PHYCS 102 Lab.
Date:
Student Name:_____________________
ID No. :_______________
Student Partners:___________________
,_____________________
Sec. : (______)
,______________________
Experiment 9- Measuring e/m of the Electron
1- Experiment Objectives
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2- Apparatus:
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3- Brief Theoretical Introduction:
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9-1
4- Data and Data Analysis:
4.1 (Part I): Proportionality of electron’s path radius (r) with current in the Helmholtz coils (I)
at constant accelerating voltage (V)


125R 2

 V
(1)
r 
2
2
 32 N (e / m)  I 2
0


 Follow the manual instructions and set the accelerating
B
2
I
voltage of the electrons constant .
 Change the current (I) in the Helmholtz coils until you
Bin

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

r
v


Front View (looking
from the left side)
Obtain a circular ring of electrons.
 Vary the current I until r changes to 2, 3, 4, and 5cm respectively.
Record your data in the following table.
Constant accelerating voltage V= ……….  ………(V)
Helmholtz coil Radius: R= 0.200 (m), Helmholtz coil No. of turns: N= 154 (turns), 0=4×10-7 T.m/A
r  ……(m)
I  …… ( )
……….. (
)
………… (
)
………….. (
)
0.020
0.030
0.040
0.050
 Plot a proper graph to determine value of e/m ratio.
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 Calculate the percentage difference in your measurement given that the known value for e/m
is1.756×1011 C/kg
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9-2
 4.2 (Part II): Proportionality of electron’s path radius (r) with accelerating voltage (V)
at constant current in the Helmholtz coils (I)


125R 2

 V
(1)
r 
2
2
 32 N (e / m)  I 2
0


 Follow the manual instructions and set the current in the
B
2
Bin










I
Helmholtz coils constant.
 Change the accelerating voltage (V) till you obtain a circular
r
v


Front View (looking
from the left side)
ring of electrons.
 Vary the voltage (V) until r changes to 3, 4, and 5cm respectively.
Record your data in the following table.
Constant current in the Helmholtz coil I= ………  …….. (A)
Helmholtz coil Radius: R= 0.200 (m), Helmholtz coil No. of turns: N=154 (turns)
r  ………(m)
V …….. ( )
………… ( )
0.020
0.030
0.040
0.050
 Plot a proper graph to determine value of e/m ratio.
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 Calculate the percentage difference in your measurement given that the known value for e/m
is1.756×1011 C/kg.
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5- Conclusions
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6-Questions
1-In part I,What will be the variation if you plot r2 vs. I?
Sketch a graph for r2 vs. I
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2-In part II,What will be the variation if you plot r vs. V ?
Sketch a graph for r vs. V
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