Parallel conducting plates create nearly uniform fields that are used to accelerate and direct beams of electrons. Typically we would free the electrons from a surface by heating it up. One way is to run a current through a wire with resistance. The power dissipated in the wire causes it to increase its temperature to the point that it can radiate that power away, mostly carried off as light. The hot electrons escape the potential barrier at the surface and are "free" to use. Consider a source of electrons that is small, like a tiny hot filament. The electrons are freed from the surface but the positive charge they leave behind holds them in a cloud near the filament. Now we add another plate at voltage VV some distance dd away. We put a little hole in that plate to enable the electrons to zip on through. The experiment looks something like this. While they seem antiquated, the concept is still useful. It is the basis of modern X-ray tubes, and until only a few years ago all television and computer screens used this technology. Of course now we have low voltage flat panels with arrays of light emitting diodes, but before then the cathode ray tube or "CRT" was the way it was done. The beam of electrons was focused and steered to excite a phosphor and we viewed the light emitted by the phosphor to see the images and text. Steering was done by parallel electric plates as we see here, or by using magnetic fields. 1. If the potential accelerating the electrons is 12,000 volts (we say 12 kilovolts (KV) ), how much kinetic energy in joules does one electron have as it leaves the "gun" on its way to the screen? 2. How fast is that electron going, as a fraction of the speed of light? PLEASE ANSWER QUESTION 2
Parallel conducting plates create nearly uniform fields that are used to accelerate and direct beams of electrons. Typically we would free the electrons from a surface by heating it up. One way is to run a current through a wire with resistance. The power dissipated in the wire causes it to increase its temperature to the point that it can radiate that power away, mostly carried off as light. The hot electrons escape the potential barrier at the surface and are "free" to use.
Consider a source of electrons that is small, like a tiny hot filament. The electrons are freed from the surface but the positive charge they leave behind holds them in a cloud near the filament. Now we add another plate at voltage VV some distance dd away. We put a little hole in that plate to enable the electrons to zip on through. The experiment looks something like this. While they seem antiquated, the concept is still useful. It is the basis of modern X-ray tubes, and until only a few years ago all television and computer screens used this technology. Of course now we have low voltage flat panels with arrays of light emitting diodes, but before then the cathode ray tube or "CRT" was the way it was done. The beam of electrons was focused and steered to excite a phosphor and we viewed the light emitted by the phosphor to see the images and text. Steering was done by parallel electric plates as we see here, or by using magnetic fields.
1. If the potential accelerating the electrons is 12,000 volts (we say 12 kilovolts (KV) ), how much kinetic energy in joules does one electron have as it leaves the "gun" on its way to the screen?
2. How fast is that electron going, as a fraction of the
PLEASE ANSWER QUESTION 2
The potential applied between the plates is 12 kV. This potential is known as accelerating potential, since as the electrons enter this potential region, they start to accelerate as they gain energy.
As they accelerate in this potential, they gain kinetic energy, which is given in terms of the accelerating potential as
For a potential of 12 kV, the kinetic energy gained by the electrons is
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