MH's ideal coil and voltage question

[Magluvin]

When I say, "The EMF and the CEMF are the same damn thing!" I am talking about the measurement of the voltage magnitude itself.  i.e.; Are the EMF and the CEMF the same value or is there a "requirement" for there to be a difference between the two values for current to flow?  The setup is an EMF source connected across a coil.  Not too many people have chimed in on that one recently.

I don't think you should take issue with the term "voltage drop" when you put your KVL hat on.  As you go around the loop there _is_ a tangible, measurable voltage drop as you spiral your way through the coil.  Granted, it is not a voltage drop like a voltage drop in a resistor, but it is still a voltage drop.  It's all a question of your perspective and your semantic approach to the issue.  The CEMF due to the changing current flow through the coil and the measurable voltage drop as you go through the loop are one in the same.  At least from my perspective there is no big issue using either term with the proviso that you establish the frame of reference for using the term.  Or arguably, both contexts are mutually understood by the parties concerned and you can use either/or without ruffling too many feathers.

Take the example of a capacitor.  If you are doing a KVL analysis of a loop in some circuit, the capacitor could represent a voltage drop or a voltage increase.  Saying, "there is a voltage drop across that capacitor" sounds pretty ordinary and mundane to my ears.

I am not really going to disagree with you here but I will just restate what I have stated before.  It is possible to put aside how closely CEMF is tied to changing current through an inductor, and simply work with the literal meaning of the term.  A voltage that is counter to my reference voltage is a voltage that is opposite in polarity to my reference voltage.  For example, say my reference voltage or reference EMF was -25 volts.  If there is a two-terminal device in the loop that is +5 volts, then that device is a counter-EMF device when I am going about my business of summing voltages in the loop.  I suppose this is an exercise in the technical use of the English language, and not really mainstream electronics.  The point being that I have license to choose to use those words if I want to and if the people you are discussing something with are all on the same page, then it works.

MileHigh

"Granted, it is not a voltage drop like a voltage drop in a resistor, but it is still a voltage drop.  It's all a question of your perspective and your semantic approach to the issue. "

Actually there should be a voltage drop for each winding of the coil. If we center tapped the coil, we should measure 5v across each half of the coil if 10v is applied.

I would say that even an ideal coil with an ideal voltage source would be measured the same, being that current will always be increasing along the way and not ever end up as steady dc current unless the current was controlled as such to be steady in an ideal situation.

But that still leads me to wonder, if the ideal voltage source were turned to 0v, like an ideal wire across the coil as described earlier, what keeps the current flowing steady in the coil? What mechanism keeps the electrons moving from atom shell to another atom shell? Does it not take energy to do such? Like why doesnt the field collapse when the source is switched out for an ideal piece of wire? What holds the field in place and not allow it to change, as in collapse? If it is a careful balance of initial emf and cemf, then that same ideal should hold for the explanation of cemf being equal to the input when we first apply the input to the ideal coil, in which there should be no current flow ever. When does that careful balance begin and/or end?

Mags

[picowatt]

What I am not saying and what is implicit is that if you measure CEMF on an inductor with your voltmeter, you are aware that increasing current is flowing through the inductor and that is the real mechanism at play.  You don't even know the magnitude of the increasing current, you are only aware that the process is taking place.

My point was that if connected to an ideal voltage source, your voltmeter will tell you nothing with regard to an inductor's CEMF.  When connected across an ideal V source CEMF can only be measured by looking at current.

The voltage across all two terminal devices connected across an ideal voltage source (as in your black box comments) will measure the same.  Measuring current can identify L or C reactance, resistance, etc.  In your particular definition of CEMF, anything and everything (or even nothing) in the black box is generating a CEMF equal to the EMF.

I think we should reserve CEMF only for use as defined regarding inductors.

I don't know if you _must_ measure the current.  If you know the inductance, and you measure the voltage you at least know the rate of change of the current, but you don't necessarily know the magnitude of the current, unless you know the initial conditions, etc.

If an inductor is connected across an ideal V source, the voltage measured across that inductor says nothing about the inductance, DCR, or CEMF of that inductor.

Again, consider your black box.  Who knows what was put in there?  Measuring current will be more informative.

I am puzzled by that statement with respect to an ideal voltage source.  Are you making reference to a real-world inductor with resistance?  I didn't state it explicitly before but I am assuming an ideal inductor unless stated otherwise.  If you do mean a real-word inductor then I agree, the CEMF could be mixed in with an IR voltage drop.

Regardless of whether or not the inductor is ideal, if connected across an ideal Vsource, the voltage across that inductor says nothing with regard to its inductance, DCR, or CEMF...

Are we back to a real-word inductor as I referenced above?

It does not matter, the question was a discussion of the hypothetical. 

Agaiin, when connected across an ideal voltage source however, the voltage across the inductor has little to do with the discussion related to CEMF.  One must measure current...

I can't see where you are coming from for this one.  In my mind the inductance value from a coil of an ideal conductor is still a function of geometry as per the derivation.  So you can go to the Hyperphysics web site and punch in the coil parameters and then get your inductance value and take it from there.  I can't envision an infinite inductance here.

When you say, "all the magnetic flux created by a current flowing thru an ideal conductor to be confined to, and cut thru, that conductor" to me that means a "perfect coil" where the magnetic flux still flows through the center of a coil, and then wraps back around the outside of the coil through all 3D space.  I just don't see an infinite inductance with that model.  I suppose if you had an infinite number of turns in the coil form that would be a different story.

I was considering a simple length of wire as an inductor.  We know that changing the geometry of the wire (i.e., winding it into a coil) increases the inductance of that same length and size of wire.  If the increased inductance is due to increased flux cutting/coupling, than one might ponder what would happen if it were somehow possible to cause all the magnetic flux to losslessly induce a CEMF into the wire, would that CEMF be equal to the applied EMF, and if so, would current be able to flow thru it.  With reference to that, I suggested that an inductor so constructed would possess infinite inductance.

An infinite number of turns would most likely result in an infinite inductance (using an ideal conductor), but then I suspect, upon first thought, that there might also be an infinite amount of loss due to radiated flux.

But these are just hypothetical discussions that are supposed to be somewhat lighthearted and enjoyable.

Your definition of CEMF derailed the train so to speak... 

PW

[Magluvin]

My point was that if connected to an ideal voltage source, your voltmeter will tell you nothing with regard to an inductor's CEMF.  When connected across an ideal V source CEMF can only be measured by looking at current.

The voltage across all two terminal devices connected across an ideal voltage source (as in your black box comments) will measure the same.  Measuring current can identify L or C reactance, resistance, etc.  In your particular definition of CEMF, anything and everything (or even nothing) in the black box is generating a CEMF equal to the EMF.

I think we should reserve CEMF only for use as defined regarding inductors.

If an inductor is connected across an ideal V source, the voltage measured across that inductor says nothing about the inductance, DCR, or CEMF of that inductor.

Again, consider your black box.  Who knows what was put in there?  Measuring current will be more informative.

Regardless of whether or not the inductor is ideal, if connected across an ideal Vsource, the voltage across that inductor says nothing with regard to its inductance, DCR, or CEMF...

It does not matter, the question was a discussion of the hypothetical. 

Agaiin, when connected across an ideal voltage source however, the voltage across the inductor has little to do with the discussion related to CEMF.  One must measure current...

I was considering a simple length of wire as an inductor.  We know that changing the geometry of the wire (i.e., winding it into a coil) increases the inductance of that same length and size of wire.  If the increased inductance is due to increased flux cutting/coupling, than one might ponder what would happen if it were somehow possible to cause all the magnetic flux to losslessly induce a CEMF into the wire, would that CEMF be equal to the applied EMF, and if so, would current be able to flow thru it.  With reference to that, I suggested that an inductor so constructed would possess infinite inductance.

An infinite number of turns would most likely result in an infinite inductance (using an ideal conductor), but then I suspect, upon first thought, that there might also be an infinite amount of loss due to radiated flux.

But these are just hypothetical discussions that are supposed to be somewhat lighthearted and enjoyable.

Your definition of CEMF derailed the train so to speak... 

PW

Not sure, but I think that was what Brad was saying a while back, that a straight ideal wire may not even have a field outside the wire itself as the cemf would keep it in check.

Mags

[MileHigh]

But that still leads me to wonder, if the ideal voltage source were turned to 0v, like an ideal wire across the coil as described earlier, what keeps the current flowing steady in the coil? What mechanism keeps the electrons moving from atom shell to another atom shell? Does it not take energy to do such? Like why doesnt the field collapse when the source is switched out for an ideal piece of wire? What holds the field in place and not allow it to change, as in collapse? If it is a careful balance of initial emf and cemf, then that same ideal should hold for the explanation of cemf being equal to the input when we first apply the input to the ideal coil, in which there should be no current flow ever. When does that careful balance begin and/or end?

Mags

Poynt already answered this stating that there is no load to dissipate the energy, but let me take a crack at it.  I am just going to give you a non-scientific seat-of-my-pants explanation.

Let's use a yo-yo as a flywheel.  So the spinning yo-yo had a shot of energy put into it from your arm.  Once it is spinning, naturally you don't question the fact that it is spinning.  You also know that you can't easily stop it from spinning, it has inertia.

So if you spin up the yo-yo, that's step one.  But then in step two, if you want to disturb the yo-yo, it's almost like the yo-yo wants to spin you.  It's almost like once you set it going, the yo-yo has a life of its own.  There is energy in the yo-yo and it has direction and magnitude.  It's hard to mess with that energy.

It's like you applied torque to the yo-yo and if you want to mess with the yo-yo it says "back off" and applies torque right back to you.

Now look at a coil.  The current flows through the wire.  The wire subtends a magnetic field around the wire into all space.  It took energy to create that magnetic field.  So that means by definition there is literally a certain number of Joules of energy in each cubic centimeter of space around the coil, and in all 3D space.

When you are right up close to the wire you see the current flowing through the wire, and the energy of the magnetic field in the space surrounding the wire.  That energy is real, it means something.  The current and the magnetic field are so tightly connected to each other that you can just about treat them as one.  That's why most people in electronics only talk about the current flow through the coil, the magnetic field is essentially the same damn thing, so you don't even have to mention the magnetic field.

So let's go back to a close up view of a wire.  You see the current flow, and you see the magnetic field wrapped around the wire.  But you can also just as easily say this:  You see the magnetic field wrapped around the wire and you see the current flow.

What are the implications:  They are as follows, and this is a thought experiment:  What if you removed the current-carrying wire and instantly replaced it with a brand new wire but with no current flowing through it?

The answer is that the very presence of the magnetic field would demand that current instantly start flowing trough the wire.  If there is a magnetic field present there must be current flow, period.

This explains why you get a shock if you cut the power to a 12-volt relay coil with your two fingers across the terminals.  You take away the source of the magnetic field, but the magnetic field is there anyways and therefore current simply must flow through the wire.

So going to a superconducting coil, obviously work is expended to create the magnetic field, and the the current flow through the coil, which is the same thing.  So when you remove the power source, the magnetic field "takes over" and current must flow through the wire.  The current flow and the magnetic field are so intimately connected it's like they are a single entity.

You spin up a heavy flywheel by hand and then if you run out of gas then the flywheel will spin your hand.  Just like if you build up a magnetic field by pushing current through a coil and then stop supplying energy, then the magnetic field will "take over" and supply the energy to keep the current flowing through the wire.

Anyway, if you get the sense of how the current flow and the magnetic field are so intimately connected that they are the same thing for all practical intents and purposes, and how energy is stored in the 3D space of the magnetic field, then maybe all of this will make sense.

MileHigh

[Magluvin]

Had not looked at this stuff for a long time, but was looking at the element chart.  I found that all conductors we typically see only have 1 electron in the outer shells, like gold, silver, copper, etc, and as I went down the line as in other elements that were metals, that outer shell tends to have more electrons in the and they are less conductive. Still looking into that stuff. And it was long ago that I knew this stuff and it wasnt like we spent weeks or months on the subject. Interesting stuff

The last one is interesting. I wonder if it has better conduction than gold?

Mags