MH's ideal coil and voltage question

[MileHigh]

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'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

I caved a long time ago on the resistor voltage drops and was just getting too analytical when I did a treatise on it anyway.

At this point I am easy and I am honestly pooped out on the whole thing.  Yes, Hyperphysics agrees with me, how about them apples?  I am perplexed how we were apparently not on the same page for this particular thing.

Just to toot my horn, from Hyperphysics, if R = 0, and using our terminology, then the applied EMF equals the CEMF.  I deserve a t-shirt.

MileHigh

[tinman]

I caved a long time ago on the resistor voltage drops and was just getting too analytical when I did a treatise on it anyway.

At this point I am easy and I am honestly pooped out on the whole thing.  Yes, Hyperphysics agrees with me, how about them apples?  I am perplexed how we were apparently not on the same page for this particular thing.

Just to toot my horn, from Hyperphysics, if R = 0, and using our terminology, then the applied EMF equals the CEMF.  I deserve a t-shirt.

MileHigh

If R=0, then the current will climb at a steady rate fo all time-right?--no-wrong.
If the flow of current from T=0 is going to rise at a steady state,then the apposing current generated from the CEMF will also rise at a steady state. So what dose that mean for the current induced by the applied voltage?

It's  like i said MH , there is a big difference between having a small amount of resistance,and none at all. It is the difference between having a time constant,and not having one.
It is the difference between having a current trace curve over time,and not having one.

It is the difference of having water at 1*C and ice at 0*C


Brad

[Magluvin]

Poynt already answered this stating that there is no load to dissipate the energy, but let me take a crack at it.  I am just going to give you a non-scientific seat-of-my-pants explanation.

Let's use a yo-yo as a flywheel.  So the spinning yo-yo had a shot of energy put into it from your arm.  Once it is spinning, naturally you don't question the fact that it is spinning.  You also know that you can't easily stop it from spinning, it has inertia.

So if you spin up the yo-yo, that's step one.  But then in step two, if you want to disturb the yo-yo, it's almost like the yo-yo wants to spin you.  It's almost like once you set it going, the yo-yo has a life of its own.  There is energy in the yo-yo and it has direction and magnitude.  It's hard to mess with that energy.

It's like you applied torque to the yo-yo and if you want to mess with the yo-yo it says "back off" and applies torque right back to you.

Now look at a coil.  The current flows through the wire.  The wire subtends a magnetic field around the wire into all space.  It took energy to create that magnetic field.  So that means by definition there is literally a certain number of Joules of energy in each cubic centimeter of space around the coil, and in all 3D space.

When you are right up close to the wire you see the current flowing through the wire, and the energy of the magnetic field in the space surrounding the wire.  That energy is real, it means something.  The current and the magnetic field are so tightly connected to each other that you can just about treat them as one.  That's why most people in electronics only talk about the current flow through the coil, the magnetic field is essentially the same damn thing, so you don't even have to mention the magnetic field.

So let's go back to a close up view of a wire.  You see the current flow, and you see the magnetic field wrapped around the wire.  But you can also just as easily say this:  You see the magnetic field wrapped around the wire and you see the current flow.

What are the implications:  They are as follows, and this is a thought experiment:  What if you removed the current-carrying wire and instantly replaced it with a brand new wire but with no current flowing through it?

The answer is that the very presence of the magnetic field would demand that current instantly start flowing trough the wire.  If there is a magnetic field present there must be current flow, period.

This explains why you get a shock if you cut the power to a 12-volt relay coil with your two fingers across the terminals.  You take away the source of the magnetic field, but the magnetic field is there anyways and therefore current simply must flow through the wire.

So going to a superconducting coil, obviously work is expended to create the magnetic field, and the the current flow through the coil, which is the same thing.  So when you remove the power source, the magnetic field "takes over" and current must flow through the wire.  The current flow and the magnetic field are so intimately connected it's like they are a single entity.

You spin up a heavy flywheel by hand and then if you run out of gas then the flywheel will spin your hand.  Just like if you build up a magnetic field by pushing current through a coil and then stop supplying energy, then the magnetic field will "take over" and supply the energy to keep the current flowing through the wire.

Anyway, if you get the sense of how the current flow and the magnetic field are so intimately connected that they are the same thing for all practical intents and purposes, and how energy is stored in the 3D space of the magnetic field, then maybe all of this will make sense.

MileHigh

Here is what I posted to Poynt yesterday....

"If so, then the ideal conductor doesnt require energy for electrons to be stripped from atoms and move them from atom to atom, jumping shell to shell.  I can understand the seeming inertial effects of an inductor on electrons in its conductors, but not inertial effects of moving electrons on their own. This might incur that electrons have mass. And the seeming inertial effects of electrons of the inductor are when the field is collapsing. In this situation there is no field collapse or motion of the fields at all. So what mechanism keeps the electrons flowing in the loop? What energy is 'stored' that keeps the flow going? What form is the energy stored as?"


Mags

[picowatt]

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.

No, I do not believe I am being too dogmatic.  I am, however, being quite firm with regard to referring to CEMF, particularly on a thread about inductors, as it is defined regarding inductors.

Sure, one can deduce what effect the inductor's CEMF must be having in the circuit given all the data you list.  But the voltage measured across an inductor connected to an ideal voltage source says nothing, directly, with regard to the CEMF.  The effects of the inductor's CEMF can only be observed by measuring current. 

PW   

[Magluvin]

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?

Yeah. Thats why I went to look and refresh on that stuff. Hadnt thought on that in many years.

Oh. Man they are big.  They seemed so small when I had copied them to post.

Getting into some eats n a shower here. Ill recopy and repost in a little while.

Mags