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

Historical Significance This experiment was thus one of the first to provide evidence of subatomic particles. 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.” “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)

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

The Apparatus Cathode ray tube Cathode ray tube Screen Screen Parallel plates Parallel plates Helmholtz coils Helmholtz coils

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

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

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

My 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 equation”: The speed:

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

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

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

References Griffiths, D.J. (1999). “Magnetostatics.” In A. Reeves (Ed.), Introduction to electrodynamics (pp ). Upper Saddle River, NJ: Prentice Hall. Griffiths, D.J. (1999). “Magnetostatics.” In A. Reeves (Ed.), Introduction to electrodynamics (pp ). 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 ). Cambridge, MA: The MIT Press. Smith, G.E. (2001). “J.J. Thomson and the electron.” In J.Z. Buchwald (Ed.) and A. Warwick (Ed.), Histories of the electron (pp ). 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: e_2/node1.html 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: e_2/node1.html