MH's ideal coil and voltage question

[tinman]

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

Indeed Poynt.
And this is why i tried so hard at the start of this thread to get you guys to take into account,that the infinite time constant plays a vital roll in MHs question.

If the CEMF is what limits the rise time of current flow,and the value of that CEMF is a constant--》what will be the value of current flow from T=0 throughout time ?.
The current can only rise in value over time if the value of the CEMF falls over time.


Brad

[poynt99]

PW hasn't acknowledged it (or perhaps not read my post), but we are saying the same thing. In short, that the cemf is not the voltage you would measure across the terminals when the inductor is connected to a voltage source, and the actual cemf is effectively shorted out by it.

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.

I'm not sure why you think I am disagreeing with you on this. Did you misunderstand my posts?

Anyway, although it doesn't make sense to me now, I am going to assume you are correct MH. Hyperphysics does seem to support you (when R=0, V=Emf), so until someone can prove it wrong, I'll go along with it.

As for the use of "cemf" for resistor voltage drops? Sorry, I'm pretty firm on that one

[MileHigh]

PW:

My comment:

"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."

Your reply:

"If an inductor is connected across an ideal V source, the voltage measured across that inductor says nothing about the inductance, DCR, or CEMF of that inductor.

Again, consider your black box.  Who knows what was put in there?  Measuring current will be more informative."

--------------------

If the setup is simple, ideal voltage source, ideal coil, you know the inductance, and you know the initial conditions, aren't you being too dogmatic here?

In the simple setup then the CEMF has to be equal to the ideal voltage source.  You know the inductance so you know the current.

Anyway, I am just about all pooped out on this stuff.

MileHigh

[MileHigh]

Had not looked at this stuff for a long time, but was looking at the element chart.  I found that all conductors we typically see only have 1 electron in the outer shells, like gold, silver, copper, etc, and as I went down the line as in other elements that were metals, that outer shell tends to have more electrons in the and they are less conductive. Still looking into that stuff. And it was long ago that I knew this stuff and it wasnt like we spent weeks or months on the subject. Interesting stuff

The last one is interesting. I wonder if it has better conduction than gold?

Mags

I think we were even taught that in General Science class about the "loose electrons" in the outer shell of the metals.  Also I believe the luster or shine to metals comes comes from the outer shell electrons bouncing up and down in response to external EM radiation (light) and re-emitting that radiation.

Can you resize the giant images?

[tinman]

Not sure, but I think that was what Brad was saying a while back, that a straight ideal wire may not even have a field outside the wire itself as the cemf would keep it in check.

Mags

Indeed.
And the same applies to the ideal coil with the ideal voltage placed across it.

Most all here fell for MHs wrong conclusion that an infinite time constant changes nothing,and a small value of resistance has very little effect on the outcome.

If the time constant is infinite,then the current will rise at a steady rate throughout time. This means that the CEMF that is acting against the induced current is also at a steady state.

So now,what will the induced current value be from T=0 ?.

Brad