Lec24 - 24-1Physics IClass 24e/m Ratio for the ElectronRev 14-Apr-04 GB24-2Joseph John(“J.J.” ThomsonJ.J Thomson1856-1940J.J Thomsonwas

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Unformatted text preview: 24-1Physics IClass 24e/m Ratio for the ElectronRev. 14-Apr-04 GB24-2Joseph John (“J.J.”) ThomsonJ.J. Thomson1856-1940J.J. Thomsonwas appointed in 1884 asthe third Cavendish Professor (head ofthe Cavendish Laboratory) at Cambridge,after James Clerk Maxwell and LordRayleigh. In 1899, his experiments withcathode ray tubes led him to postulate theexistence of a new particle with a ratio ofcharge to mass (e/m) far larger than thesame 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.24-3Calculating Change in K.E.from Electric Potential (Review)V = 50V = 0V = 100initialfinal-e-eUK=∆+∆orUK∆-=∆V)e(VqU-==orV)e(U∆-=∆J106.1)100()106.1()V)(e(K1719--×+=×=∆--=∆If the electron starts at rest (or very close to it),thenVevm221∆=24-4Magnetic Force on aMoving Charge (Review)BvqF×=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.24-5The Radius of the Circle (Review)vrFAlthough the directions of the vectors arechanging, the magnitudes stay the same.rvmamF2==BvqF=rvmBvq2=BqvmBvqvmr2==24-6Apparatus for Measuring e/mWe will set•Potential in tube (V).•Current in coils (I).We will observe•Radius of circularelectron path (r).We will calculate•e/m.24-7The Cathode Ray TubeElectronsElectrons are randomly kickedout of the metallic cathode bythermal energy....
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This note was uploaded on 12/15/2010 for the course PHYS 0001 taught by Professor Nayak during the Spring '10 term at Rensselaer Polytechnic Institute.

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Lec24 - 24-1Physics IClass 24e/m Ratio for the ElectronRev 14-Apr-04 GB24-2Joseph John(“J.J.” ThomsonJ.J Thomson1856-1940J.J Thomsonwas

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