Matt Weiner
Phys. 241 Lab
Lab Partner: Phillip Hoffman
9/26/06
Acceleration Of Electrons In A Cathode Ray Tube, CRT
Goal:
To study the motion of electrons in electric fields using a cathode ray tube.
Theory:
This experiment basically is just throwing electrons through electric
fields.
This is done inside a Cathode Ray Tube, or CRT, that was designed to be
used in an oscilloscope.
To reduce collisions between the electrons and gas
molecules, there is a vacuum inside of the CRT.
A cathode inside the CRT emits
electrons when it is heated by the heater filament.
These electrons are
accelerated along the axis of the CRT by an electric field.
Because the electrons
are nearly at rest initially, one can calculate the velocity of eh electrons after this
acceleration from conservation of energy.
.
.
.
.
E
P
E
K
∆

=
∆
(1)
Where
K.E.
is the kinetic energy and
P.E.
is the potential energy.
This
gives:
z
z
e
eV
v
m
=
2
2
1
(2)
V
z
is the potential difference or voltage difference that the electron has “dropped”
through;
e
is the charge of the electron
19
10
6
.
1

=
x
Coulomb; and
m
e
is the mass
of an electron
31
10
11
.
9

=
x
kg.
An intuitive way of understanding this, is to think of
the electron as a marble rolling down the voltage curve shown at the bottom of
Fig. 1.
After this initial acceleration, an electrostatic lens focuses the electrons so
that they will hit the screen in a small region.
Next the electrons pass between two pairs of deflection plates; see Fig. 1
and 2.
Each pair of plates is like a parallel plate capacitor and there is an electric
potential,
V
, between the plates.
These plates are separated by a distance,
d
.
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The deflection angle ,
θ
, is given by:
z
final
y
final
V
V
,
,
)
tan(
=
θ
If
L>
l
then:
)
tan(
θ
≈
L
D
This approximation can be improved if L is replaced by
L’= L+
l
/2
.
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 Spring '08
 Milsom
 Electric Fields, Electric Potential, Acceleration, Volt, Oscilloscope, Cathode Ray Tube, deflection voltage

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