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lab 4-241 - Matt Weiner Phys 241 Lab Lab Partner Phillip...

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Matt Weiner Phys. 241 Lab Lab Partner: Phillip Hoffman 9/26/06 Acceleration Of Electrons In A Cathode Ray Tube, CRT Goal: To study the motion of electrons in electric fields using a cathode ray tube. Theory: This experiment basically is just throwing electrons through electric fields. This is done inside a Cathode Ray Tube, or CRT, that was designed to be used in an oscilloscope. To reduce collisions between the electrons and gas molecules, there is a vacuum inside of the CRT. A cathode inside the CRT emits electrons when it is heated by the heater filament. These electrons are accelerated along the axis of the CRT by an electric field. Because the electrons are nearly at rest initially, one can calculate the velocity of eh electrons after this acceleration from conservation of energy. . . . . E P E K - = (1) Where K.E. is the kinetic energy and P.E. is the potential energy. This gives: z z e eV v m = 2 2 1 (2) V z is the potential difference or voltage difference that the electron has “dropped” through; e is the charge of the electron 19 10 6 . 1 - = x Coulomb; and m e is the mass of an electron 31 10 11 . 9 - = x kg. An intuitive way of understanding this, is to think of the electron as a marble rolling down the voltage curve shown at the bottom of Fig. 1. After this initial acceleration, an electrostatic lens focuses the electrons so that they will hit the screen in a small region. Next the electrons pass between two pairs of deflection plates; see Fig. 1 and 2. Each pair of plates is like a parallel plate capacitor and there is an electric potential, V , between the plates. These plates are separated by a distance, d .
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The deflection angle , θ , is given by: z final y final V V , , ) tan( = θ If L> l then: ) tan( θ L D This approximation can be improved if L is replaced by L’= L+ l /2 .
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