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Problem Set with Answer Key - Electromagnetism and Optics | PHYS 132, Assignments of Physics

Material Type: Assignment; Professor: Collins; Class: Electromagnetism and Optics-GTSC1; Subject: Physics; University: Mesa State College; Term: Fall 2008;

Typology: Assignments

Pre 2010

Uploaded on 08/19/2009

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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
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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

b b b b b b b b b b b b b b b b b b

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?

d) Suppose that the length of the deflecting plates is 2 cm, the distance between them is 0 .50 cm and the distance from the deflecting plate to the screen is 37 cm. An electrostatic potential of 125 V is applied across these so that the electron beam will be deflected upwards. Indicate which deflecting plate is at a higher potential and determine the electric field between the plates. Determine the vertical deflection of the electron by the time that it hits the screen. Hint: the motion of the electron while it is between deflecting plates is different to that once it leaves.