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# lec24 - Physics I Class 24 e/m Ratio for the Electron Rev...

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24-1 Physics I Class 24 e/m Ratio for the Electron Rev. 14-Apr-04 GB

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24-2 Joseph John (“J.J.”) Thomson J.J. Thomson 1856-1940 J.J. Thomson was appointed in 1884 as the third Cavendish Professor (head of the Cavendish Laboratory) at Cambridge, after James Clerk Maxwell and Lord Rayleigh. In 1899, his experiments with cathode ray tubes led him to postulate the existence of a new particle with a ratio of charge to mass (e/m) far larger than the same ratio for a positive hydrogen ion. The word “electron” was coined in 1891 by G. Johnstone Stoney. Today we will measure e/m for the electron.
24-3 Calculating Change in K.E. from Electric Potential (Review) V = 50 V = 0 V = 100 initial final - e - e 0 U K = + or U K - = V ) e ( V q U - = = or V ) e ( U - = J 10 6 . 1 ) 100 ( ) 10 6 . 1 ( ) V )( e ( K 17 19 - - × + = × = - - = If the electron starts at rest (or very close to it), then V e v m 2 2 1 =

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24-4 Magnetic Force on a Moving Charge (Review) B v q F × = q: charge of the particle (C; + or –) v : velocity of the particle (m/s) B : magnetic field (T) Force is at a right angle to velocity. Force is at a right angle to magnetic field. Important: If q is negative, that reverses the direction of force.
24-5 The Radius of the Circle (Review) v r F Although the directions of the vectors are changing, the magnitudes stay the same. r v m a m F 2 = = B v q F = r v m B v q 2 = B q v m B v q v m r 2 = =

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24-6 Apparatus for Measuring e/m We will set Potential in tube (V). Current in coils (I). We will observe Radius of circular electron path (r). We will calculate e/m.
24-7 The Cathode Ray Tube Electrons Electrons are randomly kicked out of the metallic cathode by thermal energy.

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