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Author Topic: MH's ideal coil and voltage question  (Read 477582 times)

verpies

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Re: MH's ideal coil and voltage question
« Reply #180 on: May 10, 2016, 02:47:10 PM »
However it is possible to externally change the magnetic flux penetrating a shorted ideal inductor. Doing so will instantaneously cause a current to circulate through it *, in order to maintain the previous flux level penetrating its windings.  This is a voltageless current! - it cannot be measured by a voltmeter and it was not caused by a voltage source.
Better yet, the current induced in the above scenario is independent of the rate of change of flux (dΦ/dt) penetrating that inductor.

On the other hand, the emf induced across an open inductor is not independent of  the rate of change of flux (dΦ/dt) penetrating that inductor.

poynt99

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Re: MH's ideal coil and voltage question
« Reply #181 on: May 10, 2016, 03:37:54 PM »
Brad,

To be clear, I am in agreement with MH. It makes sense to me now, and apparently when I answered the question years back on OUR, I also got the answer correct.

Regarding my simulation, when using such small resistance values without changing some settings in SPICE (LT Spice must already be set to handle this), the simulation engine runs out of computational precision, which is why it "flatlines" with very low values. When the value is too low, the sim runs out of gas and starts making gross approximations, which is evident below with R=1f Ohm. But you do see that it is honing in on the 2.4A value? Any smaller in value and the trace just flatlines.

For an ideal inductor, yes Tau is infinite, but this has does not preclude current flow through the inductor. If however the inductance was an unrealistically large value like 1 million Henries, then yes the current would essentially be zero for a relatively long duration of time. That was my confusion. Tau simply determines the rise time, and since it is infinitely long, the trace becomes a nice straight line rather than the curve we normally see.

I was wrong in my analysis, but it is clear to me now.

tinman

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Re: MH's ideal coil and voltage question
« Reply #182 on: May 10, 2016, 03:47:09 PM »
Better yet, the current induced in the above scenario is independent of the rate of change of flux (dΦ/dt) penetrating that inductor.

On the other hand, the emf induced across an open inductor is not independent of  the rate of change of flux (dΦ/dt) penetrating that inductor.

That is because one is closed,and the other open.

If-as in the question i posted in my last post,the inductor is a closed loop,in that it has an ideal voltage source that completes the loop. Being an !ideal! voltage source,it too must be void of resistance and resistive losses,and so becomes part of the ideal inductor.

So now that the inductor and voltage source are a complete ideal loop,void of resistance,across no two points in that loop can a voltage exist.
And hence,once again,you cannot place a voltage across an ideal inductor,when current is flowing through that closed inductor loop.


Brad

So the question asked is referring to a closed loop scenario.

tinman

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Re: MH's ideal coil and voltage question
« Reply #183 on: May 10, 2016, 03:55:16 PM »
Brad,

To be clear, I am in agreement with MH. It makes sense to me now, and apparently when I answered the question years back on OUR, I also got the answer correct.

Regarding my simulation, when using such small resistance values without changing some settings in SPICE (LT Spice must already be set to handle this), the simulation engine runs out of computational precision, which is why it "flatlines" with very low values. When the value is too low, the sim runs out of gas and starts making gross approximations, which is evident below with R=1f Ohm. But you do see that it is honing in on the 2.4A value? Any smaller in value and the trace just flatlines.

For an ideal inductor, yes Tau is infinite, but this has does not preclude current flow through the inductor. If however the inductance was an unrealistically large value like 1 million Henries, then yes the current would essentially be zero for a relatively long duration of time. That was my confusion. Tau simply determines the rise time, and since it is infinitely long, the trace becomes a nice straight line rather than the curve we normally see.

I was wrong in my analysis, but it is clear to me now.

Rise time is an actual value,not a trace on a scope.
Is verpies also wrong? Quote: Since an ideal inductor must have a zero resistance, this means that it must be shorted (if it ain't shorted, it ain't ideal) and it becomes physically impossible to connect any real voltage sources in series with it.

Otherwise, I agree with the above statement.  Not only an ideal inductor is devoid of an asymptotic V/R current limit but also the current through an inductor of infinite inductance, that is somehow connected to an ideal voltage source, could never change because of the implied zero di/dt at any voltage
.
How long will it take for the current to reach it's peak in an ideal inductor(regardless of inductance value),when that inductor has no resistance,and is supplied with an ideal voltage?

Once you have this value,you can divide it by 5 to obtain your time constant for the rise of current in that ideal inductor. Mh is using math that applies to an inductor on the understanding that that inductor will reach a maximum current value in a finite time. This cannot be applied to an ideal inductor,where the current will never reach a maximum value when an ideal voltage is placed across it.


Brad

allcanadian

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Re: MH's ideal coil and voltage question
« Reply #184 on: May 10, 2016, 05:48:33 PM »
 . . . “Mad Hatter: “Why is a raven like a writing-desk?”
 “Have you guessed the riddle yet?” the Hatter said, turning to Alice again.
“No, I give up,” Alice replied: “What’s the answer?”
 “I haven’t the slightest idea,” said the Hatter”

poynt99

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Re: MH's ideal coil and voltage question
« Reply #185 on: May 10, 2016, 08:27:35 PM »
Brad,

I don't understand what verpies is trying to say, so I can't say if he is in agreement with MH or not.

You could ask him.

Magneticitist

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Re: MH's ideal coil and voltage question
« Reply #186 on: May 10, 2016, 08:59:43 PM »
hmm, I thought the EMF followed current. It's the electric field that's the precursor. No EMF if the current is not allowed to change. Electric field see's whats going on the entire time tho.



- question:

is a rise in current a change in current, or not?


the inverse exponential curve to straight line makes perfect sense though..
that is, if current were allowed to flow or change. we have to assume that
is the case first.

*unlike a capacitor, imaginarily we have started at source voltage in our scenario across the inductor. well, again, unlike the capacitor, mission accomplished. no need for current flow because no lenz effect.

verpies

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Re: MH's ideal coil and voltage question
« Reply #187 on: May 10, 2016, 11:18:14 PM »
is a rise in current a change in current, or not?
It is

allcanadian

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Re: MH's ideal coil and voltage question
« Reply #188 on: May 10, 2016, 11:18:46 PM »
Magneticitist
Quote
hmm, I thought the EMF followed current. It's the electric field that's the precursor. No EMF if the current is not allowed to change. Electric field see's whats going on the entire time tho.

You may want to rethink your statement. The Electro-Motive Force is the force which causes the charges to move and this motion is called an electric current. The EMF is a Force on the charges and without a force acting on something it has no reason to move.

I cannot even begin to explain how completely messed up your post is... review Coulombs Law, Faradays Law, Electron flow notation, an Electro-Motive Force (Emf).

AC

Magneticitist

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Re: MH's ideal coil and voltage question
« Reply #189 on: May 10, 2016, 11:54:12 PM »
I don't understand what you mean. I'm saying EMF is a result of current. Well, 'result of' is not quite what I mean but it's hard for me to word it otherwise.

How can there be an electromagnetic field without current? There is however an electric field, the voltage.


-maybe I have this wrong but I think you may be missing that we are essentially saying the same thing. damn maybe MH is right and I need to start honing my word choices but can see how I was confusing EMF magnetic field, but I thought I was being clear by also mentioning the electric field as being the established circuit voltage. I believe someone else put it, a currentless voltage.

Pirate88179

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Re: MH's ideal coil and voltage question
« Reply #190 on: May 11, 2016, 12:13:14 AM »
I don't understand what you mean. I'm saying EMF is a result of current. Well, 'result of' is not quite what I mean but it's hard for me to word it otherwise.

How can there be an electromagnetic field without current? There is however an electric field, the voltage.


-maybe I have this wrong but I think you may be missing that we are essentially saying the same thing. damn maybe MH is right and I need to start honing my word choices.

EMF=Electro-Motive Force not electromagnetic field...maybe this will help your confusion?

Bill

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Re: MH's ideal coil and voltage question
« Reply #191 on: May 11, 2016, 12:14:15 AM »
EMF=Electro-Motive Force not electromagnetic field...maybe this will help your confusion?

Bill

you are right thank you I was just realizing that

(i will admit I really need to brush up on what to call certain things MH! but in theory I feel
I understand what is going on.. for example I want to call the voltage the electric field in nature when that's not even correct after checking)

Pirate88179

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Re: MH's ideal coil and voltage question
« Reply #192 on: May 11, 2016, 12:19:59 AM »
you are right thank you I was just realizing that

(i will admit I really need to brush up on what to call certain things MH! but in theory I feel
I understand what is going on.. for example I want to call the voltage the electric field in nature when that's not even correct after checking)

No problem.  This can be very confusing...and...just when I think I am starting to understand something...I find out that I really am not, ha ha.

Bill

Magneticitist

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Re: MH's ideal coil and voltage question
« Reply #193 on: May 11, 2016, 12:27:18 AM »
No problem.  This can be very confusing...and...just when I think I am starting to understand something...I find out that I really am not, ha ha.

Bill

it is indeed very confusing and hard to convey thoughts without looking like an idiot if you're like
me and have not dedicated enough time to properly learning the right terminology for everything.
but in an even harder way to explain it seems like this argument involves a basic paradox that
was at least evident enough to warrant many pages of debate. at the end of it all while I personally
struggle to convey my thoughts, I just think the damn thing will do absolutely nothing. Sit at source voltage and act as if it wasn't even there to begin with.

verpies

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Re: MH's ideal coil and voltage question
« Reply #194 on: May 11, 2016, 12:52:16 AM »
I thought the EMF followed current.
An EMF measured across an open inductor is proportional to the rate of change of flux penetrating that inductor (dΦ/dt).
EMF measured across an inductor is not proportional to the current flowing through it and in an open inductor the current cannot flow at all.

It's the electric field that's the precursor.
Precursor of what?