MH's ideal coil and voltage question

[minnie]

What have I learned today?
EMF is not a force.
      J.

[picowatt]

What have I learned today?
EMF is not a force.
      J.

May the EMF be with you...

[MileHigh]

They measure the same, but not because the cemf is equaling the emf. It is because the emf source is holding the voltage constant. again, I think it is a very bad idea to equate a voltage drop with a cemf. They just simply are not the same, and historically have never been used interchangeably.

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 disagree. One is called cemf because that is precisely what it is; i.e. it is a generated voltage in this case. Going around the loop is simply confirming KVL, and it always holds.

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

[MileHigh]

PW:

Your posting is a doozie and I am always amazed how sometimes it feels like you a permanently prepped to write a final exam because you have all of the hard-core theoretical and technical information at your fingertips, which is something that I don't have.

You responded to my statement, "The EMF and the CEMF are the same damn thing!"  Just a quick recap from what I just posted:  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?

You bring the focus of the CEMF discussion back to inductors which is fine.  I have conceded that point and said for resistors "potential difference" is more appropriate and less confusing.

When connected across an ideal voltage source, the voltage measured across the inductor has little to do with the defined mechanism and actions of an inductor's CEMF.  In fact, by using only a voltmeter to measure across the inductor, it is impossible to measure, or identify, any parameter related to the inductor's CEMF.  By definition, the CEMF will oppose _current_ and in order to see the effects of CEMF (as defined), one must measure current.

I agree and I cited Wikipedia and that is mentioned in the first sentence of the article, "The counter-electromotive force (abbreviated counter EMF, or CEMF), also known as the back electromotive force, is the voltage, or electromotive force, that pushes against the current which induces it."

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.

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.

Whether or not the induced voltage related to an inductor's CEMF is realized externally across the inductor's two terminals will depend heavily on the impedances external to the inductor.  In the case of being connected across an ideal voltage source, the effects of an inductor's CEMF will not manifest externally as a voltage.  Only by measuring the rate of change dependent opposition to the flow of current can the effects of an inductor's CEMF be realized.

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.

Tinman's question, "what if the EMF and CEMF were equal", with regard to an inductor connected to an ideal voltage source, had little to do with any _voltage_ measured at the two terminals of the inductor.  In this instance, CEMF will only manifest by measuring current.

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

As I responded to Tinman, if it were somehow possible to cause all the magnetic flux created by a current flowing thru an ideal conductor to be confined to, and cut thru, that conductor in such a way as to make the inductor's CEMF be equal to the EMF, I believe that inductor would have infinite inductance.  To avoid the "chicken or egg paradox" in answering whether current could flow thru such an inductor, I stated that I believed that an infinitely small current would flow over an infinitely long period of time.

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.

MileHigh

[Magluvin]

Good question - I don't know. The scenario you paint seems a bit like those dastardly 'which came first, the chicken or the egg' situations
Will have to sleep on that.
Cheers

Back some many pages ago I posted a possible scenario for this. 

There must be something that happens first, as in applying input and that input must create some mag field before cemf is created. So we have to figure if just the tiniest amount of current from the input begins, the field should induce cemf instantaneously.

If at T0 there is no current, then nothing has happened yet. Why is that? Is there cemf at t0? It is an ideal conductor which should have immediately taken on current.
But the T0 point in time is supposedly a point that when the input is connected to the ideal inductor, but if we stop time at the point of connection, then current should not have happened yet, which is sort of hard to conceive considering ideal conductors.  But I would have to say that at T0, the ends of the coil leads have charge potential from the input equaling the input.  This is where it gets tough for me I admit.  I may go as far to say that the inductor leads get some sort of say static charge even just before the contact is made, no matter how small the input voltage is. If so, could that static charge be enough to get anything moving in the inductor before the full connection is complete? Moving as in some electron flow that may set up a bit of cemf just before the connection is completed?

If so, then when the connection is finally made, could the cemf possibly be already in opposition to the input? Egg before chicken?

Again, we will most likely never know for sure.  When we see these demos of super conductors, they are most likely still far from ideal yet, as they need to be super cooled. Why? Because they still dissipate heat, thus the cooling, of which takes the heat away that IS being produced. I had seen a demo of a copper coil inductor with enough resistance that in series with a small light bulb and a source, the bulb was dim as compared to the bulb direct to the source. But then they dipped the coil in liquid nitro and the bulb seemed to come to full brightness.  As I see it, the cooling helps to keep the heat generated by the coils resistance was just continuously taken away instead of being allowed to heat up more as current is flowing causing the resistance to become higher with the build up of heat. 

A real world inductor has also capacitance. Even if it is super tiny amount of capacitance, that capacitance takes on a charge in the coil nearly immediately. I will be doing a test of that with the scope here when I get time.  A bifi coil should have a larger spike of input when the source is applied as the capacitance is greater than a single winding coil, like Tesla said, the bifi windings take on input current as if the inductance was not there at all. I want to see if it is true.


Mags