Physics 2, Lab 11 - Physics Lab 11 Measurement of e/m...

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Physics Lab 11 Measurement of e/m Joshua Budhu Lab Partner: Andrew Nouri Date Performed: 4/29/09 Date Due: 5/6/09
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1 Theory and Objective Our theory in this lab is to determine, experimentally the e/m of an electron by observing the electric potential used to propel the electron. We will also be looking at the magnetic field strength and the radius of the circular path that the electron will trace out. By using the electron gun apparatus, and changing the voltages, and the amps, we can determine the correct values to create radii of varying sizes. By using various equations we can solve the e/m, which is 2 2 2 B r V m e = . We know the number of coils in our apparatus, (N = 119) and we can then solve for a general equation, by using the magnetic field strength. We arrive at ) ( ) ( 10 5 . 2 2 2 12 I V Nr R m e × = . We already have N, and r, and we can measure the other quantities. 2 Data and Calculations ) ( ) ( 10 5 . 2 2 2 12 I V Nr R m e × = R = 10 cm N = 119 Radius 0.005m 0.01 m 0.015 m 0.02 m Volts 28 V 75 V 103 V 68 V I (Current) 5.65 A 5.32 A 4.51 A 1.76 A = m e kg C 10 10 194 . 6 × kg C 10 10 678 . 4 × kg C 10 10 973 . 3 × kg C 10 10 689 . 9 × Standard Deviation kg C 10 10 545 . 2 × Part 6 Questions 1. Why does the final electron energy not depend on the grid voltage? The final electron energy does not depend on the grid voltage because partly the grid voltage focuses the beam of electrons and conservation of energy. It is only the change in voltage that matters as the electron is accelerated upward from the electron gun and gets deflected by the magnetic field between the coils. The change in voltage is not path dependent, and conservation of energy shows that it is only the final and initial state of the electron that matters.
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