MH's ideal coil and voltage question

[picowatt]

    It looks as if by what PW is saying that for the 5hy inductor with
    4 volts applied what MH is saying is true. Result!!!!
         John.

There was never any dispute that the voltage across the inductor was 4 volts when it was connected across an ideal voltage source of 4 volts.

However, the induced voltage associated with the CEMF of the inductor, as connected, can only be observed indirectly by measuring its effect on the current flowing thru the inductor and noting that the current increases at a specific rate.

The dispute was more so with regard to also referring to the voltage across a resistor as CEMF.  Although I understand what MH was saying, I believe "CEMF" should be used only as defined with regard to inductors.

PW

[picowatt]

Yes I am familiar with the dynamics of negative feedback loops and agree with what you are saying here.

This is where I have a problem when we "sum" a positive applied voltage or Emf with any value of - voltage or Cemf other than zero and still maintain the fixed applied Emf. Are we saying that +4 + (-4) = 4?

Respectively this seems contradictory to me.  How can we have any dI or rate of change of current if Emf=Cemf? Forgive me but I just can not wrap my head around that. If this is true, what mathematical expression will support this condition?

To respond to this, I will repeat my last paragraph of my previous post-

"The current in an inductor is in phase with the applied voltage to the inductor which fits the Emf equation above and this would seem to indicate that the EMF wins in the production of output current over the Cemf. If the two are in a feedback interaction as the amplifier analogy implies, what is the ratio that would produce these results? What magnitudes or Emf and Cemf would have to exist to satisfy the EMF equation? IMO, Cemf would have to equal zero or the equation is invalid!

So, IMO Cemf does not exist in a single inductor but Cmmf does as I posted earlier."

I might add here that IMO the nexus or feedback summing point in a single inductor is at the physical interface between turns involving the bucking or cancellation of the H field or flux field.

pm

My apologies Partzman, but I have done about the best I can to describe CEMF in terms of being a rate of current change dependent feedback mechanism.   

Perhaps someone else can do better, I'll just be repeating myself...

PW

[poynt99]

PW understood when I mentioned negative feedback, that it was a similar process, not a process reflecting exactly what happens in an amplifier. I even stated that it was "similar". PW got it, others apparently took it literally, current summing nodes and all.

CEMF appears to behave as a negative feedback process. How is the nfg applied? It is applied via reverse induction caused by the circulating E field, which drives the inductor current in a direction opposing the current caused by the applied Vin.

And yes, it does appear that current can and does flow when the cemf=Vin.

Current flows if the inductor is replaced with a resistor. I think we all agree with that. The voltage across the resistor is a voltage drop. It is both a voltage drop and an induced emf in the case of the inductor however. The two values happen to be the same. It is not the induced emf that is driving the opposing current though. Simple proof supporting this is if the inductor was replaced with a 4V supply, there would be no current flow, period. The mechanism limiting the current therefore must be via an induced opposing current. The feedback mechanism is via the magnetic and electric fields, which relate directly to the current in the inductor. The two are inter-linked. If the current rises, the B field rises.

Clearly cemf is not the most suitable term for this scenario, it only confuses the issue. Even the official definitions don't make sense; "Increasing current in a coil of wire will generate a counter emf which opposes the current." How does an emf oppose a current? An emf can only oppose another emf. Unless we are misinterpreting what "they" are saying. Opposing may not mean what many of us first and may still be assuming; that oppose as to actively "DRIVE" the circuit. Perhaps what they really mean is that the voltages oppose, just as the case with a voltage source and a resistor; i.e. it is a "passive" voltage drop only. The end result of the inductor's behaviour is that it behaves exactly like a dynamic resistor.

[picowatt]

The end result of the inductor's behaviour is that it behaves exactly like a dynamic resistor.

I am a bit uncomfortable with this statement.

The CEMF is a generated effect due to induction.  A better visualization for the inductor's CEMF might be as a variable voltage source in series with a conductor.  The voltage of the variable voltage source is constantly adjusted as necessary to maintain the .8 amps per second rate of change to the current flowing thru the conductor (with regard MH's applied 4 volts and 5H inductor).

PW

(Added:  I am uncomfortable with the use of "dynamic resistor" as it seems to imply a dissipative mechanism)

[tinman]

The end result of the inductor's behaviour is that it behaves exactly like a dynamic resistor.

I think we have to leave resistors of any kind out of this,as there is no such thing as an ideal resistor.
If a resistor did not dissipate energy,then it would have a resistive value of 0 ohms,and so,you have no resistor.

A resistor has no inductance,dose not store energy,and has resistance.
An ideal coil has inductance,dose store energy,and has no resistance.

The applied EMF see's the number of turns in the coil. The CEMF also see's the same number of turns in the coil.
As soon as the EMF induced current starts to flow,an equal and opposite CEMF produced current will start to flow,due to the fact that there are no losses associated to R.
The only reason the self induced current is of a lesser value to that of the EMF induced current in a real world inductor,is due to the losses associated to R in that inductor.


Brad