Questions:7. Car, you conclude that

$rhy?[rt, * =,{oTl-othe rod is no longet expanding when it reaches a coflstant tempetature?4 bT=O,' LL='o

&cb(?


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Name SecUon Date Received by

PreLab:quiz : Specific Heat and Heat of FusionRbad the manual and answer the following questions:.{r. Specific heatl. Differentiate between heat capacity and,specific heat.

Descdbe a simple ptocedue to measure the specific heat of, an unknown metal. W'hatquantities must be determined to measure specifiChea-t?

A 50-g chunk of metal is,heated to 200oC and then &opped into'1 beaker containing 4009of watet mltially at20oC: If dre final eq-uilitdum tq4pemtue of the.miied systern,ii,'22.4"Cfindthespecrfigleaqofthemetal,,, .,, t ,. , ' ', . j

B. Heat of fusion4. Why does ice melt?

5. What do you call the energy needed to change the phase of a substance?

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Heat arrd Heat of, Fusion

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Physics 73,{

aa

ObjectivesAt the end of this activity, you should be able to:

Determine the specifi.c heats of aluminum, copPer and lead.Determine the heat of fusion of ice.

lntroductionMolecules of various matedals have diffetent weights and sizes. Hence, theamount of energy tequired to speed up of slow down those molecules willdepend on the type of material. This activity is important because mosdthermal devices are made up of diffetent materials and working substances.I(nowledge of how varj.ous matedals respond to heat will be critical in thedesign of thetmal systems and also fot the safety of the oPerators.The energy transferred as a consequence of temPeratufe difference or phasechange is called heat. When studying thermal systems involving differentmrt.iiulr, it is useful to define quantities of heat in'tetms of a particularplocess. When it.involves temPefatute difference, the quantity called speciftheat drs(ngushes one material from anothet. W.hen it involves phase change,ofle can make a distinction between matedals using the quantities called beatof fusion and heat of [email protected] A of this activity is about calculation ofspecific heats of pute metals (aluminum, coPPef and lead) and pat B is aboutcalculation of heat of fusion of ice.TheoryPart A. Specific heatThe quantity of heat tequked to taise the temPelatute of a g1vefl mass of asubstanqe by some amount varieS from one substance to anothet. Forexample, the heat required to raise the temperature of 1 g 9f water by -1Celsius degree is 1 calode (definition of 1 cal). Fot 1 g of carbon it is only

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Specific Heat and Heat of Fusion Physics 73.{0.72 cal. T)he beat caPaci\ C of any substance is defined as the amount of heatneeded to mise the tempetature of that substance by one Celsius degtee. Init, is often tnorytxeful to work with gecific heat c defined as heatcapacity per unit mass:

c- Ctnh,eat capacitltmass (1)

Q)

Ftom the definition of heat cap:aCity, we can exptess the heat enetgy Qtmnsferted between a system of mass m and its surroundings fot atemperature change AT as Q=CLT +mcLTSpecific heats of all matedals vary somewhat with temperatqte. If thetemperature intervals are small, the temperature vatiation can be ignored andr can be treated as a corstaflt. For example, the specific heat of water (1 cal/gCo) varies by only one percent ftom 0o to 100oC at atrnospheric pressure.When specific heats are fneasured, one also finds that the amount of heatneeded to taise the temperature of a substance depends on other conditionsof the measurement. In general, measurements made at constant pressure(designated as c) are different from those measured at constant volume(designated as c). Table 1 gives the specific heat of some solid elements.Note that these values are given at room temperature and atrnospheriipfessure:

Table t. Specific heat of some solids at 25"C and at atmospheric pressure.Substance Soecific heat. c^ (ca!/q G')Aluminum o.215c 0.0924Lead 0.0305

For part A of this acdiity, the tec\nique that will be used to rneasure specificheat is to simply heat the substance to some reference temperatwe, place it ina vessel containiag watet of known rnas$.and temperature, and measure thefinal temperature after equilibrium is teached. For simplicity, the referencetemperature will be the boiling,:teihp.etahrre o-f:,watet. Sihce'a hegligiSleamouflt of mechanical work is done in this process, the law of conservationof enetgy implies that the heat that leaves tlie warmer body (of unknown r)must equal the heat that entets the'watet. Assuming no heat is lost tosurrounding air, the heat lost by the metal sample must equal the heat gainedby the water:

fr,t*r*nr\r*-T**)=*;c*u,Q'iuia'-r*,)1. (3)Ftom equation (3) the specific heat of-the sa-Fle c*ocanbe determined.Part B. Heat of fusion

A substance usually undergoes a change in tempentute when heat is

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IttitrfifitIlnIB!r*:r]Bt,

Physlcs 73.1 lSpecific Heat and Heat of Fusiontransferted between the substance'and its surroundings. Thete ate situations,however, where the flow does not result in a change in temperatue. Thisoccurs whenever the substance undergoes phase change. Some commonphase changes are solid to liquid (metting) and liquid to gas @"ili"g).All suchphase changes involve a change in intemal energy. The energy requited tochange phase is called the heat of trantfomation. The heat tequire d p to changeth-e phase of a given frrass n of a pute substance from solid to liquid (ot visevetsa) is given by O=mL,I (4)where I, is called latent heat of fusioa $ridden heat) ,of the substance. Fotwatef, \= lg.l cal/5.On the molecular lwel, phase change is a result of the reaffangemeflt ormolecules when heat is added or temoyed from a substarice. At the meltingpoint of a solid, the amplitude of vibrations of the atoms about theiiequilibtium position becomes latge enough to overcome the attractive fotcesbinding them together. The heat required to totally melt a $ven mass of solidis equal to the work requfued to break the intermolecular bonds and totransform the phase ftom the otdered solid structure to the disordered hquidphase.

For pafi B of this activity, warm water and ice will constitute the system ofinterest and will be isolated ftom its sonounding ^i, by the calodmetet.Assuming no heat is lost to surtoundings, energy is exchanged only betweenthe wamr water and the ice. Sorhe of the energy absorbed by iCe will be usedto change its phase ftom solid to liquid and somc to raise its tempetature.The amount of heat absorbed by ice as it melts and then as it reaches finalequilibrium temperature must equal the quantity of heat released by thu waknwatet as it cools do"wn to final dquilibtium tempemture. Theotetical meltingtemperature of ice at OoC will be usedfor simplicity. Mathematically,

M*L r . *rlt#)t *- o"c) = " *l#)e* - ro*,)On the Ieft side of equatron (5),.th" firct term reptesents the heat tequired tochange phase ftp4,"sfr{,i& to:liquid water and the second term representsthe heat teqrrired to taise its tempemtue ftom zeto to final equilibriumterqxratute. The tight side of equation (5) tepresents the heat removed ftomthe miiially vanD,lr&tr in the Styrofoam cup. Ftom equation (5) the Iatentheat of'fosioo I*of ice can be determinedEquipment and MaterialsGlass th:rmometer or digital thetmometer with thetmocouple ptobe, 2Stytofoa# cups, stove aridboiler

(s)

IIII(IaIaa

aII

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Specific Heat and Heat of Fusion Physics 73,{Part A: cold water, metal samples. (aluminum, copPer, lead), stringsPat B: glass or digrtal thetmometer, ice chunks, warm water

ProcedureGAUTION: Steam from boiling water reaches {OO"G. Toprotect your hand from scalding use mittens when holdingsensors near steam sources.Part A. Specific he4t

1. Measure the combined mass of the Styrofoamthermonretet. Record all. data in Data Table 1Figure 1 shows the labeled expedmental setup..op, ,rrh the glassof yout worksheet.

3.

4.

Figure L Experimental setup for determination of specific heat. (a) Styr.9t9.apcuips and glass thermometer, (b) metal samples, (c) stove and boiler, (d) digitalthermometer and Probe, (e) strings2. Measure the masses of the aluminum, copper and lead samples.Record these masses in Data Table 1.

Attach a thread to each of the metal samples and suspend each of thesampies in boiling'water. Measute the actual boiling temperature ofwater. Allow few minutes fot the samples to heat thoroughly'Fill the calorimetet approximat ely '/z fi.rll of cool watet - use enoughwater to easily cover afly one of the metal samples.

5. Measute T,,,,,,on temperature of the cool water, and record youtmeasutement in Data Table 1.

6. Immediately following youf tempefatufe measurement, temove themetal sample from the boiling water, quickly wipe it dry, and then-25- @ Lab Manual Authors 2004


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Physics 73.{ Specific Heat and Heat of Fusionplace it in the cool water in the calorimeter. Use the secondStyrofoam cup as a cover as shown in Figure 1.Stir the water with your thermometer and record Tpt, the highesttemperature attained by the water as it comes into thermalequilibrium with the metal sample.Immediately after the t kirg the temperature, measure and recordM,o,o, the total mass of the calodmetet, water and metal sample.Determine mass of water.

9. Use equation 3 and collected data to solve the specific heats of thesamples.

10. Compare calculated value with standard value.Part B: Heat of fusion

1. Measure the combined mass of the Styrofoam cups and the glassthetmometer. Recotd ail, data in Data Table 2. Figure 2 showsexperimental setup and indicated are the equipment and materials.2. Fill one Styrofoam cup approximately t/z frrll with warm water. Thetemperatue of the water should be about 40-50'C. Use the second

Stytofoarrr: cup as a covef as shown in Figure 2.Detetmine the mass of the warm watet.Measute the initial tempetatue of the system right before the chunksof ice are added to the watet-styrofoam calorimetet system.

Figure 2. Experimental setup for determination of heat of fusion of ice. (a)Styrofoam cups and glass thermometer, (b) digita! thermometer and probe, (c)stove and boiler, (d) ice bucket

5. Add small chunts.of ice, wiping the excess water right before adding.

7,

8.

3.4.

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Specific Heat and Heat of Fusion P rysics 73.1Stir continuously with the thermometet until each chunk of ice melts.When the system has reached thermal equilibrium, i.e., tempetatute isalmost coflstant, measure the final temPeratwe.Measute the mass of the system to determine mass of ice.Calculate heat of fusion of ice using equation 5.Compare calculated value with standard value.

Detenlle the specific heats of sevetal water coolants.Determine the specific heats of coins and other metal alloys.Compare with a pure metal.

o Cdmpare heats of fdsion, of ice, ftozen mixtuqe of water andsalt and frozet soft cola {dnk.o Measure heat of vapoization of watef and other substances,i.e., heat tequired pet unit mess to change phase from liquidto gas (and vise versa).


6.

7.8.9.

Suggested Extension ExPerimentsoo

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worksheet: specific Heat and Heat of FusionData Table 1. Specific heat Aluminum Copper LeadUL.PD ailq glass tnermometgr

tttt,I;Cl lrsalnplsrPrrta.ure gI SISI|TI Ii2X;21Finat temperature of systemfl",rrrrttttitt InaSS Of SlStGlTl M16131

YrdalBt lltweterePrrt,ttlu ttEat C,I mglalPercent errorSample calculations:

Questions: Specific heat. 1. Describe the heat exchanges involved in prrt A of this activity

2. Based onheat?

yout calculations, which metal has the greatest specificI rf et;fic hu4 + rw@ hd,.l t* +t* scot,Lheat? The least specificfu *^r

3. Compare the specific heat of the meals with that of water.

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Worksheetu Specific Heat and Heat of Fusion Physics 73.14. What ate the possible sources of error in the experiment? In which direction do they affectthe measured values of specific heat?

irr,H'l e,oca+t *(*. c rr ),*.-^ = t t'lt) ut r, * ktaT),r. u n

41. { A lo4,Jthr-o!bata Table 2. Hqat of fusiontl.L %.vw'-A

Sample calculations:

Mass of cups and glass thermometerMass of warm waterffitem (warm water, cups and thermometer)Finaltemperature of system (including the melted ice)Mass of iceHeat of fusion of icePercent eiror

Questions: Heat of fusionl. Descdbe the heat exchanges'involved,in Part B of this activity.

2. What are the possible sotrces of erot in the expedment? Ho* can they be avoided otminimized?

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PreLab Quiz : Gas LawsRead the manual and answet the following questions:1. W.hat is an ideal gas?

Z. What is the equation of state for an ideal gas? What ate some of its applications?

3. In the Boyle,s law expetiment, how is tempetatute kept coastaflt?

4. In the Chadeq't w exp.tm.nt,ho; is pressure kept constant?

(Continued at the back)



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Prelab Quiz: Gas Laws5. lfhy is it necessary to use absolute tefnPefatue when using the equation of state?

Physics 73.{

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Physics 73.1

ObjectivesAt the end of this activity, you should be able to:. Expedmentally determine the relationship betrveen the ptesswe and

volume of'a sample of at atconstant temperature.'. Experimentally detennine the relationship between the temperatuteand volume of a sample of ai at constant pressufe.

lntroductionAn ifual gas is a low-ptesslue (ow.densiry) gas with tempetature sufficiendyhrgh that it does not condense into liquid. Most gases at roijm temPemtureand atrnospheric pressure behave as ideal gases. The equation of state for anideal gas relates the pressure P, volume Z, numbet of moles of gas n, andabsolute temperatrue Tand is given by

PV=nRT.where Ris the universal gas constant G=8.3t J/mol).Thete ue many applications to the equation of state:1. Monitoring of a gas system that operates undet coflstant conditions,knowing three of the fout vadables allows you to detetmine thefourth vadable;

Determination of the density of gas;Convetsion of a volume of gas under constaflt pressure andtemperature to number of moles;Determination of partial pressrrre of a known amoust of gas io , gmmixtue; andDerivations of the idividual gas hws: Boyle's law (relates P md V),Clrarle5'5 law (telates V md.T), Gay-Lussac's law (dates P nd 1)and Avogadro's Iaw (telates V md n).

(1)

2.3.

4.

5.

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Gas Laws Physics 73.1Part A of the activity is about Boyleis lavr where the relationship betweenpressure and volume of a confined gas will be investigated. Gas ptessute willbe monitored using a seflsor intetfaced to a computer as the volurne of gas isdecteased. Patt B is about Chatles's law where the volume of a gas will bemonitoted as the tempetature is deceased

PrinciplesPart A. Boyle's lawBoyle's law states that for a fixed tempetature the product of the pressureand volume of an ideal gas is a coflstant:

PV = mnttantIn this experiment, the pressute inside a sytinge is measuted using a gaspressure sensor that is intetfaced to a computet wbjle volume of the syringeis being decreased at constant temperature. The temperatute can be assumedconsant if the vdlume change is not abrupt and the system is allowed toequilibrate which takes about a few seconds.Part B. Charles's lawChatles's law states that at constant pressrue, the temperatue of an ideal gasis proportional to its volume:

V =iTwhere r is the constarit of ptopottionality and T is exptessed in lQbin.Absolute temperatute is used in the equation of state because it is alwayspositive. In.this expedment, the volume of air confined in the gas lawapparatl,ts is monitored a s its tempetature is decreased and pressure heldconstant. The gas ptesslue is kept constarit by positioning the gas lawappafatus hotizontallar. In this position, the ptessure on the apPafatus pistonis the atrnospheric pressure which can be assumed constant at equilibriumconditions.MaterialsPatt A: syringe, gas pressure-sensor, Vemier Labproru intetface module andcomputer with I .oggerPtoru softwarePart B: Pascoru Heat engine/gas law apparatus (see Appendix A), airchamber can and rubbet tubing, cold watet bath ahd crushed ice, glassthermometer or digital thermometet with thermocouple probe

Q)

p)



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Physics 73.{ Gas LawsProcedurePart A. Boyle's lawFigure 1 shows the components of the Boyle's law expedment.

Figure'1. Boyle's law experiment: (A) syringe, (B) gas pressure sensor,and (C) computer interface module.

GAUTIONT Do not use forcesyringe to the gas pressureforce can cause air leakage. when connectingthesensor. Any excessive

1.2.3.

!7ith the sydnge piston raised to its uppermost position, connect thesyringe to the gas pressrue sensor.Connect the gas pressure sensor to the Vernier interface module and,lr"dy, the module to the computer.Setup the LoggetPro software in the computer to record pressrrre asa function volume. A shortcut of LoggerPro is foirnd at computeldesktop. Once the gas pressure ,*ror is deteited LoggirProautomatically launches the .Boyle's law ptogtam.You ate now teady to collect data fot volume and pressute. Click the"'Collect" button" Then click the "Keep" button to enter the syringepiston position. Tlpe in the actual piston position of your syringe.You will obsewe data points appeadng on Table window and Gr@hwindow. {Vary ,the syringe position by regular intervals Gry OtI-). Thaindicated unit in the spinge is in cubic centimeter (1ml,=1cc). Runserretal trials to fimiliaize with the operation before 1ss61ding yourfinal data.Gathet about 8-10 data points.Copy ptessute and volume data from Table uhdayto Table 1 in yourwotksheeL Shut dourn fig somputer

4.

5.

6.7.

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Gas Laws Physics 73.18. Calculate reciptocal of volume.9. Ptot pressure versus volume in Graph 1.10.Ptot pressute versus reciptocal of volume in Graph 2.

Patt B. Chades's lawFigure 2 shows the configuration of the setup for Chatles's law experiment.

l,iillitl

lllttl

-dl

Figure 2. Gonfiguration for Charles's law experiment. (A) Gas law apparatus, (B) airchimber can, (C) water bath, (D) thermometer and probe' (E) crushed ice bucket.

CAUTION : Do not use force when connecting therubber tubing to the gas law apparatus' Any excessiveforce can cause air leakage.Connect the gas law apparatus to the rubber tubing and ak chambercafl.Turn the gas law apparatus on its side. In this configutation pressweon the piston at equilibrium can be assumed constaflt.Place the air chamber in a containet of hot water, near boiling point.Aftet the chamber equilibtates to the temPerature, record thetemperature and the height of the piston in Table 2 in yourworksheet.

5. Add small chunks of ice to the containet and record the piston hetghtfot a given temperatue reading. Gathet 8-10 data points for a teguiartemperature interval.

1.

2.

3.4.



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Physlcs 73.1 Gas Laws6. Calculate the gas volumes for various pistondiameter of piston is 32.5mm).7. PIot volume versus temperature in Graph 3 in yout worksheet.

(FIint:

Suggested Extension ExperimentsVeri$r relationship of gas pressule and temperature. Investigate forethyl alcohol and acetone.Verify conditions whete ideal gas law does not hold.

Appendix : The Gas Law Apparatuso The Gas Law Apparatus is used for quantitative expedmentsinvolving the ideal gas law and investigations of a working heatenglne. Figure A1 shows a photo of the zppara;tus and the ait

chamber with their pats indicated.The Gas Law appryatus is designed with two pressure ports withquick-connect fittings for connecting to the air chamber tubing andto the pressure sensor.Do not use force when connecting to the pressure ports and whenregulating the. shut-off valves.Loosen the piston-holding thumb screw when not in use.

Figure Al. Gas Law Apparatus. A) pnessure ports; B) air chamber can; C)rubber tubing with clamp; D) shuf-off valves; and E) piston-troldingthumbscrew



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Gas Laws

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Name Section Date ScoreGroupmates

Worksheet: Gas LawsTable 1. Boyle's law

Pressure Volume I iTotume

Graph 2. P - l/V plotraph 1.P- Vplot

: Questions: Boyle's lawWhat is the shape of the resulting curve in Gtaph 1?what is the shape of the resulting curve in Gtaph z? canyou fit a trend line?can you say that the system obeys Boyle's law? Exprain your answer.

-i',

From the P versus t / v grryhwhat o'ther information can you obtain?

7.

2.

3.

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Worksheet! Gas Laws Physlcs 73.t1Table 2. Charles's law

Temperature (K) HeiEht (mm) Volume (mm")

Graph 3;.V- f plot

Questions: Charles's law1. What is the shape of the tesulting cuwe in GaPh 3? Cao you fit a tend line?

2. Canyou say that the system obeys Chads's fugr? EsPhin yoru anslret.

3. Frcm &reVlT gFaph wt t otherinfomretbn gaqyou'obtaio?

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I,dlpr'nj,Name Section Date Received by

PreLab Qu,iz : Heat EngineRe"ith" mautalafld answer the following questions:I !7hat is a heat eagine? Wh,et are sotne important applications of heat eagi4esP

2. What is the difference betureen:a stearn eogae and an intemal combrrstion engine?

3. V/hat is an isobadc ptocess?

4. Wbat is an adiatrlralicptocess?


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Prelah Quiz: Heal Engfne Phyoics 73,1(Contiaued at the back)

ltrUhat is the work done in li{ting an obiect of mass rzz through a vertir,al herght }?

How is the thermodynamic work done obtained ftom a pressute-volume diaqfan of an enginecycle?

lfhat ate possible reasons fot loss of ertergy in real heat engines?

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Physics 73.1

aa

lntroduction


Describe qualitatively the cyclical operation of a heat engine.Determine the net thermodynamic wotk for an engine cycle byfiodirg the enclosed arca. of a ptessure-volume (P-V) dtagam.Vedfy experimentally that the usefirl rnechanical wotk done in [ftinga mass is equal to the net thermodynamic wotk done during a cycle.

A heat engine is a device that converts thermal energy into other usefulforms of enetgy, such as mechanical or electrical erergjr. Internal combustionengines, which ptopel automobiles and airctaft, extract heat ftom e burnin8fuel and coovetr a ftaction-of this enetgy ioto mechanical eoergy. Powe(plaots geoerate dectticity by converting the potential energy stored in nucleatfuels into thetmal eflergy. This thermal eoergy is, in tum, converted into themechanicel energjr used to ddve a4 electtical generator. AII heat enginesoperate in the same pdnciple. In effecg a heat engine catdes a wotkingsubstance through a cydic ptocess involving hot aqd cold teservoirs. Figwe 1shgws a schematic representation of a heat engine. In the operation of anyheat enging a quantity of hatQrris extracted ftom the hot teservoir, somemechaaical wotkZis done, and some heat;Q.is tej,ected to a cold resetvoir.

FigtrE l. Schsnffc represerilafion of a heat engine-

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Heat engine Physlcs 73.{In the case of a steam engine, the working substance is water. The water iscaried through a cycle in which it evapotates into steam in a boiler, thesteam expands against a piston, and then it condenses and tetums to itsinitial state. In th" ""." Of * intemal combustion engine, the working;:'H::;fffi:::'#i[n,,,*, wi], be used,o simu,a,e andinvestigate the different pfocesses in an engine cyc1e. Hot and cold waterbaths will constitute the heat tesewoirs and mechanical wotk is obsewed asup and down motions of the aPParatus piston. The heat engine appamtus will.orr,r"rt the thermal enetgy ftom a hot watet bath into usefrrl mechanicalenefgy in lifting a test obi""t. The pressures and volumes of the wotkingsubstance (confined ait) dudng specific stages of the cycle will be recordedand used to calculate the thermodynamic wotk duting ooe cycle,Thermodynamic work will then compated to mechanical work done dutingthe opetation.

Principles of operationHeat Engine Apparatus

Figure 2. Experimental setup. (a) Heat engine apparatus, (l) gas.prgssu.re sensor, (c) airch-amber can, 1d1 Vernier t-ilpr6 Interface module, (e) cold and hot water baths, (f) testobject.Figue 2 shows the experimental setup. The heat engine appafatus is attachedto two flexible ru\ber tubings. One is connected to a gas Pfessrile sensor andan interface module and thi othet to an ak chamber can. The ait chambercan be alternately placed in the cold and hot water baths. The apparatus has apiston with platfonn attached to it for lifting a test object'

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I

Physlcs 73,{ lleat englneIf the ait chamber is placed in the hot bath, tlre temperatuie of ait trappedinside will inctease and consequentty its volume will also inctease. This effectwill be observed as a tise io tt. position of the object resting on theplatform. If the object is temoved, the platform will rise a bit more as thepressrre inside the appatatus decreases a bit. Finally, if the air chambet isplaced in the cold bath, the temperah:rre and volume of ait will decrease. Thiscauses the piston to descend to its otiginal position completing one cycle.The pressure in the confined air is consant when the mass on the'platform isnot changed. Any process that akes place while the mass is not changed caf,be considered ircbaric (constant pressure). There is no heat flow when mass isbeing added ot being removed ftom the platfo;rm. The ptocess of -ete1yadding ot removing masses can be considered an adiabatic (no heat flow)

lPfocess.

Equipment and MaterialsPascom{ Heat engine apparatus, rubber tubings, air chamber can, VernietLabProrM intetface module and computer, Logger Ptoru software. (Figure 2shows labeled setup.)Hot and cold water baths, test object and weighing scale.

ProcedureA. PREDTCTIONS, OBSERVATIONS & ANIALYSES1. Fill the cold bath with watet and crushed ice and the hot bath withpreheated watet at about 60-70'C.

2. Set the initial height of the piston by mising it a few centimetersabove tlre bottom of the cylinder before fitti"g in the rubber tubinginto the apparatus. For the test obiect, limit the mass to about 100-2009 to avoid air leakage. Record in Table 1.3. Fami]7anze first with the operation of the heat engine. Place the canaltemrtely in the hot and cold baths. Place and then remove the testobject on the platfomr. Make some initial observations.4. The following table is a summary of the various stages of the engine

cycle..Iodicated ate the positions of the test object on the platformand locations of the air chamber can in the water baths for the fourstages of the cycle A, B, C and D. Figue 3 shorus the photogtaphs ofthe vadous sages of the engine cycle.



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Heat engine Physics 73.{Table 1. Four stages of the engine cyclel A - D. lndicated are the status of test object on platformand location of air chamber can during the various stages,

Figure 3. Photographs ofttrc various stags ofthe engine cycle-

Start the engfile cycle at stage A with the can placed m the cold bathand with the test object removed ftom the platfom. Prcdict andobserve what happens dudng transitions between vad.ous stages ofthe cycle. Determine which ftaasitioos are approximately isobadc aadwhich are approximately rdiabaic processes.

A4lb-o oca90-a Acia.Wcinaic,"1^"fu'fi'ciW*vq

5.

A B c DTest object onplatform Removed Added Added RemovedLocation of airchamber can Cold bath Cold bath Hot bath Hot bath

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Physics 73.{ Heat engineIn this activity, wdte all predictions with shot explanations, actualobsenrations and identified process involved dudng ftansitions in Table2. (It is all tight if yout predictions and observations diffet. No pointdeductions.) ,

TransitionA+B6. Predict: What should happen to the height of the platform when youadd a test object? Should the height inctease or dectease? Explain the- basis of your predictiion.s..' Observe: Add the test object on the pladorm. What happens to theheight when y6u ackled tlre imiss to the platform?.Identif the pnocess: Approximately what thermodynarnic process'

Ttwtsition Bx7. Predict: What should happen'to fhe heigft of ph*orm urhen.tle aiichember is placed',iri thg hot ;bath? ,E{plr- the basis of'ptedlctipirs.bbr.lrr", viitn tle *"dr ;dil on ttie plaifo"ni, ptr"L the irn in thi hotbath. !(hit happens to tbe heigt* of the platform when you placed itin the hot bath? (fhis is the engine power stokb.)Identifr the process: Approxirnately what thermodynamic processhappens dqflng this transition?

Transition C-+D8. Ptedict: While the can is still in the hot bath, what should happen tothe height of the platform if the test object is removed? Explain yourpredictions.

Observe: !7hile the can is still in the hot bath, remove the test object.What happens to the height of the platform?Identi!, the process: Approximately what therrnodynamic processh^ppens during this transition?

Transition D+A9. Predict \What will happeh to the height of the platform when the canis placed back to the cold bath? Explain yoru ptedictions.Observe: This ttansition s6mFletes the cycle. Place the can back tothe cold bath. What happens to the height of platform? Is it the sameas the initial height at point A or has some ait leaked out?Identify the process: Apptoximately what thermod;mamic processhappens during this transition?

your

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l{eat engine Physics 7alB. MEASUREMENTS OF VOLUME AND PRESSUREIn this activity, the engine cycle will be studied quantitatively. Fot all tbcstages of the engine cycle, the volume of air will be determined based oo rheheight of the platform. The pressure of the confined air will be obtainedusing a gas pressure sensor connected to the apparatus and intedaced to acomputef.GAUTION ! Do not use force when connecting therubber tubing to the gas pressure sensor. Anyexcessive force can cause air leakage.

1. Calculate the volurne of ak using the piston height h *re specifiedvalue of the inner diameter d of the apparatus, and the equationV = rdb for each stage of the cycle. Record in Table 3.Connect the gas pressure sensor to the Vernier LabPtom computetintetface -odrrl..-S"ttrp the LoggerPtoru software to record ptos*.as a finnction volume. A shortcut of LoggerPro is found at computerdesktop. Once the gas pressute sensor is detected LoggetPtoautomatically launches the Boyle's law progtam:For each of the stages of the engine cycle, click on the "Collect''button. Then click the "Keep" button to enter the calculated volume.You will observe data points appeating on Table uindou ard, GrEbwindov.

4. Obtaifl the values of the pressrue fot each stage of the cycle. Copythe generated pressue data from the softrxrate to Table 3C. P.V DIAGRAM AI{D CALCULATTONS OF WORK

Plot a P-V diagtam in Gmph 1 ptovided in the lTotksheet.Calcu]ate the net thetmodynamic wotk done ftom the atea enclosedby the P-V curse. Device yorr own ways to measure the eacloseduer of the cycle. Recotd calculations in Table 4.Compare the thetmodynamic work with the mechanical wotk doneobtained ',sing the equation V/ = mgh where m ts the combined massof the obiect and the piston (specified on the apparatus), g is theaccderadon due to gta"ity and / is change in the piston height

2.

3.

1.2.

3.

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Physics 73.1 Heat engine

Suggested Extension Experimentso Construct and test an engine protoq?e.o Investigate reftigerator cycle.

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Heat engine Physics 73.1

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Name Seclion Date Score

Groupmates

Worksheet: HeaNngineTable 1.

lnitialheightMass of test object + platformDiameter of engine apparatus

Table 2.

Table 3.

Engine cycle Predictions and shortexolanations Observations Procesi involvedTransition At B

Transition B) C

Transition C)D

Transition D)A

A B c DHeightVolumePressure


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Worksheet: Heat Engine Physics 73.{Graph 1. P-V diagram

Table 4. Net thermodynamic workThermodynamic work (area enclosed by curve)Mechanicalwork (total mass x gravity x height)

Percentage errorQuestions:1. Explain why tansitions from,Z + B and ftom C+ D xe approximatgly adiabatic.

2. Explain why tansitions from B -+ C and ftom D + A are apptoximately_isobanc.

3. Theoretically, the pressure of the confined air should be the same after the system cools backto its original tefnperature. Why?

4. \ilhat are the possible soruces of eror in the calculations of thermodynamic and mechanicalwork?



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Name 5ection Date Received by

Prelab Quiz : Photoelectric EffectRead die manual and answer the following questions:1. Describe the photoelecric effect phenomenon.

2. Descdbe the na11se 6f light as a paticle.

i 3. ltr(hat will be the maximum kinetic energJr of the electtons ejected in a photoelectric effect setupfor a meal (work function = 2.0 eY) illuminated by a monochtomatic Iight of wavelength 450nm?

4. tU7hat are the pdn.rprl emission lines in the speitrum of metcury?



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Prelab Quiz: Photoelectric Effect Physics 73.{

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Physics 73.{

aaa


Obsewe the particle property of light.Detetmine the work fi,rnction of a materialDetermine Planck's constant

lntroductionThe photoelectric'effect is a phenomenon that ptovides ofle of the sftoflgestexperimental dernoastrations of the particle nature of light. When lightimpinges upon a metal surface, electtons ate liberated ftom the surface toptoduce a photoelecttic cutentThe dual wave-particle nature of light has been a plzzle for scientists in the1800s. When photoelectdc effect was observed around 1900, it forcedscientists to fotmulate the physics of quantum mechanics versus classicalphysics that could not cleady explain this phenomenon.

TheoryThe classical vrave model of light predicts that as the intensity of the incidentlight is increased, the arnplitude and thus the energy of the wave wouldincrease. Ftom this .point of view, more eoergetic photoelecttons should beemitted when light stdkes a metal sutf,ace in photoelectric effect. Also timelag should be olsewed as electtons are emitted condnuously. But thesepredictions ftom the classical model were absolutely contrary with actualobservations in photoelectric emission.Ahert Einstein attempted to explain the photoelectric effect. He firstassumed that light, according to Max Planck's published Law of Radiation,occrrrs in discrete quantities called photons having eoergy dependent on the



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Photoelectric Effect Physics 73.1ftequency, i.e.

E=buwh91e "E is eneggy, vis the ftequency of radiation, and lt is a fundamentalconstant of nature. The cdnstaot D is known as Planck's constant.When light energy reaches the metal in photoelectdc effecg this energy maybe enough to eject an electton out of its orbit around the patent atom, andmove it towxds tl'te sutface. If this elecfton is ftom an atom at the surface ofthe metd, t cettain amount of energy is still tequited to liberate the electronfrom the metal surface. T'his energy, known as the wotk function IYo, is ameasute of the minimum amount of *ork needed to escape electrons ftomthe metal sutface. The escaped electrons in tutn gain a maximum kineticelrergy RE*,Each metal have a chatacteristic wotk function that does not change fotdiffetent ftequencies of light. The alkali meals (ithium, sodium, potassium,rubidium, etc.) have the smallest work functions which ate in the tange ofvisible light. Fot other metals, such as those used in the early expetiments(coppet, nickel, zinc) the work functions are in the uluaviolet. -Any ftequency

(1)

of lisht with insufficient enerw to overcome the work function of a metalcamot ffil-uce cutrent to flow throush the exoerimental cfucuit. Workfunction values of some metals ate listed in the appendix.The photoelectric phenomenon can be cleady uoderstood from the point ofview of the neuz quantum model Wh* of light with sufficientstrike a

The kinetic."@ ciepencient onthe wavelength or ftequency of the light butindependent of its intensity. The increasedintensity would only 'inctease thephotoelectric curtent -- the numbet ofelectrons emitted ftom the metal surface.Finstein applied Planck's theory and."pl"ifl.d the photoelectdc effect in tetmsof the quantum modd using the law ofconsetvadon ofenergy:E: bu = I(E* +W, a)where KE*is the maximum kinetic eretgyof the emittedphotoelectrons, and V"isthethe work firnction of the metal. E is theerergy supplied by the photoo

The kinetic erergy KE*of the electrons emitted ftom the metal surface canbe detetmined by apptying a minimutn reyerse potential to stop the

Figure 1. Experimental circuit of tfiephotoelectric e,trect erpedmenlffitlr enough ene_rgt/ imparEd .byphotons on ttre photoctr&ode boverEonre its wort function, anelectnon is liberabd. lf OEopposing Yoltage is sufficient tostop lfte photoelectrcns frornreaching Ote inode, rpphotoelectric current ls mecurcd.

Reverse Bias Voltage

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Physics 73.1 Photoelectric Effectphotoelectrons, thus teducing the photoelectric cufient to zero (Figure 1).Relating kinetic eflergy to stopping potential V gpves the equation:KE* =Vewhere e is the chatge of the electron.Therefore usrng Eiostern's equaton,

Ve +WoSolving for V, the. eguation becomes:

(3)

(s)=(L\,-(%\\e) \' )Plotting v vs. V for different ftequenciesof light, we will get a graph similar toFigue 2. By putting a linear fiq weobain a, V intercept equal ,to -lYo/e arrd aslope of lt/e. ,ln'experimental value forthe work functioa of the metal of thephotocathode, as well as Planck'sgbnstant 4 with the accepted value of4.735 x 10-1s eV.s, can be obtained fromthe slope and y-intetcept of the ploL

Flgure 2. A typical graph of therclationshlp of frcquency qndstopping potential in photoelectriceffect experiment

d,EooqbD.soro{oa

The Photoelectric SetupThe setup illusftated in Figuqe 3 is composed of a mercury light source wheqea,difftaction grating is attached io ftont of it to sptead the light into a discretespectrum. Anh/e apparatus'attached to the base of the light source can beadfusted at the spectal color undet investigation. Tmosmission filtets ateptovided to tegulatq intensity of light that passes through the opening of theh/e apparetusMaterialsMetcury liglt soutcg diftaction gratin& h/e appatatus, tansmission filtes;greeo and yellow filters, digital muhimeter and alligator clips.

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Photoelectric Effect Physics.73.1Figure 3. The PhotoelectricSetup. The diffraction gratingspreads the light from themercury lamp into its discretespectra. The h/e apparatus canbe adjusted such that onlY onecolor from the spectra imPingeson the opening. The reverse biasvoltage can be measured usingthe voltmeter. Different filters areprovided to modulate.the lightstriking the opening of the h/eapparatus. These filters can beattached on the white reflectivemask of the h/e apparatus.

Part ABlock the mercury light and measrue the voltage reading. Recotd as"dark voltage" in yout wotk sheet. In succeqding voltage ls2dingsalways subttact the dark voltage value.Adiust the h/e r1pplrtzrws zuch that only the fitst ordet yellow spectmlline falls on the opening. Place the yellow filte1 o1. the whitereflective mask. The.output voltage is sensitive to the alignment ofthe h/e .apparatus with the incident light. Slowly t'uist the appafatusabout its base until maximum voltage teading is obseived.

1.

2.EfA:-ffiransmission F-,ti""I

lFigure 4. Filters. Thetransmission filter isused to regulate theintensity of lightstriking the opening ofthe h/e appafiltus. Tofilter only green andyellow light green andyellow filters are used.

3. Position the transmission filter over the colored filtet so that the lightpasses thrdugh tfg section marked *70uo/i':

4. . Recotd the voltage teading in Table 1 of yout wotksheet.5. Press and release the discharge button on the side of the h/i

apparatus and obserrre apptoximalely hour much time is tequired totechatge the:instrument to the maximum volage.

6. .Repeat steps 3 to 4 fot 80, 60, 40 afld 207o uansmissions.

ProcedureNOTE :- Avoid touching the suilaces of the diffraction grating andfilters.

Perform this activiQl in a darkened environment tominimize ambient tight on photoserisitive equipment.



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Physics 7&{ Photoelectric Effect7 . Repeat the procedute,usirg the gteen band of the spectrum.Part B1. choose one color in the first order spectrum and adiust the h/eapparatus so that only that color falls ,rpon the opening of the h/eappafatus.

Recotd the volt4ge re.lding in Table 2 of your worksheet.Repeat the ptocess fot atleast five colors in the spectrum.Move to the second order and repeat thc process.

2.'3.

4,

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Photoelectrlc Efiect,, :,t.:': , i ,

Physics 73.i

AppendixTable l. Wavelengths of the spGtral llnes of Mercury

Table 2. Wor* Functions for Some tletals

*Hondbr)ok of Chemistry ond Physics

Color Waoele*gh (nm)

Yellot 578rGrcen 546

Blue 436Violet 405

Ulttaviolet 365

Metal WorkFunaba (eV)Aluminum 4.08

Carhou 4.81Coppet 4.7

Gcrld 5.1lton 4.5I-ead 4.14

Nic&et 5.01Silvs 4.73

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Name Seclion Date Score

Groupmates

Worksheet: Photoelectric EffectData Table 1. Part ADarkVoltage:

How did you arrive at the best estimated values of the stopping potential and chatge time? Defendby reporing the uncertainties of the values.

What can be deduced from the tesults of Part A?

Color #1 %Transmission Best Estimate of StoppingPotential(V) Best Estimate of Charge Time(s)

Yellow

10080406020

Golor #2 %Transmission Best Estimate of StoppingPotential(V) Best Estimate of Charge Time(s)

Green

10080406020



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Workshee* Photoelectrac Effect Physics 73.{Data Tabl 2. Pat B

First Order' Colors Wavelength (nm) Frequency (xl0r4 Hz) Stopping Potential(V)

Second OrderColors Wavelength (nm) Frequency (xlol'Hz1 Stopping Potential(V)

Ho.ur did you arive at the best estimated values of the stoPping potential? Defend by tepotting theuncertainties of the values.

!7hat can be dpduced ftom the tesults of Part B?



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Physics 73.1 Worksheet: Photoelectric Effect

Sample calculations:

Questions:1' Plot the.stopping potentia! versus ftequency for the fust and second order specftal lines andderive a hneat relationship. Detetmine the slope and y-intercept. \)7hat can be implied from theplot?

2. Get the actual values of. b arrd lZo. Determine what metal is used in the photocathode.

3. What is the implication of having a higher or lower value for the wotk function?

a' 11 thg SaPh, intelpolate the value of the frequency when stopping potential is zero. What is thesignificance of this value?



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Workshee$ Photoeleotrlc Effect Physies 73.15. P.redict tle range of wavelengths of light that will induce photoelectdc emission in the mqtalsutface of the setup. Defend yout prediction.

6. Describe the effect on the stoppmg potentid of passing different arnounts o6 "o1o1sd lightthgugh the vadabie transmission filter. Relate this to the maximum energy of thephotoelectrons.

7. Does the intensity of the colored light have an effect on the chatging time? Explain.

8. Does thig experiment support L wuye or a qrufltum model of light based on your lab results?Explain.

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PreLab Quiz : The SpectrometerRead the manual and answer the questions below:1. Nflhat are the indications that the diffraction gtating is pelpendicular with the berm of light,fromthe collimatot slit?

2. Based on the image below what is the angular reading?

3. How are pamllel beams ensured in aligning the spectrometer?


(Continued at the back)

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Prelab Quiz: The Spectrometer Physics 73.1

4. In the image below identify the parts found in the student spectrometet and speci$ what they arefor.

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Physics 73.1

ObjectivesAt the end of this activity, you should be able to:o Identi& the basic parts of a spectrometef.

Align a student spectrometer.Determine the numbet of lines per millimeter in a ruled transmissiongrating.Compute the Jight wavelength fiom angle in a specftometer.Relate observed color to electromagnetic radiation wavelength

I

lntroductionlhe .snec{ometer is one of the most vitar apparatus in optics and modemphysics. Through this device many signifrcani theodes ,bort the structureand behavior of atoms wele conceived and varidated. To this day thespecftometer remains_an important tool for diverse fields such as asttonomy(e.g. for detertnining the composition of objects in outet space), industry (e.j.for-materials testing), remote sensing (e.g. for classi$ring ,"a .ro-ru"* ilrr?and sea cover), and biology (e.g. for classi$,ing ma{nant versus h"ealthycells). In this expedment, we'shall learn the proper ,rr^g. of a student versionof the spectrometer as well as the -rr.".rrtoi. oltight.-Light DiffractionTake a compact disc (cD), turn the readabre side up to a source of light andone sees a spectrum of color reflected from the disc. The colors"alwaysoccur in a certain sequeflce - red, orange, yellow, green, blue, indigo, violet _that an acronym helps to remember trr.* - Roy G BIV. This cornmoflobservation is one of the strongest evidence that light has wave properties.The cD behaves like an optical element known ^ri dfirrrtion grating-and itsaction is the same as that of a double slit in young's DJrbl. Sliioxp"eriment.Iike water waves passing tkough two smal opening, in a dpple tank,

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The Spectrometer Physics 73.{monochromatic light (single-colored light) passing thtough sevetal equally-spaced slits will diffrort ot sptead out from each s]it and, because ofconstructive and destructive intetfetence of light waves, will fotm patterns ofdatk and bright bands when viewed on a screen placed a short distance ftomthe slits. rWhen white light is passed thtough the slits it breaks up into itsconstituent colors, each colot intedering constructively at diffetent positions.Consider trxzo light tays emerging ftom a pair of grooves A and B in thedifftaction Satiflg as shown in Figure 1. 0, actual observation, the ray offuht will fan out tn r rardril dfuection. Hete we considet one emergent rayamong the many.) The gtooves ate sepatated by a distance d.

Figure 1. Two rays emerging from two grooves in a diffraction grating.Sirppose the path of light with wavelerlgth X difftacts or bends by an angle 0from A. Light emerging ftom B vrill interfere constructively with A if thepath diffetence PD between the two rays is an integer numbet ofwavelengths. That is, the condition for constructive intetference is

PD=dsinO=m).where m is the order of tbe difraction and is integet-valued (m =0, t1, t2,...).The case when m=0 is when the light is undifftacted, ot the emergent light isstaight through. Positive and negative values of m refer to diffracted light atopposite sides of the zetoth-order ot dirssl image. Equation (1) is vital inthat the wavelength of the difftacted Iight can be determined if we know theslit or gtoove separation d and the angle of diffraction 0.

The SpectrometerThe specttometer is a device for obsen ing and quantifying specua. Thestudent spectrometet has thtee main parts: (1) a collimator, (2) a light-dispersing element, and (3) a telescope which can rotate about a base that hasangle matkings. Shown in Figute 2 is a student spectrometer.

(1)

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Physics 73.1 The Spectrometer

tersinq element

Gollimator

Figure 2. Student spectrometer.The collimator is a tube with an adjustable slit and lens. The slit ispointed at the light souce and is used to ptoduce a naflow beam of light.The lens is used to shape the beam such that it will emefge as patallel rays ztthe exit end of the collimatot tube.The disperstng eiement is used to cliffract light into itsconstituent colors. In this activity, we will be using affansmission difftaction Satiflg. A difftaction Sating(Figute 3) is a ftansparent optical element whose surface isetched with very fine, equally-spaced glooves numbedng

from 60 to 600 lines pet millimeter. The diffraction gatingis held in place by an adiustable holdet on a black circulatplate which can be rotated and locked in place. Thenumber of grooves or lines pet millimeter is called thegrating constant D. The inverse of the Satiflg coristant gresbetween grooves which is the distance d tn Equation (1). Thus

The plate is attached to a platforrn base that has two viewing windows actosseach other which allows the reading of a main angulat scale ftom 0 to 360".Adiacent to the main scale is a Verniet scale which allows for finet angularreading.Attached to the main scale is a telescope that can rotate about the axis of theplatform. Light dispersed by the gating i.s scanned thtough the telescopeeyepiece which has a ctosshait such that the slit image can be aligned with it.Since the main angular scale rnoves with the telescope the angle of diffractionof light can be measuted from its position.

Figure 3.Transmissiongrating.the,sepatation

Q)1D



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The Spectrometer Physics 73.{Diffraction Angle MeasurementAngle measurement is a crucial part of spectrometer usage. From Equation(1) knowing the angle of diftraction and the slit separation allows us tocalculate the wavelength of the difftactid light. The circular main scale onthe spectrometer platform is graduated ftom 0 to 360o in steps of th degree.Against the movable main angle scale is a fixed vernier scale with 30divisions. To read the angle, fust locate the angle the zero point of thevemier scale aligns with. If the zero point is benvelen two lines, ore the lbweroire. Record this'angle. Then find the mark on the Vernier scale that alignsvlth any line on the main scale. Add this to the eadiet angle marking. Eachnotch on the Vernier scale corresponds to a minute of arc.Fot example, the readiug in Figure 4 below.is 90o 14'.

/ 100

100 9590o on Main Scale

Figure 4. Example of angular main and vernier scale showing a reading of 90o l4'.

If the apertures are parallel to the slit, diffracted light may be viewed oneithet side of the normal image. The specra lying flearest the direct imageon either side are called the first (1st) order while those successively moredistant are called second order, third order, and so on.MaterialsStudent spectromete\ arty 3


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Physics.73.l The Spectrometer

ProcedureCAUTION: 'Never stare directly at the high powermercury lamp. Prolonged exposure of the eye toultraviolet light can cause retinat damage. Usesunglasses to shield your eyes.A. Alignment

Peer through the telescope and slide the eyepiece in or out until thecrosshairs are cleady seen.Focus the telescop e tt far objects such as those outside the windowto bdng the telescope focus at approximately infinity. For thePASCO SP-9268 Student Spectrometer, the focusing knob is at theright side of the telescope barrel while for older models the barrelmay be pushed in ot out.Open the collimatot slit to a small a width is possible and align theteGscope and collimatot directly opposite each other.Peer through the telescope and adjust th. collimator focusing knob(not the telescope's!)' ,otil the slit image appears sharp. You mayneed to rotate the slit mechanism if it appears skewed.Tighten the telescope rotation lock screw and use the fine-adiustknob to align the crosshair to one of the edges of the slit image.Thertelescope and collimator are now aligned such that rays reachingthe telescope from the collimator are parallel.Insert a gndngon the central platform with its ruled lines verticaland its face perpendicular to the beam of light from the iollimatot.Make sute that light ftom the slit impinges in the middle of theglating. To do so adjust the platform height by loosening the lockingscrew on its side.Waming: For this procedure wear sunglasses if the light is toointense. B.irg the MetcurT light source about one centimetet fromthe slit and direct the telescope to find its ditect image. Read theangle on both windows and tabulate in the worksheet under "DirectImage" angle. Sweep the telescope clockwise until you obsewe thefirst ordet bright gteen line of Metcury. Measute and tabulate the

1.

2.

I3.

4.

5.

6.

7.

8.

I Tel"s"op" -urt be focused at infinity. Focusing the slit image using the collimator focusing knob ensures thatrays reaching the telescope are nearly parallel.

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The Spectrometer Physics 73.1angle. Sweep the telescope counterclockwise and find the green.lineto the right of the direct image. Measure the angle.

9. Subtract the ditect angle measuremeflts ftom the green line angles. Ifthe angular displacements between clockwise and counterclockwiseteadings are different, re-align the grating and repeat steps 8 to 9.10, Once the angular displacements between clockwise andcounterclockwise are neady equal, tighten the screw of the gratingplatform to lock it in place. The plane of the gtating is nowpeqpendicular to the incident light and the specftometer is atigned.

Record the aligned direct image aflgle.B. Measurement of Grating Constant D and Observation ofDiscrete Spectra

1. With the Mercury [ght source still on, locate a#d record in Table 2the angular displacements of each visible bright line in the first orderon each side of the direct image. Remember to subtract the ditectimage angle. In the space provided io yogt worksheet, reproduce asaccutately as possible the observe spectra of mercurT.2. PIot the sine of the average angulat position versus the standardwavelength of ihe disctete lines. Compute D from the slope of the. g^ph. Compare with the declared value of D.of the gratings. Dosteps 1 and 2 for 2 more gtatings. Each time you change gratingspedorm the alignmeirt procedures in Part A.

NOTE: For some gratings, light is allowed to pass thtough in only one side.If no image seems to appear, try reversing the gating.

Suggested Extension Experimentso Determine the range of wavelength of any of the following light

soufces:t. your cellphone's backlight2. different coloted light emitting diodes (LEDr).3. Fluorescent lampResearch the different specftometer designs. How is calibtationpedotmed for those designs?

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Physies 73.{ The Spectrometer

AppendixSandard wavelengths of the emission lines of Mercuf

Golor lntensity Wavelengtth (nm)Violet Faint 404.66Violet Faint 407.78

Blue-violet Bright 435.83Bluegreen Faint 491.60

Green Bright 546.07Yellow Bright 576.96Yellow Bright 579.06

Red Very faint 671.64Red Very faint 690.54

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The Spectrometer Physics 73.1


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Direct lmageAngle (CW) Green Line CW Difference Direct lmageAngle (CCW) GreenLineccw Difference

Name secrirn Date Score

Groupmates

A. AlignmentTable 1.

Aligned Condition: Final Value of Ditect Image Angle: C!7:

B. Grating ConstantTheoretical Grati.g Constant 1:

Worksheet: The Spectrometer

CCW:

3::Table 2. (Subtract the direct image angle from the teadings fot angular displacemeat).

Grating 1Golor StandardWavelenqth Angular'Disolacement GW AngularDisplacementGGW Ave. AngularDisolacement

Grating 2Color StandardWavelenoth AngularDisolacement GW AngularDisolacementCCW Ave. AngularDisolacement



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Phystcs 73.{ Worksheet The SpectrometerGrating 3Golor StandardWavelenoth AngularDisplacement GW AngularDisolacementCCW Ave. AngularDisplacement

Gfaphl . Qveday the plot of sin g vs. wavelength for the 3 diffraction gratings on the graphbelow. Estimate the grating constant ftom the slope of each gtaph.

I.t"'d"rd (mm)

Computed Gtating Constant 1:



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Worksheet: Tha Spectrometer Physics 73.{

In the space below, reptoduce in colot the observed specttum of Mercuy as accwately as possible.ra

ll

420 480 s20 slm 560 580wavelength (nm)

620 580

Sample Calculatioris

Questions:1. If the gating is not propedy aligned what ate the consequences on the avetage angulat

displacement and ultimat"ly th" wavelength teading? Explain.

2. How do the Sating constants affect the obsetved spectml lines?



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Physics 73.{ Worksheet: The Spectrometer3. IThy must the spectrometd 6s aligned each tirne a grating is teplaced?

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Nam Section Date Received by

Prelab Quiz : Spectral FingerprintingRead the manual and answet the follouring quesdons:X. What causes light?

:

2. How can elernents be identified ftom fieir qp6tte?

PREI-AB ACTIVITY

Bring the solution to class.

Do one of the following:Chlorophyll1. Collect leaves of any plant and chop finely-2. Soak the chopped leaves in alcohol overnight.3. Collect the juice in a test tube and seal with a cotk.Neon matkers1. Bring to class different colored neon markets.Gemstones1. Bring a ruby to class.

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Prelab Quiz: Spectral Fingerprinting PlryrE



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Physics 73.{

ObjectivesAt the end of this activity, you should be able to:o Differentiate betq/een condnuous and line sPecfta of light sources.

o Identi$, elements ftom their line spectra.o Obsewe and quanti$ absoqption sPecta in liquids.o Observe fluotesceace in solutions aod solids.

lntroductionThe human fingerprint is a pattem so unique to an individual that if there is adatabase of fingelptints of all persons it an. zreq the identity of an individualmay be ascettained ftom his ot her fingelpdnts alone. Atoms have a similat"finge1pdnt" in the colot of light they emit. This is why atoms or moleculesifl the s-un, plan.ts, minetal ote, .., or atnbsphere rnay be deduced from thespectra or Iight they erriit, absotb or teflect. This expedment explotes howmattef in different phases emits ot absotbs light and how we can determinethe presence of an element ot molecule based on "spectral fingeqprinting".

TheorySurrounding us afe various Iight sources both natural and man-made. Theteis sunlight, stadight, flame, lightning and bioluminescence ftom terrestriaiand aquatic cteatutes. Man-made light souces include gas discharge tubessuch as fluorescent lamps and neon light, incandescent lamps and flashlights,lasets and light-emitting diodes pED's). Whether natuial of mafl-made, Iightis produced when.a valence electton of an excited atom returns to a state oflower enefgy. From quartum mechanics we know that the energy levelsoccupied by an atom are quantized, meaning they are limited to specificvhlues. For diiferent enefgy levels its electrons occupy some specific otbitalconfiguration. If undistubed, an atom exists in the gtound state with itselectrons in a stable otbital of lowest energy.



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Speetral Fingerprinting Physics 73.{Atoms can absorb enet$y only in specifir ahour-rts. If the disturbance is ftoman elecftomagnetic wave, the absotbed energy E is exptessed as

E =hfHete, h is Planck's constaot kL = 6.626 x tO-*J'9 and f is the frequency ofthe electtomagnetic wave. Recall that given the wavelength 1, of light, onecan calculate its ftequency ftom c = ,2,f ,whete c is the speed of light (c = 3x 108m/s).An atom reacts to enbrgy in several ways. It may bounce atound like a billiardball, vibrate, bteak up, heat up of emit light. If the imp.arted enefgy is exactlyequal to the difference between two atomic enefgy levels it quickly absotbsthib energy and jumps to an excited state, its elecftons tapidly moving to theorbital configwation of the higher enetgy-level This condition is veryunstable ,t d t"*pomry, lasting oflly around 10-8s, and the . electronsinstantaneously retlrn to a lowet state otbital. As they deceletate they emitelecttomagneLic radiation: Depending on the levels where they wete excitedto and the states they land in, some of the tadiation may tum out asulUaviolet mdiation, visible light, inftated ot heat. The amount of enetgyabsorbed by the atom is quantized since it is equal to the enefgy diffetencebetween the initial and final states of the atom. Therefote, if the conditionsate right, the atom can reemit light.But something else affects the emitted light and it depends on the amount ofatoms there are.in a given sPace.

(1)

When the spacing between atgms is large, such as gas undetlow pressrue, atom-to-atom collisions ate infrequent.Consider a glass tube, evacuated and sealed with conductorsat both end. If a gas made of a Prrre element such ashydrogen or vapoiz.ed metcury is introduced into the tubeattd a, high voltage is placed across it, the gas will glow. TheIatge potential diffetence will cause elecffons ftom one endof the tube (cathode) to acceletate to the mote positiveelecttode (anod

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