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Physics Lab 11
Measurement of e/m
Joshua Budhu
Lab Partner: Andrew Nouri
Date Performed: 4/29/09
Date Due: 5/6/09
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View Full Document1 Theory and Objective
Our theory in this lab is to determine, experimentally the e/m of an electron by observing
the electric potential used to propel the electron. We will also be looking at the magnetic
field strength and the radius of the circular path that the electron will trace out. By using
the electron gun apparatus, and changing the voltages, and the amps, we can determine
the correct values to create radii of varying sizes. By using various equations we can
solve the e/m, which is
2
2
2
B
r
V
m
e
=
. We know the number of coils in our apparatus, (N =
119) and we can then solve for a general equation, by using the magnetic field strength.
We arrive at
)
(
)
(
10
5
.
2
2
2
12
I
V
Nr
R
m
e
×
=
. We already have N, and r, and we can measure
the other quantities.
2 Data and Calculations
)
(
)
(
10
5
.
2
2
2
12
I
V
Nr
R
m
e
×
=
R = 10 cm
N = 119
Radius
0.005m
0.01 m
0.015 m
0.02 m
Volts
28 V
75 V
103 V
68 V
I (Current)
5.65 A
5.32 A
4.51 A
1.76 A
=
m
e
kg
C
10
10
194
.
6
×
kg
C
10
10
678
.
4
×
kg
C
10
10
973
.
3
×
kg
C
10
10
689
.
9
×
Standard
Deviation
kg
C
10
10
545
.
2
×
Part 6 Questions
1.
Why does the final electron energy not depend on the grid voltage?
The final electron energy does not depend on the grid voltage because partly the
grid voltage focuses the beam of electrons and conservation of energy.
It is only the
change in voltage that matters as the electron is accelerated upward from the electron gun
and gets deflected by the magnetic field between the coils.
The change in voltage is not
path dependent, and conservation of energy shows that it is only the final and initial state
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 Spring '10
 HoggBudickAdler
 Physics, Electric Potential

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