- #71
Ratch
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sophiecentaur,
You will never observe me using the phrase "current flow". That literally means "charge flow flow", which is redundant and ridiculous. Charge does not flow twice, so you are correct to wonder about it. I always refer to current as "existing" or "passing through" or having a "direction", but never flowing. Charge can flow, however.
Electrons cannot be ignored. They are the primary charge carriers in metals. When electrons move, current exists.
Kirchoff's Current Law (KCL) is not violated in either case. Here's why. KCL simply says that all charges have to be conserved. They cannot travel down a wire and just disappear. There is no problem with charges passing through a resistor, but it is a little more subtle when charges encounter a capacitor. A capacitor is a energy storage element, and it stores charges on one side of the dielectric insulator and supplies electrons from the opposite of the dielectric, as I said several times before. This separation and accumulation of electrons causes a back voltage to form which diminishes the current in the branch containing the capacitor. It takes energy to accumulate and deplete the electrons, and this energy is stored in an electric field within the dielectric. Nevertheless, every electron is accounted for according to KCL. There is also a transitory current in the circuit branch containing the capacitor.
It is good to know how a capacitor works, even if it is different than generating high voltage by rubbing a comb.
A capacitor is going to follow its energizing voltage. Unless there is no resistance in the circuit, it will have a time delay that a resistor does not have. This time delay is caused by having to imbalance or even out the charge between its plates. A capacitor energized by a step voltage of constant amplitude will have an impedance whose magnitude is infinite, but with an orthoginal orientation. That makes it different than just an open circuit. Mathematically, it is described as -j∞.
We are using a minimum of mathematics, because for nonsinusoidal circuits, differential equations (DE) are necessary to calculate and understand the response. Not everyone is up to speed on DE.
I think we are closer to the micro level than the quantum level. I do think it is necessary to understand what really goes on rather than using hydraulic analogies and other fool's aids to describe what is not really happening.
Ratch
Umm. I'm not sure what current flowing "physically" means.
You will never observe me using the phrase "current flow". That literally means "charge flow flow", which is redundant and ridiculous. Charge does not flow twice, so you are correct to wonder about it. I always refer to current as "existing" or "passing through" or having a "direction", but never flowing. Charge can flow, however.
Let's ignore the electrons bit because that just clouds the issue.
Electrons cannot be ignored. They are the primary charge carriers in metals. When electrons move, current exists.
A current flows in one end and out of the other end of a resistor or a capacitor. Kirchoff's laws work perfectly well in most circuits. How are the two cases different?
Kirchoff's Current Law (KCL) is not violated in either case. Here's why. KCL simply says that all charges have to be conserved. They cannot travel down a wire and just disappear. There is no problem with charges passing through a resistor, but it is a little more subtle when charges encounter a capacitor. A capacitor is a energy storage element, and it stores charges on one side of the dielectric insulator and supplies electrons from the opposite of the dielectric, as I said several times before. This separation and accumulation of electrons causes a back voltage to form which diminishes the current in the branch containing the capacitor. It takes energy to accumulate and deplete the electrons, and this energy is stored in an electric field within the dielectric. Nevertheless, every electron is accounted for according to KCL. There is also a transitory current in the circuit branch containing the capacitor.
Is it really worth labouring the point that 'charging a capacitor' is not the same thing as 'charging' a comb by rubbing it?
It is good to know how a capacitor works, even if it is different than generating high voltage by rubbing a comb.
I think we agree that 'yer actual DC' does not exist, because that would involve infinite time for it to be established. So, if DC is a pragmatic term for 'constant value for long enough', then the reactance (let's just call it Impedance, in fact) to DC is just as meaningful as at any frequency of AC. At our newly defined version of DC frequency (<0.0001Hz, say), the impedance is (to all intents and purposes) infinite.
A capacitor is going to follow its energizing voltage. Unless there is no resistance in the circuit, it will have a time delay that a resistor does not have. This time delay is caused by having to imbalance or even out the charge between its plates. A capacitor energized by a step voltage of constant amplitude will have an impedance whose magnitude is infinite, but with an orthoginal orientation. That makes it different than just an open circuit. Mathematically, it is described as -j∞.
But I still don't see why you guys don't want to use Maths (or at least refer to it) to describe what goes on. The exponential charge / discharge of a CR network describes exactly what goes on and that can be re-stated in terms of frequencies and Impedance. The results of experiment agree so well with that simple theory and it isn't difficult to approach the 'ideal case' in practice. That's why we can design filters and other circuits to work in such a predictable way.
We are using a minimum of mathematics, because for nonsinusoidal circuits, differential equations (DE) are necessary to calculate and understand the response. Not everyone is up to speed on DE.
Discussing "what's really happening" is not really getting one any closer to an understanding unless you really want to get into QM and how materials behave.
I think we are closer to the micro level than the quantum level. I do think it is necessary to understand what really goes on rather than using hydraulic analogies and other fool's aids to describe what is not really happening.
Ratch
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