Download Name: Cathode Ray Tubes – Points of Clarification

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Cathode Ray Tubes – Points of Clarification
Deflection in cathode ray tubes (CRT’s) can be produced by charged plates that create a uniform eletric
field that in turn causes the electron beam travelling through the field to experience a force.
r
In the situation depicted in the diagram below, any electron travelling through the electric field ( E ) will
experience
r a vertical force directed upward. At any point in an electron’s trajectory through the field, the
force ( Fe ) experienced by the electron will be directed upward.
r
E
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The following question was looked at in class and merits further comment:
A cathode ray tube is adjusted so as to deflect the beam as shown.
If the deflecting voltage is held constant and the accelerating voltage is then decreased, which diagram
displays the new deflection?
Since the deflecting voltage is held constant, we can conclude that the force experienced by each electron
will remain the same, in both direction and magnitude. Since the accelerating voltage is decreased, we can
also conclude that the speed of each electron travelling through the field will be reduced as well. This
means that any given electron will spend more time in the deflecting field, and will experience the same
deflecting force for a longer period of time. The new deflection will therefore be more pronounced, in the
same direction. Answer: A
Deflection in cathode ray tubes (CRT’s) can also be produced by solenoids and other devices that create
magnetic fields. Electrons travelling through a magnetic field will experience a force that is determined by
their velocity as well as the magnitude and direction of the magnetic field (see RHR for charged particles).
The following question was looked at in class and merits further comment:
A solenoid placed beneath a cathode ray tube as shown below produces a magnetic field of 0.011 T on the
electron beam causing it to hit the screen at position 1.
The electron beam is then made to strike the screen at position 2.
What two changes were made to the current in the solenoid?
The most obvious change made to the current was a reversal in direction. A reversal in the direction of the
current leads to a reversal in the direction of the magnetic field created by the solenoid (see RHR for
solenoids). This change in the direction of the magnetic field leads to a force experienced by the electrons
that is now in the opposite direction, yielding the shift from 1 to 2 (see RHR for charged particles).
The second change in the current is not so obvious. Position 2 is the result of less deflection, and the
deflection is the result of the force experienced by each electron travelling through the magnetic field. Note
that an electron
r inra magnetic field experiences a force that causes it to deflect in an arc (part of a circle).
Therefore, Fc = Fm . This principle allows us to derive an equation for the radius of an electron’s path in a
magnetic field.
So,
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mv 2
= qvB
r
and therefore
r=
mv
.
qB
In addition, the link between radius and actual deflection must be understood. The diagrams below show
how radius and deflection are related. Note that a smaller radius leads to more deflection.
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smaller radius
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deflection
plane
larger radius
In order to reduce the deflection experienced by the electrons, the path radius must be larger. According to
our equation for radius above, a larger radius can be achieved by decreasing the strength of the magnetic
field, since r is inversely proportional to B. The strength of the magnetic field is decreased by decreasing
the current in the solenoid, since B = µ0 nI .
Answer: i. The direction of the current was changed, and, ii. the current was also decreased.
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