This bothered me all night. According to this, an isolated in-vacuum incandescent lamp filament (or thermionic cathode) emits a constant flow of electrons. Until what, the bulb is fully packed with electrons and bursts?
George's experiment gives a hint:
Heat will loosen electrons from a solid (a cathode), but external field will repel them back to the cathode. Dushman has a detailed explanation, too dense for me. In short a material has a Work Function saying how hard it is to extract an electron to an infinite distance. This also varies with temperature, from several Volts cold to about a Volt hot for our more favorable materials.
I believe this is George's -0.7V. When enough liberated electrons fill the bottle to the Work Function of the cathode stuff, electrons stop liberating. Current drops to zero. Power drops to zero (not counting the incidental pass-through from the heated cathode).
In typical internet style, this zero-current condition completely misses the OP's question.
This topic should be left to die ... there are not 2 alternatives to debate with examples and calculations . In the first place he just doesn't seem to know that the current is the flow of electrons , tiny number or big number , as he said 😀 Maybe he missed that first class .
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This argument has become very polarised.
Isn't the definition of current 'the flow of charge per unit time'? That can be electrons one way or positive ions the opposite way.
Also the flow of electricity is a bit like those Newton's cradle executive toys where you lift a ball at one end of a row of balls, and the one at the other end lifts off when the other collides again. The drift speed of electrons in a wire is surprisingly low - just a few metres per second, but the signal (charge) velocity is close to the speed of light.
In between those speeds, there is the speed of the free electrons in the wire, moving in a Brownian motion in the absence of an external charge, which is about 1/100 of the charge velocity.
When an external charge is applied the movement of the electrons becomes more polarised and energetic, leading to collisions and loss of kinetic energy as heat.
My understanding of a tube is that the space charge creates the environment for a charge to flow, and an electron from the cathode may be lost when an electron arrives at the anode, but it is more complicated than just a stream of individual electrons.
"The drift speed of electrons in a wire is surprisingly low - just a few metres per second, but the signal (charge) velocity is close to the speed of light."
You are right in saying that the electron drift velocity when current flows through a wire is remarkably low; in fact very much lower than you said, even. In some typical example, it is of the order of a few inches per hour. But this is not like what is going on in the vacuum tube.
"My understanding of a tube is that the space charge creates the environment for a charge to flow, and an electron from the cathode may be lost when an electron arrives at the anode, but it is more complicated than just a stream of individual electrons. "
Yes, there are always complications when one delves more deeply into things. But it is often helpful to keep one's eye on the essentials first, and not turn every question into an endless litany of caveats and quibbles. The essential point in this case is that if an anode current I is flowing, then this necessarily means that I/e electrons are arriving at the anode, from the cathode, every second, where e is the charge on an electron. And each of these electrons has been accelerated through a potential difference V (the anode voltage relative to the cathode), and so has acquired a kinetic energy e V.
It is the flow of this very large number of electrons per second that is responsible for the anode current, and it is the energy deposited on the anode when they slam into it that is responsible for heating it up.
You are right in saying that the electron drift velocity when current flows through a wire is remarkably low; in fact very much lower than you said, even. In some typical example, it is of the order of a few inches per hour. But this is not like what is going on in the vacuum tube.
"My understanding of a tube is that the space charge creates the environment for a charge to flow, and an electron from the cathode may be lost when an electron arrives at the anode, but it is more complicated than just a stream of individual electrons. "
Yes, there are always complications when one delves more deeply into things. But it is often helpful to keep one's eye on the essentials first, and not turn every question into an endless litany of caveats and quibbles. The essential point in this case is that if an anode current I is flowing, then this necessarily means that I/e electrons are arriving at the anode, from the cathode, every second, where e is the charge on an electron. And each of these electrons has been accelerated through a potential difference V (the anode voltage relative to the cathode), and so has acquired a kinetic energy e V.
It is the flow of this very large number of electrons per second that is responsible for the anode current, and it is the energy deposited on the anode when they slam into it that is responsible for heating it up.
So what if the current in some cases can be made from positive ions ... not the case here . And even when the speed of electrons is low , a classic resistor can disispate thousands of watt because of the collisions between those slow electrons and atoms inside .
In a tube the current is a stream of electrons passing from cathode to plate , what else can be ? Don't make statements without any explanations so we have to guess what do you want to say .
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Not to cloud a current discussion, but . . .
If the tube is gassy, I think there may be some positive ions.'
Perhaps that will light-en things up.
If the tube is gassy, I think there may be some positive ions.'
Perhaps that will light-en things up.
Electron charge flows OK in a vacuum. The usual "space charge" is a cloud of electrons which typically repels and turns-back electrons.
In an *electron* tube, positive ions cancel electron space charge and electron current flows REAL WELL (and may arc).
The speed of electrons in a wire is no clue to the speed of electrons in empty(or ionized) space. A wire is like a pinball game: full of "bumpers". (Our younger members may search YouTube for a clue.)
What you've said is, in fact, true, i.e. "if" they return to the Anode. That was the one failing of W.H. Schottky's 1919 invention of the Tetrode. The Screen Grid was an excellent Faraday shield between the Control Grid and Plate. but in over-accelerating electrons striking the Plate, it aggravated Secondary Emission.
Many of those Secondary Electrons left, via the Screen Grid. That's why God created the Suppressor Grid.
Actually, it was B.T.H. Tellegen's 1926 invention of the Pentode. The Suppressor Grid repelled and slowed Secondary Electrons leaving the Plate. They didn't have enough energy to reach through the Suppressor Grid, now acting as a Faraday shield for the Screen. All of those tired, slow Electrons returned to the Plate, with insufficient energy to create further trouble.
The Pentode is a virtually perfect device. It has almost zero Control Grid-to-Plate capacitance, very high Plate resistance, for high gain, and almost zero Secondary Emission.
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Concerning Vacuum Tube or Valve power dissipation, here are some points, based on basic Physics.
In this discussion. Element currents are Primary i.e. without significant Secondary Emission. Unlike the true Laws of Physics, the Positive Element Current Split "Laws" have still been supported by numerous experiments and are called "Laws" for convenience's sake.
While Grids are Elements, not all Elements are Grids. Grids are porous and generally allow the passage of Electrons.
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1) if the net or effective E-field seen by the electron cloud, just outside the Cathode is positive, the tube will conduct what is known as Space or Cathode current, i.e. Ispace or Is in the days before solid-state electronics.
If the net E-field is zero or negative, Is will be "cut off".
2) all tube Elements, positive with respect to the Cathode, including the spiraled or meshed Tetrode & Pentode's Screen Grid or the Class C's Control Grid, conduct current and dissipate some power. If positive, a Suppressor Grid will also dissipate some power.
3) hopefully, the greater majority of the power is dissipated by the (usually) solid plate. On some very high power tubes, part of the plate surface may be outside the envelope, significantly improving cooling by forced air, liquid, evaporative or change-of-state methods.
4) those internal dissipating Elements or Grids cool by radiation and the tiny amount of conduction out their leads, the glass or ceramic envelope. Too much power will easily melt any dissipating Grid.
5) using the downstream-flowing water analogy, with the Cathode at the highest relative water pressure head or negative Voltage, the cumulative (multiplicative) Faraday shielding effect of successive downstream positive and negative Grids, each Positive Element or Grid's current, calculated by its own Current Split Law, times that Element or Grid's Voltage, will determine its dissipation.
6) if an overheating Element or Grid has a series external resistance, the resistance may open first , protecting that Element or Grid and shifting the dissipation to another positive Element or Grid.
7) if the Plate Voltage is too low, zero or negative, the Positive Element Current Split Law, times each Element's Voltage will calculate the dissipation of each Element and, if dissipation limits or thermal data are available, will also determine which will melt first. After that, the current and dissipation will shift to the next candidate, Elements shorted together, etc.
Ron
In this discussion. Element currents are Primary i.e. without significant Secondary Emission. Unlike the true Laws of Physics, the Positive Element Current Split "Laws" have still been supported by numerous experiments and are called "Laws" for convenience's sake.
While Grids are Elements, not all Elements are Grids. Grids are porous and generally allow the passage of Electrons.
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1) if the net or effective E-field seen by the electron cloud, just outside the Cathode is positive, the tube will conduct what is known as Space or Cathode current, i.e. Ispace or Is in the days before solid-state electronics.
If the net E-field is zero or negative, Is will be "cut off".
2) all tube Elements, positive with respect to the Cathode, including the spiraled or meshed Tetrode & Pentode's Screen Grid or the Class C's Control Grid, conduct current and dissipate some power. If positive, a Suppressor Grid will also dissipate some power.
3) hopefully, the greater majority of the power is dissipated by the (usually) solid plate. On some very high power tubes, part of the plate surface may be outside the envelope, significantly improving cooling by forced air, liquid, evaporative or change-of-state methods.
4) those internal dissipating Elements or Grids cool by radiation and the tiny amount of conduction out their leads, the glass or ceramic envelope. Too much power will easily melt any dissipating Grid.
5) using the downstream-flowing water analogy, with the Cathode at the highest relative water pressure head or negative Voltage, the cumulative (multiplicative) Faraday shielding effect of successive downstream positive and negative Grids, each Positive Element or Grid's current, calculated by its own Current Split Law, times that Element or Grid's Voltage, will determine its dissipation.
6) if an overheating Element or Grid has a series external resistance, the resistance may open first , protecting that Element or Grid and shifting the dissipation to another positive Element or Grid.
7) if the Plate Voltage is too low, zero or negative, the Positive Element Current Split Law, times each Element's Voltage will calculate the dissipation of each Element and, if dissipation limits or thermal data are available, will also determine which will melt first. After that, the current and dissipation will shift to the next candidate, Elements shorted together, etc.
Ron
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Speaking of the pinball machine and downstream water flow analogies, early vacuum tube experiments rolled tiny bearing balls down a slanted, stretched rubber sheet, with the Cathode represented by the raised end. Regularly spaced rows of pegs pushing against the sheet from above represented electron-attracting, conducting, positive Grid wires and pegs from below represented repelling negative Grid wires. The Plate was at the lowest end of the sheet
Small tilt angles and small peg deflections approximated Poisson's, Laplace's and other Physical equations closely.
Ron
