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Unformatted text preview: a spot that you can ad just w ith the knob labeled “Focus”.
N ote the d etails of all the connections in your lab notebook. (If your connections are
correct and the spot still d oes not appear, inform your lab instructor.)
Discuss the follow ing in your group and note your responses in your lab notebook. D o you expect the gravitational d eflection to vary as a function of the angle of the
CRT w ith the horizontal? Try d ifferent orientations in the horizontal plane to see if
you can observe any difference. Does the observed behavior of the electron d eflection
agree w ith your pred iction? For w hat orientation of the CRT is it im possible for the gravitational force to d eflect
the electron? This is the location of the beam spot w hen there is no gravitational e ffect
on the m otion of the electrons and should be used as the origin (and N OT the arbitrary
origin on grid ). D o you observe any deflection of the electron beam ? H ow can you d eterm ine if this
d eflection is or is not caused by the gravitational force? If it is not, how can you
m inim ize such effects on your m easurem ents? D evise a m easurem ent schem e to record the angle of the CRT an d the position of the
beam spot and record you m easurem ent plan. (In general, a m easurem ent plan
m inim ally consists of three labeled colum ns of nu m bers w ith units: (1) the
ind epend ent variable that you vary (you should pre-d eterm ine at w hich values of the
ind epend ent variable you w ill perform your m easurem ents; also, w hat is the
ind epend ent variable in this experim ent?), (2) the pred icted values of the d epend ent
variable (i.e., use your pred iction equation to com pute w hat m easurem ent you expect
to observe at each valu e of the ind epend ent variable; w hat is the d epend ent variable 21 GRAVITATIONAL FORCE ON THE ELECTRON – 1302Lab1Prob3
for this experim ent), and (3) the actual, m easured values of the ind epend ent variable.
Try d oing this in Excel, as the com putational pow er of using a com puter program w ill
m ake d oing labs easier as the labs them selves becom e m ore d ifficult.) M EASUREMEN T
Follow ing your m easurem ent schem e, m easure the position of the beam spot at an
orientation of the CRT for w hich you expect the gravitational d eflection to be zero and
then at the orientation for w hich you expect the gravitational d eflection to be
m axim um . Finally, m ake m easurem ents at several d ifferent interm ed iate angles of
N ote: Be sure to record your measurements with the appropriate number of significant figures
and with your estimated uncertainty Otherwise, the data is virtually meaningless. If necessary,
read the suggested appendix material. A N ALYSIS
U se your d ata to d eterm ine the m agnitud e of the d eflection of the electron. Make a
graph of the position of the electron beam spot as a function of the angle that the CRT
m akes w ith the horizontal for both your predicted and m easured d eflection values.
If y ou observe a d eflection, how can you tell if it is caused by the gravitational force? If
the d eflection is not caused by gravity, w hat m ight be its cause? H ow w ill you d ecid e? CON CLUSION
D id you observe any d eflection of the electr on beam ? Was it in the d irection you
expected d ue to the gravitational force? Did you observe any aberrant behavior? What
could account for this? H ow d id you cond uct the experim ent to m inim ize any aberrant
Can you m easure the effect of the Earth's gravitational force on the m otion of the
electrons in the CRT? Why or w hy not?
Based on your results, d o you think you need to take gravitational d eflection into
account w hen using the CRT? Why or w hy not?
Ov erall, w as your pred iction successful? Why or w hy not? 22 PROBLEM #4: D EFLECTION OF AN ELECTRON BEAM
BY AN ELECTRIC FIELD
You are attem pting to d esign an electron microscope. To precisely steer the beam of
electrons you w ill use an electric field perpend icular to the original d irection of the
electrons. To test the design, you m ust d eterm ine how a change in the initial velocity of the
electrons a ffects the p osition of the beam spot. A colleague argues that an electron’s
trajectory through an electric field is analogous to a bullet’s trajectory through a
gravitational field . You are not convinced but are w illing to test the id ea. One
d ifference that you both agree on is that the electrons in the microscope w ill pass
through regions w ith an electric field and other region w ith no electric field , w hile a
bullet is alw ays in a gravitational field . You d ecid e to m od el the situat ion w ith a
Cathod e Ray Tube (CRT) 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...
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This document was uploaded on 02/23/2014 for the course MANAGMENT 2201 at University of Michigan.
- Spring '14