MH's ideal coil and voltage question

[ramset]

Loner
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Take an "Ideal" voltage source, of any value.  Connect to "Ideal" switch of SPST design.  Connect this to an "Ideal" inductor.  I hope I have not lost anyone, yet.  This is for the "Minds" eye, as such components don't actually exist.  Close to these "Ideals" is of no use in this explanation, just as 0 is 0, not some infinitely small number...
If the switch closes at Time = 0, as was mentioned here by someone, what is the situation that is REALLY happening.  ( Common sense applies here, try to stay in the minds eye and remember "Imagination is more important than knowledge" )
At t=0, you have a voltage "appearing" at the leads of the inductor, before current has started to flow, and therefore BEFORE the CEMF has been generated by self inductance.
If someone wishes to argue that the current would "instantaneously" appear, then you wish to state that a current can appear that way in an inductor, which violates the "Laws" of an inductor.  You can have it both ways at the quantum level, but not at the macro scale.
THIS is one of the errors in current thinking and math, because as t increases, the current increases.  as current increases, it is the "Rate of change", NOT that actual amount, that is producing the CEMF.  Again, if the CEMF were EXACTLY equal to the applied EMF, the "Rate of change" would be equal to 0, which then forces the CEMF to also drop.  I am ignoring ALL external influences here.  We ARE NOT talking about a wave or cyclic event, just the initial magnetic charge from 0.  To put it another way, simply try to see HOW a totally empty inductor, with NO field, can produce any CEMF.  There IS NONE UNTIL the current flows.  It's not even a "Chicken and Egg" idea, it is just the reality.
Are we there yet?  I am NOT talking about a large difference, nor could anyone on this forum (I'll wager...) say with absolute certainty WHERE that difference could be measured,  Even if you used real circuit components, this Micro difference exists.  It is how much of the math could be easily derived, as a figure so low can easily be ignored for any realistic calculation of components or values, but to blatantly state, "It does not exist" is to deny reality and common sense for people like me.
Snip
You can accept it or not, but the actual reality is different and NOT covered in any textbook due to lack of need, and basic understanding.  2 other PhD's agree with me, but then quoted me one of Murphy's laws.  "Never argue with a fool, people might not know the difference."  In the context of this situation, they say I am the fool for expecting someone else's viewpoint to be even close to mine.
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It was good to read all your contributions over the years , Thanks for all the work you shared and the level headed mindset
you brought along with it.

Loner has deleted all his posts and his acc't [or it will be shortly]

A sad day indeed.

ChetKremens@Gmail.com

[tinman]

If not, then what is it,, if yes,, then why?

This part is not covered so well,, so I ask.

I think that is what I ended up saying,, that so long as there is an R in the inductor the CEMF from the inductor will be less than the source EMF

.

Exactly,and the R is the very reason the CEMF is less than the applied EMF,as some energy is lost due to resistive heat from both the applied EMF,and resulting CEMF.
The applied EMF looses some energy to R,and then the self induced EMF(the CEMF) looses some more to R-->this is why there is a value difference between the EMF and CEMF. We see this every day in transformer losses.

Does an ideal inductor have a relaxation time period?

Not sure what you mean there--relaxation period?
If you mean,is there a time delay between the applied voltage and current starting to flow,well we are dealing with propagations that happen at the speed of light,and it only seems like there is a delay between applied voltage and current flow,because the CEMF is creating a current flow in the opposite direction to that of the induced current flow,and so keeps the induced current flow down very low for a short time.

We can watch this all happen in real time--a time period as long as you want it,with a PM DC motor.
You place a voltage across that motor,and let it get up to running speed. We now have the magnetic fields greatest change in time in regards to the rotor windins. As we slow the motor down(place a load on it),we decrease the value of the magnetic fields change in time,as now the rotor windings are passing slower through the PM fields. We also know that the current flow through the rotor windings(coils) will increase when the rotor windings pass through the PMs magnetic fields slower--the change in the magnetic fields in time has decreased-it's the same dam thing. If we were to spin that motor faster than it's maximum unloaded speed,we would get a CEMF that is greater than the applied EMF,and the current would start to flow in the opposite direction. We know this is true,as we already have DC generators. Infact,i am rebuilding such a generator ATM,where it is both a motor,and a generator--see pics below.

We can take an inductor,and simulate a CEMF in that inductor on the bench-no problem at all,and very simple to show and do.


Brad

[poynt99]

Loner has deleted all his posts and his acc't [or it will be shortly]

If you look at his profile, he has 994 posts, so no I don't think he has deleted any of his posts.

[ramset]

Poynt
please read here

Loner
Quote

I will not "Discuss" this and will probably not receive any reply as I have requested to Stephan that both my accounts and all posts be deleted, so this may not even get posted


Removing posts takes time !!

[MileHigh]

No,this has not been covered already.

The CEMF is created by the changing magnetic field,which is due to the increasing current over time,that was induced when the voltage was placed across the inductor
This CEMF creates a current flow that is in the opposite direction to that of which the applied voltage induced. The value of the current flow produce by the CEMF is less than that induced by the applied voltage. It you take the peak current value that will be flowing at the 5th time constant,and you subtract from that the peak current value of the first time constant,you are left with the calculated reverse current produced by the CEMF. As you  can see,the current produced by the CEMF ,is less than that of the induced current by the applied voltage. This means the remaining difference is flowing through the coil at the end of the first time constant. The greatest amount of CEMF is produced at T=0,and so the greatest amount of reverse current is produced. This is why the inductor draws the least amount of current at T=0-because the difference between the revers current from the CEMF,and the induced current from the applied EMF ,is at a minimum.

The CEMFs value,and so the value of reverse current,is dependant on how much the magnetic field is changing over time. As the magnetics field change over time slows,less reverse current is produced,as the CEMF value is less. This is why the current induced by the voltage starts to rise over time.

If the current is going to continue to rise-such as it would in your question,then the magnetic field would continue to increase at a steady rate. If the magnetic fields change in time is a constant (as it would be for your ideal coil),then the CEMF would also be at a steady value-as Poynt has answered.

The magnetic fields rate of change over time  in a coil with no resistance,remains at a constant value,and that value is what it was at T=0--the instant the ideal voltage was placed across the ideal coil.
The current value will never increase from what it was as T=0,as the magnetic fields change over time remains at a constant value,and so the self induced EMF also remains at a constant value-->and there for,the reverse current also remains at a constant value.

So,regardless of what you believe, the answer to you question is--the current will not increase any higher than it was at T=0--regardless of the time the voltage is applied to the coil for.

Brad

It's like you have regressed and you are back to some kind of whackadoo "Great Pumpkin" fantasy.

It you take the peak current value that will be flowing at the 5th time constant,and you subtract from that the peak current value of the first time constant,you are left with the calculated reverse current produced by the CEMF.

So like if the 5th time constant current is 100 amps and the 1st time constant current is 25 amps then the reverse current produced by the CEMF is 75 amps.   So does that mean when you first apply the voltage across the coil the current is -75 amps?  It's "Attack from Planet Bizarro and the Pumpkin Patch Creatures."

As you  can see,the current produced by the CEMF ,is less than that of the induced current by the applied voltage.

I can't see anything.  I thought the current was -75 amps at the start.

The greatest amount of CEMF is produced at T=0,and so the greatest amount of reverse current is produced. This is why the inductor draws the least amount of current at T=0-because the difference between the revers current from the CEMF,and the induced current from the applied EMF ,is at a minimum.

I am totally confused.  I tried every setting on my secret decoder ring and it's not able to unscramble what you are saying.

The magnetic fields rate of change over time  in a coil with no resistance,remains at a constant value,and that value is what it was at T=0--the instant the ideal voltage was placed across the ideal coil.
The current value will never increase from what it was as T=0,as the magnetic fields change over time remains at a constant value,and so the self induced EMF also remains at a constant value-->and there for,the reverse current also remains at a constant value.

It's like you have completely regressed and this entire thread never happened.

Listen, when you apply voltage across a coil, the coil integrates the voltage with respect to time and the result of the integration is current flowing through the coil.  It's like pushing on a shopping cart.

If the coil is a real coil then you have to account for the resistance.  Then it becomes a slightly more difficult problem.  Using standard mathematical techniques you solve for the circuit and you get the standard exponential equation that we all know that with a tiny bit of algebraic rearranging of some variables gives you a nice convenient time constant to work with.

That's all there is too it, a coil integrates on voltage to give you current just like a shopping cart integrates on force to give you velocity.  It's just Mother Nature in action.

All of the stuff in your head about "battling currents" is a model that simply does not work.  It's crazy talk.  It's like something that you found in a pumpkin patch.

MileHigh