MH's ideal coil and voltage question

[Grumage]

" In saying that,do you know how to cut Tungsten carbide ? "

Dear Brad.

You can use any Diamond edged cutting disc and plenty of water.

One of those motor powered ceramic tile cutters should do the trick.

Cheers Grum.

[MileHigh]

During the course of the rioting more than 1000 oscilloscopes were destroyed.

[picowatt]

Regarding the Emf (applied voltage) equaling the Cemf (induced counter-emf) in an inductor with the analogy of an amplifier with feedback raises some questions IMO. Having designed, built, and manufactured many types of amplifiers with various types of feedback loops thru the years, they all have a common summing point somewhere in the circuitry. This summing point is actually a current node where the input and inverted output voltages are converted to opposing currents in ratios to yield an overall amplifier gain that one desires. The higher the open loop gain, the more accurate the designed gain.

This all seems to fit our Emf=Cemf situation at first glance but I would like to know "where is the summing point" in a single inductor? If it exists it should be able to be shown mathematically.

We know that Emf (applied voltage to a coil) = L*dI/dt as we can easily prove this with experimentation. We can rearrange the formula to solve for any unknown variable with the remaining known factors, so we are again assured this formula is correct.

We also know that Lenz dictates that with an "induced" current, Cemf = -L*dI/dt and this again can be proven experimentally.

What is difficult for me to accept and understand is to say the the Emf=Cemf in a single inductor with a voltage applied for if they are equal mathematically, they cancel mathematically and no current flows. I have to agree with TM on this. I would be happy to see mathematical proof to the contrary.

Partzman,

I would have thought anyone familiar with the concept of negative feedback would realize that my step wise description of the action of an inductor's CEMF was actually a smooth and continuous, self-regulating process.

Looking at slices in time, I could as well describe the action of the feedback resistor of an amplifier, emitter degeneration, or various other electronic, chemical, or mechanical feedback mechanisms in a step wise fashion, even though they may actually be a smooth and continuous process.

Regarding where the "summing junction" is will depend on your measurement reference point.  The rate of current change dependent voltage that is the inductor's CEMF, appears across the inductor's terminals and is "summed with" the EMF applied across those terminals.

The two formulae you posted exactly describe the feedback mechanism related to CEMF.  As well, both you and Tinman are indeed correct when you state that if the CEMF equals the applied EMF, no current will flow.

However, (in the 4V across 5H example discussed) the only time the CEMF is equal to the applied EMF of 4 volts is when the RATE OF CHANGE of the current flowing thru the inductor is .8 amps per second.

If, for example, the rate of current change were to become even slightly less than .8 amps per second, the generated CEMF would also become less than 4 volts, which would allow the current flow to increase until the .8 amps per second rate of change is again achieved.  It is this feedback mechanism that regulates the rate of change of the current flowing thru the inductor to be .8 amps per second.

Although discussed in a rather step-wise manner, like the negative feedback in an amplifier or the degeneration of an emitter, it is a smooth and continuous process.

PW

[minnie]

    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.

[partzman]

Partzman,

I would have thought anyone familiar with the concept of negative feedback would realize that my step wise description of the action of an inductor's CEMF was actually a smooth and continuous, self-regulating process.

Looking at slices in time, I could as well describe the action of the feedback resistor of an amplifier, emitter degeneration, or various other electronic, chemical, or mechanical feedback mechanisms in a step wise fashion, even though they may actually be a smooth and continuous process.

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

Regarding where the "summing junction" is will depend on your measurement reference point.  The rate of current change dependent voltage that is the inductor's CEMF, appears across the inductor's terminals and is "summed with" the EMF applied across those terminals.

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?

The two formulae you posted exactly describe the feedback mechanism related to CEMF.  As well, both you and Tinman are indeed correct when you state that if the CEMF equals the applied EMF, no current will flow.

However, (in the 4V across 5H example discussed) the only time the CEMF is equal to the applied EMF of 4 volts is when the RATE OF CHANGE of the current flowing thru the inductor is .8 amps per second.

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?

If, for example, the rate of current change were to become even slightly slower than .8 amps per second, the generated CEMF would also become less than 4 volts, which would allow the current flow to increase until the .8 amps per second rate of change is again achieved.  It is this feedback mechanism that regulates the rate of change of the current flowing thru the inductor to be .8 amps per second.

Although discussed in a a rather step-wise manner, like the negative feedback in an amplifier or the degeneration of an emitter, it is a smooth and continuous process.

PW

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