Phys 132
Fall 2008
Electromagnetism and Optics: Class Exercise 1
Cathode Ray Tube
The heart of older (non-LCD flat screen) television sets and computer monitors was a cathode
ray tube (CRT), which is a sealed glass container through which electrons are fired towards the
screen. A cross sectional schematic diagram is illustrated below.
Filament Accelerating
plates
−+
Deflecting
plates
bbbbbbbbbbbbbbbbbb
The interior of the container is at a near vacuum, thus allowing the easy passage of electrons.
The filament is heated, causing electrons to leave its surface. These are then accelerated via
an electrostatic potential difference applied across the accelerating plates (in real designs these
are configured differently). Those that pass through the hole in the rightmost accelerating plate
proceed toward the screen. An electrostatic potential can be applied across the the deflecting plates
to direct the beam of electrons up or down.
1 Questions
a) Suppose that the electrostatic potential across the accelerating plates is 1250 V.If the
distance between the accelerating plates is 2.0 cm,determine the direction and magni-
tude of the electric field between the accelerating plates.
b) Consider an electron which is at rest at the hole in the negative accelerating plate
electron. Ignoring gravity, determine the velocity of the electron as it emerges from the
hole in the positive accelerating plate.
c) Suppose that the distance from the positive accelerating plate to the screen is 40 cm.
Taking gravity into account, determine the distance through which the electron will drop
as a consequence of the action of the earth’s gravity on the electron from the moment
that it leaves the positive accelerating plate until it hits the screen. Is this appreciable?
1
