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Unformatted text preview: nitial velocity?
7. W rite an equation relating Vplates t o the electric field betw een the plates, and substitute
it into your d eflection equation. Your final d eflection equation should involve only
quantities that can be m easured in lab or found in the textbook or in append ix.
H int: the electric field betw een the plates equals V plates d ivid ed by the d istance betw een
the plates. PRED ICTION
D eterm ine the physics task from the problem statem ent, and then in one or a few
sentences, equations, d raw ings, and / or graphs, m ake a clear and concise pred iction
that solves the task. (H int: H ow can you m ake a qualitative prediction w ith as m uch
d etail as possible?) EXPLORATION
WARN IN G : You w ill be w orking w ith e quipm ent that generates large electric
v oltages. Im proper use can cause painful bu rns. To avoid d anger, the p ow er m ust
b e turned o ff a nd you m ust w ait a t least one m inute before any w ires are
d isconnected from or connected to the pow er supply. N ever tou ch the cond u cting
m etal of any w ire.
Follow the d irections in the a ppend ix for connecting the pow er supply to the CRT.
Check to see that the connections from the pow er supply to the high voltage and the
filam ent heater are correct, before y ou turn the p ow er supply on. Apply betw een 250
and 500 Volts across the anod e and cathod e. After a m om ent, you should observe a
spot on the screen that can be ad ju sted w ith the knob labeled “Focus”. If your
connections are correct and the spot still d oesn’t appear, inform your lab instructor.
TAKIN G EXTREME CARE!, change the voltage across the accelerating plates, and
d eterm ine the range of values for w hich the electrons have enough energy to prod uce a
spot on the screen. Changing this voltage changes the velocity of the electrons as they
enter the d eflection plates. What is the range of initial electron velocities correspond ing
to this range of accelerating voltages? Which of these values w ill give you the largest
d eflection w hen you later apply an electric field betw een the d eflection plates? 25 DEFLECTION OF AN ELECTRON BEAM BY AN ELECTRIC FIELD – 1302Lab1Prob4
Before you turn on the electric field betw een the d eflection plates, m ake a note of the
position of the spot on the screen. The d eflections you m easure w ill be in relation to
this point. Make sure not to change the position of the CRT since external field s m ay
affect the position of the spot.
N ow apply a voltage across one set of d eflection plates, noting how the electron beam
m oves across the screen as the voltage is increased . Find a voltage across the d eflection
plates that allow s the d eflection for the entire range of initial electron velocities to be
m easured as accurately as possible.
Devise a m easuring schem e to record the position of the beam spot. Be sure you have
established the zero d eflection point of the beam spot.
Write d ow n your m easurem ent plan. H ow w ill you d eterm ine the strength of the
electric field betw een the d eflection plates? H ow w ill you d eterm ine the initial velocity
of the electrons? What quantities w ill you hold constant for this m easurem ent? H ow
m any m easurem ents d o you need ? M EASUREMEN T
M easure the position of the beam spot as you vary the electric field applied to the
d eflection plates, keeping other param eters constant. At least tw o people should m ake
a m easurem ent at each point, so you can estim ate m easurem ent uncertainty.
N ote: Be sure to record your measurements with the appropriate number of significant figures
and with your estimated. Otherwise, the data is virtually meaningless. If necessary, refer to the
suggested appendix material. A N ALYSIS
G raph the m easured d eflection of the electron beam as a function of the voltage
d ifference across the d eflector plates. Display uncertainties on your graph. CON CLUSION
D id your d ata agree w ith your pred iction of how the electron beam d eflection w ould
d epend on the initial electron velocity? If not, w hy? H ow d oes the d eflection of the
electron beam vary w ith initial electron velocity? State your results in the m ost general
term s supported by your d ata. 26 PROBLEM #5: D EFLECTION OF AN ELECTRON BEAM AN D
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 (CR...
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- Spring '14