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Electron Gun
An electron gun is used to produce a stream of electrons with a well defined kinetic energy. They are commonly found in all vacuum tube applications such as TV pictures tubes.
There are two parts to an electron gun; 1) a heated filament, and 2) the accelerating region, which is bounded by two electrodes, known as the cathode and the anode. The filament consists of a piece of wire, commonly made of a refractory material such as tungsten, which is heated by an electric current. Electrons leave the surface of the filament by a process known as thermionic emission, but with very little energy. They drift through a small hole in the cathode, into a region where there is an electric field, which accelerates them across the gap to the anode. They then pass through a hole in the anode, with a final energy which is determined by the applied voltage.
Voltage control
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The easiest way to understand the electron gun is in terms of conservation of energy. In crossing the gap, the electrons potential energy decreases by an amount equal to eV. This potential energy appears as the kinetic energy and we can write
½mv2 = eV
from which the speed can be calculated. |
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In this simulation, the electrons are emitted from the filament. Initially there is no electric field between the cathode and the anode, and the electrons just drift acroos the gap. You are however provide a means by which you can control the applied voltage, and see the effect on the electrons.
When you have set the voltage, click on the diagram to start and stop the electron gun. Note that, for the gun to operate, the filament must be heated, and glows red. Note also the direction of the electric field (from right to left). Since the charge of the electron is negative. the electric field must be in this direction in order to provide an accelerating force to the right. |
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