Please correct me if I'm wrong, but this is how I understand things so far:They are measures of different things. A farad is a measure of how much charge a capacitance can hold for each volt of potential difference across it. It describes the structure and applies whether or not there is any charge stored there at all. A coulomb is a measure of how much charge is actually stored on that capacitance at a particular time (or under a particular set of conditions).
Neither is a measure of energy, though both can be a proxy in a given context. If I tell you that one capacitor has 1 mC of charge on it and another has 2 mC of charge on it, can you tell me which one has more energy stored in it? Of course not, since the energy stored on a capacitor is CV²/2. Since Q = CV, this can be written as Q²/(2C). So if the 1 mC charge is on a capacitor that has ten times the capacitance, the energy stored in it will be many times smaller than the capacitor charged to 1 mC due to the much higher voltage required to place the smaller amount of charge on the smaller capacitor.
As for transistor ratings, I've only seen capacitance values given since they are descriptive of the structure. But in cases where you have a relatively fixed voltage, such as the base-emitter junction of a BJT transistor, I can see it being convenient to describe the capacitance in terms of the amount of charge that is stored there at that operating voltage. In designing imagers we commonly did the same thing -- the pixel capacitance was often discussed in terms of the number of electrons (not coulombs, but the number of electrons) it could hold at it's saturation level (usually defined as the point at which it either became too nonlinear or the point at which charge would start to be lost through some mechanism).
Boy, you've wandered off into Disneyland now! I can't see how random motion relates to differing concentrations of charge. Take milk, for example. It is not a solution, it is a colloid. The particles remain suspended by random (Brownian) motion. How much voltage does a milk molecule have? Does the voltage change if you throw the glass of milk? Look up posts by Gerry Rzeppa to learn about the relationship between velocity and several aspects of electrical circuits.somehow I don't think that their voltage difference relates to a kinetic characteristic, such as their relative speed.
Will do, thanks for the tip... and yes, maybe I did go a little hyperbolic for a moment there, but I'm trying to visualize things in terms that I can understandLook up posts by Gerry Rzeppa
No problem for me accepting those two points.One volt is, by definition, a potential energy of one joule per coulomb of charge.
Voltage is inherently differential (or referential).
Let's work two analogous situations.What I'm trying to visualize here is, what's the physical difference between two single electrons with a potential of one volt between them.
You mean like electron density? More electrons in less volume? If that is so, then I think that my mind has just gone click.Voltage is about a difference in concentration of electrons
You need to brush up on the notions of "potential" versus "kinetic".A volt is a unit of potential. But potential can also move. The velocity of potential adds to the original potential.
If the potential is static, the voltage is related to charge location.
If the potential is moving, the voltage is related to charge velocity.
To some degree you can think of it this way, but you can have two capacitors with the same volume and the same configuration and the same number of electrons stored yet have different voltages and different amounts of energy stored. All that is needed is to use a different dielectric between the plates.You mean like electron density? More electrons in less volume? If that is so, then I think that my mind has just gone click.
Yes, that different materials have different dielectric properties, I get, thank you.To some degree you can think of it this way, but you can have two capacitors with the same volume and the same configuration and the same number of electrons stored yet have different voltages and different amounts of energy stored. All that is needed is to use a different dielectric between the plates.
An electron-volt is the product of an amount of charge (one electron charge, which is 1.602E-19 coulomb) with an amount of potential energy per charge (one volt, which is one joule per coulomb). Thus the result has to be energy since it is expressed in joules (1 eV = 1.602E-19 J).Yes, that different materials have different dielectric properties, I get, thank you.
One of the reasons for my initial confusion is that in a particle accelerator, for instance, energy is expressed in electron-volts. And since an electron has a fundamental charge, and one farad=1*coulomb*volt, I got kind of mixed up. But then it occurred to me that an electron also has mass, and that's why an electron-volt is not the same as a fraction of a farad, but rather a unit of energy.
No, one farad of capacitance will store one coulomb of charge PER volt. C = Q/V, not C = Q*V.And since... one farad=1*coulomb*volt...
Yes, I got that from the start, got a little mixed up there while typing.No, one farad of capacitance will store one coulomb of charge PER volt. C = Q/V, not C = Q*V.
It shows that I didn't do my research very well before my last post... how embarrassing. Anyway, rest assured. After all of your patient explanation, I think I got it now. So thank you all.A farad is simply not a measure of energy and is simply not the same as ANY measure that is.
Then, if we consider that energy is equivalent to mass, then I might have the base for an argument. But I don't want to go there... 'cause I don't want to start looking like a Troll hereAn electron-volt is the product of an amount of charge (one electron charge, which is 1.602E-19 coulomb) with an amount of potential energy per charge (one volt, which is one joule per coulomb). Thus the result has to be energy since it is expressed in joules (1 eV = 1.602E-19 J).
Gophers, moles, all the same to me.And therein lies the nature of my question...
Sometimes in our aim for simplification, we overdo it and end up creating a cartoon of the reality we're trying to describe.
ACK! Do not confuse the volume of a cylinder with the amount of capacitance. It just muddies the waters. In this conversation, it is sufficient for capacitors of equal size have equal capacitance.you can have two capacitors with the same volume and the same configuration and the same number of electrons stored yet have different voltages and different amounts of energy stored. All that is needed is to use a different dielectric between the plates.
Maybe you could review this simplistic model. Starting at First page, halfway down, "The first thing to do is to get some electrons in a pile."You mean like electron density? More electrons in less volume? If that is so, then I think that my mind has just gone click.
Electrons repel each other so if you apply a voltage to the electrons in a conductor then they will move slightly closer to each other until their mutual repulsion matches the added voltage.You mean like electron density? More electrons in less volume? If that is so, then I think that my mind has just gone click.
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