l9 - Forces: q and m The electron Plum pudding Electron...

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Unformatted text preview: Forces: q and m The electron Plum pudding Electron charge Radioactivity Rutherford Thomson scattering Rutherford scattering Experimental confirmation Closest approach Electrostatics and gravity Recall electrostatic force: F = 1 4 q 1 q 2 r 2 with = 8 . 85 10- 12 (mks units). Recall gravitational force: F = G m 1 m 2 r 2 with G = 6 . 67 10- 11 (mks units). Ratio is 4 G m 1 m 2 q 1 q 2 . Two protons: 4 G m 1 m 2 q 1 q 2 = 4 G ( 1 . 7 10- 27 ) 2 ( 1 . 6 10- 19 ) 2 = 8 . 1 10- 37 Two electrons: 4 G m 1 m 2 q 1 q 2 = 4 G ( 9 . 1 10- 31 ) 2 ( 1 . 6 10- 19 ) 2 = 2 . 4 10- 43 Gravity is a wimp! Most things must be electrostatically neutral. Forces: q and m The electron Plum pudding Electron charge Radioactivity Rutherford Thomson scattering Rutherford scattering Experimental confirmation Closest approach The discovery of the electron The American Institute for Physics has a good web site on this. Remember that much was swirling about with cathode ray tubes: first the discovery of X rays, then the discovery of electrons, then the photoelectric effect, all within a decade. Cathode rays: drawn from regular matter (the cathode). Are they particles? Constituents of atoms? Do they have some elementary charge? J.J. Thomson, director of the Canvendish Lab at Cambridge, thought so, but wanted to prove this. J.J. Thomson (18561940; Nobel Prize 1906) Forces: q and m The electron Plum pudding Electron charge Radioactivity Rutherford Thomson scattering Rutherford scattering Experimental confirmation Closest approach Thomsons experiments 1. cathode rays carry negative charge (electrometer). 2. with good enough vacuum, electric field deflects cathode rays 3. use an accelerating voltage to give corpuscles with the same charge the same velocity qV x = ( 1 / 2 ) mv 2 or v = radicalbig 2 qV x / m Forces: q and m The electron Plum pudding Electron charge Radioactivity Rutherford Thomson scattering Rutherford scattering Experimental confirmation Closest approach Magnetic field alone Thomsons third experiment with a magnetic field alone: m v 2 r = qvB q m = v rB q m = radicalbig 2 qV x / m rB = radicalbigg q m 2 V x rB giving q m = 2 V x r 2 B 2 (1) so Im mystified why Thomson didnt consider this sufficient! Note that this ignores relativity; it would have us start out with mv 2 / r = qvB (1909). Forces: q and m The electron Plum pudding Electron charge Radioactivity Rutherford Thomson scattering Rutherford scattering Experimental confirmation Closest approach Electric field alone Acceleration due to voltage V y over plate separation d : a y = qE / m = qV y / ( md ) Velocity in y changes from 0 to v y = a y t = qV y md v x (2) Recall velocity in x direction: v x = radicalbig 2 qV x / m Angle upon leaving electric field region: tan = v y v x = qV y md v x 1 v x = q m V y dv 2 x (3) = qV y md m 2 qV...
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This note was uploaded on 05/28/2011 for the course PHY 251 taught by Professor Rijssenbeek during the Fall '01 term at SUNY Stony Brook.

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l9 - Forces: q and m The electron Plum pudding Electron...

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