Electronics > Metrology
Coulomb's law and a voltage frame of reference
Zeranin:
--- Quote from: John Heath on May 10, 2016, 11:13:19 am ---Great PEA rendering . I am getting better at predicting what it will look like. I had the middle part right but goofed up the outside . I was sure the field attenuation would be more round but it turned out to be more of a square with round edges.
--- End quote ---
Like you, I expect that if we modelled out further that the mod B contours would start to look round.
--- Quote ---Mr monopole has been reduced is size from 2 inch square to 1 inch square. A second monopole is on its way that will complement the first , north facing out and south facing out. It is a given that there will not be an attractive force between the + and - monopoles but have to verify.
--- End quote ---
Yes and no. Because it is difficult to accurately build a 'monopole', especially using round magnets, you will not get zero field outside of the cube, though it will be less than for a single magnet alone. The cube with the S poles facing outward will have a total of 6 'South Poles', but will also have 6 North poles - you can see this clearly on the FEA vector plot. Therefore, you will be able to get your to monopoles to weakly attract or repel, depending on their relative orientation.
IanB:
--- Quote from: Zeranin on May 10, 2016, 06:01:02 am ---Question. Where does the energy come from to accelerate these electrons, while they are in transit, before they strike the phosphor anode? The grey matter is pleasantly tickled.
--- End quote ---
In a CRT a large potential difference is created between the anode and the cathode, producing a strong electric field. It takes a power input to charge up this field, and once in place it stores energy. When electrons travel from the cathode to the anode they are accelerated through the potential gradient created in the field, drawing energy from it, and upon striking the anode they discharge it a little. If we continued without providing a replenishing current between the anode and cathode to keep it charged up the system would become discharged and would stop working. The power input to store potential energy in the electric field is where the energy comes from to accelerate the electrons.
RIS:
--- Quote from: IanB on May 10, 2016, 01:29:54 pm ---
--- Quote from: Zeranin on May 10, 2016, 06:01:02 am ---Question. Where does the energy come from to accelerate these electrons, while they are in transit, before they strike the phosphor anode? The grey matter is pleasantly tickled.
--- End quote ---
In a CRT a large potential difference is created between the anode and the cathode, producing a strong electric field. It takes a power input to charge up this field, and once in place it stores energy. When electrons travel from the cathode to the anode they are accelerated through the potential gradient created in the field, drawing energy from it, and upon striking the anode they discharge it a little. If we continued without providing a replenishing current between the anode and cathode to keep it charged up the system would become discharged and would stop working. The power input to store potential energy in the electric field is where the energy comes from to accelerate the electrons.
--- End quote ---
now thats a very nice explanation I like it and that would be nice and a closed system
but I wonder Why is the outer glass of the CRT has a charge.
I mean that is a technical problem but I somehow see some interaction between the closed and open systems.
so What would be a good explanation or opinion for that
Zeranin:
--- Quote from: IanB on May 10, 2016, 01:29:54 pm ---
--- Quote from: Zeranin on May 10, 2016, 06:01:02 am ---Question. Where does the energy come from to accelerate these electrons, while they are in transit, before they strike the phosphor anode? The grey matter is pleasantly tickled.
--- End quote ---
In a CRT a large potential difference is created between the anode and the cathode, producing a strong electric field. It takes a power input to charge up this field, and once in place it stores energy.
--- End quote ---
Yes. We can go further, and calculate the stored energy from E=0.5CV^2, where 'C' is the stray capacitance between the electrodes to which the EHT power supply is connected. So far, so good.
--- Quote ---When electrons travel from the cathode to the anode they are accelerated through the potential gradient created in the field, drawing energy from it, and upon striking the anode they discharge it a little.
--- End quote ---
Well yes, but what I actually asked was where the energy came from to accelerate a small bunch of electrons, before they strike the anode.
--- Quote ---If we continued without providing a replenishing current between the anode and cathode to keep it charged up the system would become discharged and would stop working.
--- End quote ---
We all agree with that, but here you are presumably referring to the steady state situation, where electrons are returned to the anode, and then ‘pumped’ back to the cathode by the EHT power supply. My question was, where does the energy come from to accelerate a single bunch of electrons, while they are in transit, before they strike the anode.
--- Quote ---The power input to store potential energy in the electric field is where the energy comes from to accelerate the electrons.
--- End quote ---
That does not really answer the question. Creating the accelerating electric field in the first place is a one-off. Thereafter the electric field strength (V/m) is constant, and does not ‘wind down’ while the bunch of electrons is in transit. Does anyone disagree that the only place that the energy can come from to accelerate our bunch of electrons while in transit is the EHT power supply. Thus the question becomes, how can current be drawn from the EHT power supply during this transit time, when the bunch of electrons does not touch and is not collected by any electrode?
Of course, we all understand how the EHT power supply provides the current and power to accelerate CRT electrons when there is a constant electron beam current, but that was never my question.
My question relates to the case where the CRT control grid is used to produce a small bunch of electrons, that are then accelerated towards the anode. We all understand that once this bunch of electrons strike the anode, then the EHT power supply will need to provide a small pulse of current (and thus energy) to pump those electrons back to the cathode, but that also has nothing to do with my question.
What I am asking, is where does the energy come from to accelerate a single bunch of electrons, while they are in transit, before they strike the phosphor? I have put forward the proposition that the only place this energy can come from is the EHT power supply, which means that current would need to be drawn from the supply during the transit time. Assuming you agree with this, how can such a current exist, given that during transit, the electrons do not touch and are not collected by the anode or any electrode. It’s a fair question, and as yet we have no answer. Where is Catalina to WOW us with the answer?
IanB:
--- Quote from: Zeranin on May 10, 2016, 11:54:17 pm ---What I am asking, is where does the energy come from to accelerate a single bunch of electrons, while they are in transit, before they strike the phosphor? I have put forward the proposition that the only place this energy can come from is the EHT power supply, which means that current would need to be drawn from the supply during the transit time. Assuming you agree with this, how can such a current exist, given that during transit, the electrons do not touch and are not collected by the anode or any electrode. It’s a fair question, and as yet we have no answer. Where is Catalina to WOW us with the answer?
--- End quote ---
Firstly, the electrons left the cathode before they started on their journey. Since the electrons carry charge, they discharged the cathode a little at this point.
Next, the electrons are accelerated through the electric field towards the anode, gaining energy as they go. Since the electrons in motion form an electric current, the moving electrons are actually creating a magnetic field. In this process, some potential energy from the electric field is being converted to stored energy in the magnetic field around the electrons. As the electrons are accelerated, the electric field is weakened (by whatever minuscule amount). Given sensitive enough measurements, the potential difference between the anode and the cathode while the electrons are in transit could be observed to be decreasing. It is as if an inductor has been connected in parallel with a capacitor and some energy is transferred from the capacitor to the inductor.
It might be imagined that the voltage on the capacitor remains constant during the journey of the electrons and drops suddenly when the electrons strike the anode. But this will not be found to be the case. What will be found is that the potential difference remains the same at the moment the electrons leave the cathode, it will then drop gradually as the electrons are accelerated, and it will stop dropping once the electrons strike the anode.
(It might be asked what happens to the stored energy in the magnetic field created by the fast moving electrons? Since there is no recovery path for this energy, it will be dissipated on the collapse of the magnetic field as heat (from eddy currents) and low energy radiation.)
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