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Author Topic: MH's ideal coil and voltage question  (Read 484406 times)

minnie

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Re: MH's ideal coil and voltage question
« Reply #315 on: May 14, 2016, 09:19:36 AM »



  tinman, did you study Lewin, Kirchoff for the birds, striped shirt about
  16 mins YouTube?
          John.
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tinman

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Re: MH's ideal coil and voltage question
« Reply #316 on: May 14, 2016, 10:30:51 AM »
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tinman

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Re: MH's ideal coil and voltage question
« Reply #317 on: May 14, 2016, 10:32:50 AM »
Quote from: poynt99 on May 14, 2016, 04:40:35 AM
Is this loop current steady, or is it continuously-varying with time?

To be more accurate,lets say the current is increasing at 800mA a second.

Brad
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tinman

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Re: MH's ideal coil and voltage question
« Reply #318 on: May 14, 2016, 11:03:15 AM »
Quote from: minnie on May 14, 2016, 09:19:36 AM


  tinman, did you study Lewin, Kirchoff for the birds, striped shirt about
  16 mins YouTube?
          John.

I guess you missed the whole thread dedicated to this very subject that Poynt started.

The total sum of the voltage around that loop = 0,and Kirchoffs law hold's.

So now you have to decide whether you believe Lewin or Poynt?

If lewin is correct,and Kirchoffs law dose not hold,then im screwed.
If Poynt is correct,then my statement also remains correct,in that there is no potential difference that can be measured at any two points across that looped ideal coil. Also remember there is no resistance value in the wire,and so the two resistors in Lewins experiment are omitted.

Below is a circuit showing a real world inductor.
Using this circuit,turn it into an ideal inductor that is looped(shorted)
Lets see if you can do that John.


Brad
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verpies

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Re: MH's ideal coil and voltage question
« Reply #319 on: May 14, 2016, 01:45:56 PM »
Quote from: MileHigh on May 14, 2016, 12:38:56 AM
When you think about that hard you realize that one Henry is one Volt-Second per Amp.
That is true, too.  This is a very useful relation for building electronic circuits.  But note, that it involves 4 variables.

For this reason, I do not consider the above relation to be most basic, like the relation of one Henry being one Weber per Amp.
Note that it involves 3 variables. It is also less useful for building electronic circuits - albeit not so for building motors.

Also, notice that it is impossible to have Amps without Webers but it is possible to have Volts without Amps.
This lack of difference makes the 3 variable relation more basic and coherent than the 4 variable relation.
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verpies

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Re: MH's ideal coil and voltage question
« Reply #320 on: May 14, 2016, 02:09:21 PM »
Quote from: tinman on May 14, 2016, 02:33:52 AM
An "ideal inductor" has inductance, but no resistance or capacitance, and does not dissipate or radiate energy.
But it stores energy in the form of magnetic flux.  Because of this an inductor can have reactance even of it does not have resistance

Quote from: tinman on May 14, 2016, 02:33:52 AM
An ideal voltage source is a voltage source that supplies constant voltage to a circuit despite the current which the circuit draws.
This means that despite the resistance which a load may be in a circuit, the source will still provide constant and steady voltage.
An ideal voltage source has the following characterstic that allows it to act as a 100% efficient source of voltage: it has zero internal resistance.
That's quite true

It is impossible to connect such voltage source across a shorted ideal inductor, because in such case  this voltage source would see a load, which does not have any resistance nor reactance

However it is possible to connect such voltage source in series with an ideal inductor.  In this case the inductor becomes shorted by the zero internal resistance of the ideal voltage source and this voltage source sees a load, that does not have any resistance but has reactance.

Quote from: tinman on May 14, 2016, 02:33:52 AM
Why a voltage cannot exist across a shorted ideal inductor that has current flowing through it>
Because there is no place to place the voltmeter probes on a shorted ideal inductor ...in practice as well as in theory.

Quote from: tinman on May 14, 2016, 02:33:52 AM
...and the current flowing through that ideal inductor is the same at any two points of that inductor.
And there is no voltage drop anywhere along the ideal shorted inductor even if there is current flowing through it.
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verpies

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Re: MH's ideal coil and voltage question
« Reply #321 on: May 14, 2016, 02:15:58 PM »
Quote from: webby1 on May 14, 2016, 02:59:15 AM
There could be voltage that you can not measure or interact with because all the way around the loop the voltage would be uniform, so current flow but no way to measure the voltage.
And in such case voltage becomes just a postulate ...since you think it's there but cannot measure it.
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verpies

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Re: MH's ideal coil and voltage question
« Reply #322 on: May 14, 2016, 02:24:23 PM »
Quote from: tinman on May 14, 2016, 03:13:43 AM
Having no internal resistance,the current flow is in no way impeded ,an so that is what makes it an !ideal! voltage source.
At T=0,the ideal voltage source is connected to the ideal inductor-->what have you just done?
Indeed the ideal voltage source has no internal resistance nor reactance.
However an ideal inductor has reactance even if it does not have resistance.
Impedance = Resistance + Reactance

That's why if an ideal voltage source is connected in series with an ideal inductor, then the current will be impeded by its reactance despite not being impeded by its resistance.

This is the reason why a simple Ohm's law i=V/R is not applicable to calculating current in an L circuit, as it totally disregards the reactance (half of the complex impedance).
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verpies

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Re: MH's ideal coil and voltage question
« Reply #323 on: May 14, 2016, 02:27:50 PM »
Quote from: poynt99 on May 14, 2016, 03:36:42 AM
What is there that is beyond a voltage source and an inductor in series/parallel?
It seems that you are conflating the series connection of the voltage source with an inductor, with a parallel connection (across a shorted inductor) stipulated by Tinman.
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verpies

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Re: MH's ideal coil and voltage question
« Reply #324 on: May 14, 2016, 02:32:14 PM »
Quote from: picowatt on May 14, 2016, 03:52:05 AM
As long as the current flowing is increasing or decreasing over time, the answer to both is yes.
I disagree.
Current can vary in an ideal shorted* inductor only when the flux penetrating it is varied externally somehow.

But despite that variance, there is no place to connect a voltmeter and measure the voltage or voltage drop.
...and presto!, you have a voltageless current - finite current flowing in zero resistance outside of the confines of the Ohm's law.



*  (shorted by an ideal wire, not by an ideal voltage source)
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partzman

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Re: MH's ideal coil and voltage question
« Reply #325 on: May 14, 2016, 02:53:21 PM »
Brad,

I am curious as to why you wish to apply an ideal voltage to a shorted ideal coil in reference to MH's original question? IMO, it really has no bearing. Perhaps this idea stems from the second condition of the question where zero volts is applied for 2 secs!?

In regards to an ideal voltage source not changing, we must qualify the change.  Assume for a moment that I am the creator of an ideal voltage source.  I am free from all known laws to set the voltage level at any magnitude I choose. The magnitude I choose however will not change with any attached load but I am still free to change the magnitude at any given time I wish. The output is still unable to change with any load variation.  This is the ideal voltage source MH used in his question.

partzman
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picowatt

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Re: MH's ideal coil and voltage question
« Reply #326 on: May 14, 2016, 02:58:58 PM »
Quote from: verpies on May 14, 2016, 02:32:14 PM
I disagree.
Current can vary in an ideal shorted* inductor only when the flux penetrating it is varied externally

Which is what I stipulated, that the current flowing had to vary over time to be able to measure a voltage as per the question.

Quote

But despite that variance, there is no place to connect a voltmeter and measure the voltage or voltage drop.



*  (shorted by an ideal wire, not by an ideal voltage source)


Consider a series string of ideal inductors connected into a loop with a shorting wire.  If you induce a time varying current into the loop, would there not be a measurable voltage drop between the inductors due to the reactance of those inductors? 

PW
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verpies

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Re: MH's ideal coil and voltage question
« Reply #327 on: May 14, 2016, 03:01:37 PM »
Quote from: MileHigh on May 14, 2016, 04:06:01 AM
I think this is an example of the root cause of one of Brad's problems:
<<< Why can a voltage not exist across a shorted ideal inductor that has current flowing through it>
Because V=IxR,and R=0  >>>
He keeps on going back to Ohm's law for an inductor.
Yes, the Ohm's law is not applicable to inductors because it totally disregards the reactance of the inductor.
Resistance is only one half of the total Impedance.  It actually is the reason why we have all these words to describe it.
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poynt99

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Re: MH's ideal coil and voltage question
« Reply #328 on: May 14, 2016, 03:04:15 PM »
Quote from: tinman on May 14, 2016, 10:32:50 AM
To be more accurate,lets say the current is increasing at 800mA a second.

Brad

It doesn't matter really. All inductors, whether ideal or real will have a voltage across them when there is current through them, regardless if the current is changing or not.
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poynt99

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Re: MH's ideal coil and voltage question
« Reply #329 on: May 14, 2016, 03:08:32 PM »
Quote from: verpies on May 14, 2016, 02:27:50 PM
It seems that you are conflating the series connection of the voltage source with an inductor, with a parallel connection (across a shorted inductor) stipulated by Tinman.
I'm not referring to tinman's circuit, but the original one stipulated by MH.
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