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More than One Way to Skin a Cat?

More than One Way to Skin a Cat?. The Charge-to-Mass Ratio of the Electron. Historical Context. Cathode rays were known to carry negative charge. The carrier of the charge was unknown. This experiment was performed to explore “the nature of the Cathode Rays.” (J.J. Thomson, 1897).

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More than One Way to Skin a Cat?

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  1. More than One Way to Skin a Cat? The Charge-to-Mass Ratio of the Electron

  2. Historical Context • Cathode rays were known to carry negative charge. • The carrier of the charge was unknown. • This experiment was performed to explore “the nature of the Cathode Rays.” (J.J. Thomson, 1897)

  3. Historical Significance • This experiment was thus one of the first to provide evidence of subatomic particles. • “I can see no escape from the conclusion that [cathode rays] are charges of negative electricity carried by particles of matter.” • J.J. Thomson (1897)

  4. The Experiment • The Apparatus • J.J. Thomson’s method • My method

  5. The Apparatus • Cathode ray tube • Screen • Parallel plates • Helmholtz coils

  6. Determined speed of particles using the force equation: and balancing the effects of E and B on the cathode ray. Thomson’s Method

  7. Thomson’s Method (cont’d) • Measured R, the radius of the beam’s path without the E field. • Calculated e/m. and

  8. I don’t know what Thomson found… But the currently accepted value of e/m is 1.7588 x 1011 C kg-1

  9. My Method • Only one high voltage supply. • Used the “gun equation” to find speed. The “gun equation”: The speed:

  10. My Method (cont’d) • Then I followed Thomson’s lead. • Measured R. • Calculated e/m using “my” equation for speed. and

  11. My Results e/m = (1.97 ± 0.133) x 1011 C kg-1 Recall that the accepted value is 1.7588 x 1011 C kg-1 That’s a difference of %Δ = 11.88%.

  12. Possible Explanations • My results are correct. • The high voltage source was not calibrated correctly. • Cathode ray gun was tilted.

  13. References • Griffiths, D.J. (1999). “Magnetostatics.” In A. Reeves (Ed.), Introduction to electrodynamics (pp. 202-254). Upper Saddle River, NJ: Prentice Hall. • Smith, G.E. (2001). “J.J. Thomson and the electron.” In J.Z. Buchwald (Ed.) and A. Warwick (Ed.), Histories of the electron (pp. 21-76). Cambridge, MA: The MIT Press. • Tuckerman, M.E. (2007). J.J. Thomson’s experiment and the charge-to-mass ratio of the electron. Retrieved December 13, 2007, from New York Univ.: General Chemistry I: Honors, Website: http://www.nyu.edu/classes/tuckerman/honors.chem/lectures/lecture_2/node1.html

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