You w ill calculate the d eflection of an electron

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Unformatted text preview: T) in w hich electrons are em itted at one end of an evacuated glass tube and are d etected by their interaction w ith a phosphorous screen on the other end . You w ill calculate the d eflection of an electron that begins w ith an in itial horizontal velocity, passes betw een a pair of short m etal plates that prod uce a vertical electric field betw een them , and then continues through a region w ith no electric field until hitting the screen. Your result could d epend on the strength of th e electric field , the electron’s initial velocity, intrinsic properties of the electron, the length of the m etal plates that prod uce the vertical electric field , and the d istance from the end of the m etal plates to the screen. Your goal is to determine deflection as a function of the initial electron speed. Instructions: Before lab, read the required reading from the textbook and the laboratory in its entirety. In your lab notebook, respond to the warm up questions and derive a specific prediction for the outcome of the lab. During lab, compare your warm up responses and prediction in your group. Then, work through the exploration, measurement, analysis, and conclusion sections in sequence, keeping a record of your findings in your lab notebook. It is often useful to use Excel to perform data analysis, rather than doing it by hand. A t the end of lab, disseminate any electronic copies of your results to each member of your group. Read : Tipler & Mosca Chapter 21 Section 21-6 and review Chapter 3 Sections 3-1 & 3-2. EQUIPMEN T You have a Cathod e Ray Tube. You also have a Cenco pow er su pply, banana cables, DMM and an 18v/ 5am p pow er su pply. The app lied electric field is created by connecting the internal parallel plates to the pow er supply. N ote: The CEN CO pow er supplies can have transient AC voltage in the DC output, m aking it less than ideal for creating an electric field – u se the 18volt/ 5am p supplies . Read the section Cathode Ray Tube and A ccessories in the Equipment a ppend ix. Read the append ices S ignificant Figures , A ccuracy, Precision and Uncertainty, a nd Review of Graphs t o help you take d ata effectively. 27 DEFLECTION OF AN ELECTRON BEAM AND VELOCITY – 1302Lab1Prob5 If equipment is missing or broken, submit a problem report by sending an email to l abhelp@physics.umn.edu . Include the room number and brief description of the p roblem. WARM UP 1. Exam ine the d iagram of the CRT in the append ix. You w ill use only one set of the d eflection plates show n. Draw a sim plified d iagram of an electron w ith an initial horizontal velocity about to enter the region betw een the plates. Draw the screen som e d istance past the end of the plates. Label the relevant d istances. Assum e that the electric field is vertically oriented in the region betw een the plates and is zero elsew here. Ind icate on your picture w here an electron experiences electrical forces. Draw a coord inate axis on this picture. Sketch the electron's trajectory through the CRT, ind icating w here the electron should accelerate and the d irection of that acceleration. Ind icate on the screen of the CRT the d istance by w hich the electron has been d eflected aw ay from its initial straight-line path. Why can you ignore the gravitational force on the electron? 2. Recall som e things you alread y know about projectile m otion. Does a force in the vertical d irection affect the horizontal com ponent of an ob ject’s velocity? In this situation, can you use the horizontal velocity com ponent to find the tim e required to travel som e horizontal d istance? 3. C onsid er the m otion of the electron in the region betw een the d eflection plates. Calculate the am ount of tim e the electron sp end s in this region. Calculate the vertical position and vertical velocity com ponent of the electron w hen it leaves this region. Rem em ber you are assum ing that only an electric force acts on the electron and are neglecting the gravitational force. (You w ill need the relationship betw een the electric field and the electric force on a charged object, as w ell as the general relationship betw een force and acceleration.) 4. C onsid er the m otion of the electron in the region past the d eflection plates. What is true about the vertical and horizontal com ponents of its velocity in this region? Calculate w here the electron hits the screen relative to w here it entered this final region. Then calculate the total d eflection of the electron at the screen from w here it initially entered the region betw een the plates. 5. U sing the equation you have found for the d eflection of the electron beam d raw a graph of the d eflection vs. the initial velocity. Treat the other quantities as constant. 6. Tw o quantities in your expression are not d irectly m easurable in lab. These are the electron’s initial velocity and the electric field strength betw een the d eflection plates. You w ill, how ever, know the voltage that accelerates the electrons, V acc , a nd the voltage across the d eflecti...
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