MH's ideal coil and voltage question

[minnie]

   Old Webby's gettin' a bit of steam up!!!
          J.

[MileHigh]

   Old Webby's gettin' a bit of steam up!!!
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Speaking of which, let's take a break and give the boys, girls, and zombies a little dose of culture.  And when I say "dose" I really mean "dose."

Sun Valley Serenade - Chattanooga Choo Choo

https://www.youtube.com/watch?v=V2aj0zhXlLA

[poynt99]

The EMF and the CEMF are the same damn thing!  The battery says, "I am imposing 12 volts across you."  The coil says, "Oh shit, then I have to let changing current flow through me at a rate where I muster up the same 12 volts."   They are the SAME THING.  They both measure 12 volts with a volt meter and have the same polarity if you use the same ground reference.  They HAVE to be the same potential because they are CONNECTED to each other.

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.

If they are the same damn thing then why is one called CEMF?  It's because you "travel though the loop" in ONE DIRECTION only.  So if you go clockwise and you go UP in potential because of the EMF, then as you continue on your journey through the coil you go DOWN in potential.  Hence the "counter."  You go up in potential and then you counter that by going down in potential.  But when you are not "in the loop" the EMF and the CEMF are EXACTLY THE SAME with the SAME polarity.

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.

[poynt99]

OK-good.
Now i want you to think about this very carefully Hoptoad-very carefully.

We have an ideal coil,and that is one free from any winding resistance. It is also void of a time constant--has none.
So from T=0,a voltage is applied across this ideal coil from an ideal voltage source-remember,no time constant,due to no winding resistance resistance.
At T=0,the current will continue to rise at a steady rate,and never reach a peak--the current rises to an infinite amount over an infinite amount of time. The CEMF as you said,is governed by the change in current flow induced by the applied voltage over time. But with our ideal coil,there is no change in current,as the current rises at a steady state for an infinite amount of time. So the current flow is the same as it was at T=0(-the moment a voltage was placed across the coil)for an infinite time.

Will the CEMF change from it's starting value(T=0),if the induced current from the applied voltage always rises at the same rate for an infinite amount of time?

P.S
To add your statement
Quote:  If the cemf was a steady value, all other factors would also be steady.


Brad

Since the amps/sec is constant, the induced cemf should be steady.

[picowatt]

The EMF and the CEMF are the same damn thing!

I am going to have to disagree with this.

I believe I understand what it is you are saying with regard to your definition of CEMF.  However, by your definition, even if your "black box" were empty (a perfect insulator), that perfect insulator would also be creating a CEMF equal to the "applied" EMF.  And, by your definition, I can even see the argument that can be made to make the case for that being true.
   
However, this thread is about inductors and the definition of CEMF, regarding inductors, is very well defined.  As defined, the CEMF related to inductors is generated by way of a very specific mechanism (induction).  With all the harping on about the use of proper terminology, and to avoid confusion, it is perhaps best to stay with the existing definition of CEMF as it relates to inductors.

In the above you state that EMF and CEMF are the same thing.  You make this case based solely on a voltage drop measured across the two terminals of the inductor.  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.

The voltage portion of the definition related to an inductor's CEMF is, rather clumsily, something along the lines of "a rate of change dependent voltage induced into a conductor that produces a _current_ in opposition to the current that induced it".  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.

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.

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.

At no time were we discussing CEMF in reference to any _voltage_ measured across the inductor.   

Just my .02...

PW