More than One Way More than One Way to Skin a Cat?to Skin a Cat?

The Charge-to-Mass Ratio of The Charge-to-Mass Ratio of the Electronthe Electron

Historical ContextHistorical Context

Cathode rays were known to carry Cathode rays were known to carry negative charge.negative charge.

The carrier of the charge was unknown.The carrier of the charge was unknown.

This experiment was performed to This experiment was performed to explore “the nature of the Cathode explore “the nature of the Cathode Rays.” (J.J. Thomson, 1897)Rays.” (J.J. Thomson, 1897)

Historical SignificanceHistorical Significance

This experiment was thus one of the This experiment was thus one of the first to provide evidence of first to provide evidence of subatomic particlessubatomic particles..

““I can see no escape from the I can see no escape from the conclusion that [cathode rays] are conclusion that [cathode rays] are charges of negative electricity charges of negative electricity carried by particles of matter.”carried by particles of matter.”

– J.J. Thomson (1897)J.J. Thomson (1897)

The ExperimentThe Experiment

The ApparatusThe Apparatus

J.J. Thomson’s J.J. Thomson’s methodmethod

My methodMy method

The ApparatusThe Apparatus

Cathode ray tubeCathode ray tube

ScreenScreen

Parallel platesParallel plates

Helmholtz coilsHelmholtz coils

Thomson’s MethodThomson’s Method Determined speed of particles using the Determined speed of particles using the

force equation:force equation:

and balancing the effects of E and B on the and balancing the effects of E and B on the cathode ray. cathode ray.

Thomson’s Method (cont’d)Thomson’s Method (cont’d)

Measured R, the radius of the beam’s Measured R, the radius of the beam’s path without the E field.path without the E field.

Calculated e/m.Calculated e/m.

andand

I don’t know what Thomson I don’t know what Thomson found…found…

But the currently accepted value of But the currently accepted value of e/m is e/m is

1.7588 x 101.7588 x 101111 C kg C kg-1-1

My MethodMy Method Only one high voltage supply.Only one high voltage supply.

Used the “gun equation” to find speed.Used the “gun equation” to find speed.

The “gun The “gun equation”:equation”:

The speed:The speed:

My Method (cont’d)My Method (cont’d)

Then I followed Thomson’s lead.Then I followed Thomson’s lead.– Measured R.Measured R.– Calculated e/m using “my” equation Calculated e/m using “my” equation

for speed.for speed.

andand

My ResultsMy Results

e/m = (1.97 ± 0.133) x 10e/m = (1.97 ± 0.133) x 101111 C kg C kg-1-1

Recall that the accepted value isRecall that the accepted value is

1.7588 x 101.7588 x 101111 C kg C kg-1-1

That’s a difference ofThat’s a difference of

%%ΔΔ = 11.88%. = 11.88%.

Possible ExplanationsPossible Explanations

My results are correct.My results are correct.

The high voltage source was not The high voltage source was not calibrated correctly.calibrated correctly.

Cathode ray gun was tiltedCathode ray gun was tilted..

ReferencesReferences Griffiths, D.J. (1999). “Magnetostatics.” In A. Reeves (Ed.), Griffiths, D.J. (1999). “Magnetostatics.” In A. Reeves (Ed.),

Introduction to electrodynamicsIntroduction to electrodynamics (pp. 202-254). Upper (pp. 202-254). Upper Saddle River, NJ: Prentice Hall.Saddle River, NJ: Prentice Hall.

Smith, G.E. (2001). “J.J. Thomson and the electron.” In J.Z. Smith, G.E. (2001). “J.J. Thomson and the electron.” In J.Z. Buchwald (Ed.) and A. Warwick (Ed.), Buchwald (Ed.) and A. Warwick (Ed.), Histories of the Histories of the electron electron (pp. 21-76). Cambridge, MA: The MIT Press.(pp. 21-76). Cambridge, MA: The MIT Press.

Tuckerman, M.E. (2007). Tuckerman, M.E. (2007). J.J. Thomson’s experiment and the J.J. Thomson’s experiment and the charge-to-mass ratio of the electron.charge-to-mass ratio of the electron. Retrieved December Retrieved December 13, 2007, from New York Univ.: General Chemistry I: 13, 2007, from New York Univ.: General Chemistry I: Honors, Website: Honors, Website: http://www.nyu.edu/classes/tuckerman/honors.chem/lecturhttp://www.nyu.edu/classes/tuckerman/honors.chem/lectures/lecture_2/node1.htmles/lecture_2/node1.html