frank hertz experiment - university of rochesteradvlab/reports/sliwa_richard_franck_hertz.pdf ·...

Frank Hertz ExperimentFrank Hertz Experiment

Ben Richard and Katrina SliwaBen Richard and Katrina SliwaPHY243WPHY243W

Preview of Things to ComePreview of Things to Come

HistoryHistory TheoryTheory ApparatusApparatus IonizationIonization ExcitationExcitation ShieldingShielding ConclusionsConclusions

History History

In 1914, by James Franck and Gustav Ludwig HertzIn 1914, by James Franck and Gustav Ludwig Hertz One year after Bohr’s theory of quantized energy states One year after Bohr’s theory of quantized energy states Nobel Prize in 1925: “for their discovery of the laws Nobel Prize in 1925: “for their discovery of the laws

governing the impact of an electron upon an atom”governing the impact of an electron upon an atom”

Photos: Copyright © The Nobel Foundation

Energy LevelsEnergy Levels

Bohr: discrete energy levelsBohr: discrete energy levels Excite electrons with electron impactExcite electrons with electron impact Hg: 80 electronsHg: 80 electrons Excited transitions from electrons Excited transitions from electrons

33PP0 0 →→ 11SS00 ((4.6 eV)4.6 eV)

33PP1 1 →→ 11SS00 ((4.9 eV)4.9 eV)

33PP2 2 →→ 11SS00 ((5.4 eV)5.4 eV)

11PP1 1 →→ 11SS00 ((6.7 eV)6.7 eV)

Electron ScatteringElectron Scattering

Why Hg?Why Hg? Monoatomic vaporMonoatomic vapor Easy to control vapor pressure Easy to control vapor pressure

Mean free path through a gas in a cavity:Mean free path through a gas in a cavity:I = (2I = (2ππnRnR00))-½ -½

Kinetic energy gained in an electric field during I: Kinetic energy gained in an electric field during I: K = eEIK = eEI

Elastic scattering: K < Elastic scattering: K < ΔΔEnergy between statesEnergy between states Inelastic scattering: K = Inelastic scattering: K = ΔΔEnergy between statesEnergy between states

Frank Hertz Vacuum TubeFrank Hertz Vacuum Tube

Anode

Cathode (VF)

Grid (VCG)

Electric field

Frank HertzBox

Control Box

Multimeter Electrometer

To Computer

A

G

F C

Experimental Setup Experimental Setup Thermometer

B

IonizationIonization

Anode at negative potential w.r.t. cathodeAnode at negative potential w.r.t. cathode Grid at positive potential w.r.t. cathodeGrid at positive potential w.r.t. cathode Electrons can ionize mercuryElectrons can ionize mercury

positive Hg ions pulled toward anodepositive Hg ions pulled toward anode SettingsSettings

Retarding voltage: 1.5 VRetarding voltage: 1.5 V VVf f = 6.3 V= 6.3 V Temperature from 100Temperature from 10000C to 185C to 18500CC

Temperature ResultsTemperature Results

Ionization ResultsIonization Results

DiscussionDiscussion

1.5 V retarding potential 1.5 V retarding potential 0 V < V0 V < Vcg cg < 15 V at 0.1 V steps < 15 V at 0.1 V steps Inconclusive resultsInconclusive results Expected value: 10.39 eVExpected value: 10.39 eV Potential sources of errorPotential sources of error

Recommended retarding voltage too lowRecommended retarding voltage too low Filament voltage possibly incorrect Filament voltage possibly incorrect

ExcitationExcitation Retarding voltage: 1.5 VRetarding voltage: 1.5 V VVf f = 6.30 V= 6.30 V

0 V < V0 V < Vcgcg < 45 V in 0.1 V steps < 45 V in 0.1 V steps Assumptions: Assumptions:

KKe cathodee cathode= 0 = 0 All electrons with KAll electrons with Kee < eV < eVretardingretarding captured by grid captured by grid Electrons with required energy travel negligible Electrons with required energy travel negligible

distances before collisiondistances before collision

Excitation resultsExcitation results

Excitation resultsExcitation results

Avg dist between peaks: 4.93Avg dist between peaks: 4.93+/- .25 eV+/- .25 eV For Hg: For Hg: ΔΔE = 4.6 eV, 4.9 eV, 5.4 eV, 6.7 eVE = 4.6 eV, 4.9 eV, 5.4 eV, 6.7 eV Lifetime Lifetime 33PP0 0 →→

11SS0 0 >> Lifetime >> Lifetime 33PP1 1 →→ 11SS00

Accepted value: 4.9 eVAccepted value: 4.9 eV h = Veh = Veλλ/c; /c; λλ = 253.6 x 10 = 253.6 x 10-9-9 m m h = 6.63 +/- .34 x 10h = 6.63 +/- .34 x 10-34 -34 Js Js Accepted value: h=Accepted value: h= 6.626 x 106.626 x 10-34-34 Js Js

Excitation ObservationsExcitation Observations

Excitation ObservationsExcitation Observations

www.physics.umd.edu/.../demos/demosn2/n2-06.htm

Electrometer Electrometer

Very sensitive to external electric fieldsVery sensitive to external electric fields Attempted to create grounded tin foil shield Attempted to create grounded tin foil shield

ConclusionsConclusions

Inconclusive ionization results Inconclusive ionization results Found excitation peaks at: 4.93Found excitation peaks at: 4.93+/- .25 eV+/- .25 eV Measured h to be: 6.63 +/- .34 x 10Measured h to be: 6.63 +/- .34 x 10-34 -34 Js Js Future work:Future work:

Fine tune parameters for ionization Fine tune parameters for ionization Obtain VObtain Vff experimentally experimentally Create better shield for electrometerCreate better shield for electrometer

References References UR Advanced Laboratory Manual, The Franck-Hertz

Experiment [Online] http://www.pas.rochester.edu/~AdvLab/2-Frank-Hertz/Lab02%20Hertz.pdf

K. M. Beck and J. Mainzer, The Franck-Hertz Experiment [Online] http://www.pas.rochester.edu/~advlab/reports/beck_mainzer_franckhertz.pdf

A. C. Melissinos, Experiments in Modern Physics Academic Press (1966)

C. J. Sansonetti C. J. Sansonetti et al.et al., Phys. Scr. , Phys. Scr. 6363 219-242, 2001 219-242, 2001 Nobel Foundation Nobel Foundation

http://nobelprize.org/nobel_prizes/physics/laureahttp://nobelprize.org/nobel_prizes/physics/laureates/1925/index.htmltes/1925/index.html