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### Author Topic: MH's ideal coil and voltage question  (Read 449081 times)

#### tinman

• Hero Member
• Posts: 5363
##### Re: MH's ideal coil and voltage question
« Reply #120 on: May 09, 2016, 08:49:28 PM »
Zero current means no load. So we have an infinite load, i.e. an "open" circuit.

Ok,im going to need help understanding this Poynt

The zero current meaning no load is no problem,but having an infinite load(a load of infinite proportions),would this not require a current flow of infinite magnitude?,and we have no current flow.

#### poynt99

• TPU-Elite
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• Posts: 3582
##### Re: MH's ideal coil and voltage question
« Reply #121 on: May 09, 2016, 08:52:56 PM »
MH,

For your test (13s of a few different voltages), the different voltages have no affect on the outcome.

So the question could have been asked with one voltage and still have the same answer, correct?

I did the simulation with 0.0000000001f (femto) Ohms, which is very very small, and ran it for 100s, far exceeding the limit of your test. The current stayed flat at 0A for the full 100s. Needless to say the voltage across the coil also remained at 4V for the entire 100s.

I think we can conclude from these results that my answer is correct.

#### tinman

• Hero Member
• Posts: 5363
##### Re: MH's ideal coil and voltage question
« Reply #122 on: May 09, 2016, 08:53:58 PM »
I am going to have to assume that you don't understand the question.  Before t=0 the voltage source and the coil are disconnected.  The only reason for putting it that way is to implicitly establish that the current through the coil is zero at the start of the test.  At t=0 you then have a 4-volt source connected to a 5-Henry inductor, and then the voltage varies in time as described.

When it is reworded as above, is it clear now?

Im going to have to correct that MH.
At T=0,an ideal voltage of 4 volts is placed across an ideal inductor of 5 Henrys for 3 seconds. This is how your question reads.

To quote again,An ideal voltage source is a two-terminal device that maintains a fixed voltage drop across its terminals. It is often used as a mathematical abstraction that simplifies the analysis of real electric circuits.

#### tinman

• Hero Member
• Posts: 5363
##### Re: MH's ideal coil and voltage question
« Reply #123 on: May 09, 2016, 08:58:00 PM »
MH,

For your test (13s of a few different voltages), the different voltages have no affect on the outcome.

So the question could have been asked with one voltage and still have the same answer, correct?

I did the simulation with 0.0000000001f (femto) Ohms, which is very very small, and ran it for 100s, far exceeding the limit of your test. The current stayed flat at 0A for the full 100s. Needless to say the voltage across the coil also remained at 4V for the entire 100s.

I think we can conclude from these results that my answer is correct.

Poynt

Thank you for your time on this,i am really enjoying this topic.
But i have to get some sleep--work in 3 hour time

I hope we can look into this further together,along with anyone else that is interested.

Cheers

#### poynt99

• TPU-Elite
• Hero Member
• Posts: 3582
##### Re: MH's ideal coil and voltage question
« Reply #124 on: May 09, 2016, 09:00:13 PM »
Ok,im going to need help understanding this Poynt

The zero current meaning no load is no problem,but having an infinite load(a load of infinite proportions),would this not require a current flow of infinite magnitude?,and we have no current flow.

Don't get too hung up on my use of the term "infinite load". In electronics when we say that we mean an "open circuit", not a "short circuit". I know it seems counter intuitive. If we mean to say a "short", we say "short, "short circuit" or "zero Ohm" load.

#### MileHigh

• Hero Member
• Posts: 7600
##### Re: MH's ideal coil and voltage question
« Reply #125 on: May 09, 2016, 09:00:26 PM »
MH,

For your test (13s of a few different voltages), the different voltages have no affect on the outcome.

So the question could have been asked with one voltage and still have the same answer, correct?

I did the simulation with 0.0000000001f (femto) Ohms, which is very very small, and ran it for 100s, far exceeding the limit of your test. The current stayed flat at 0A for the full 100s. Needless to say the voltage across the coil also remained at 4V for the entire 100s.

I think we can conclude from these results that my answer is correct.

Poynt, I am really surprised and I suspect that you are a victim of GIGO from your simulator.  You put in a resistance value that was too low for the sim and it got scrambled brains. (a bug!)  That affected you also I am afraid!  lol  I almost suspect you may have worked 36 hours non-stop or something.

Hold on, I will find the posting number of another sim also.  ===>>> Please look at post #79.

#### Magneticitist

• Full Member
• Posts: 131
##### Re: MH's ideal coil and voltage question
« Reply #126 on: May 09, 2016, 09:04:07 PM »
wait, are we now saying that the answer MH was looking for, was to clarify that no current would flow and it would act like an open circuit?
this is confusing because I recall at least a couple of us saying that and it was some ol whackadoo or crazy stuff, or whatever.

edit- nevermind I see MH is in disagreement, now idk what is going on

#### MileHigh

• Hero Member
• Posts: 7600
##### Re: MH's ideal coil and voltage question
« Reply #127 on: May 09, 2016, 09:11:42 PM »
wait, are we now saying that the answer MH was looking for, was to clarify that no current would flow and it would act like an open circuit?
this is confusing because I recall at least a couple of us saying that and it was some ol whackadoo or crazy stuff, or whatever.

edit- nevermind I see MH is in disagreement, now idk what is going on

It could be sleep deprivation, you never know.  It's like the old cliche that when you factor in all of the rewrites and recompiles and bug fixes, a software engineer can only write 10 lines of code a day.

However, the other issue is that we are getting way off the beaten track.  We still haven't really scratched the surface when it comes to actually understanding the how and why of what is going on in this trivial circuit.  I suppose we need to actually get it answered also.  The how and why is the real point in this whole exercise.

#### poynt99

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##### Re: MH's ideal coil and voltage question
« Reply #128 on: May 09, 2016, 09:18:57 PM »
Poynt, I am really surprised and I suspect that you are a victim of GIGO from your simulator.  You put in a resistance value that was too low for the sim and it got scrambled brains. (a bug!)  That affected you also I am afraid!  lol  I almost suspect you may have worked 36 hours non-stop or something.

Hold on, I will find the posting number of another sim also.  ===>>> Please look at post #79.
I saw that post.

His R value is too big. I can obtain similar results as I increase the resistor value. 1p Ohm is nowhere near 0 Ohms. Have Partzman try 0.00000000000000000000000000000000000001f Ohms. He will see a flatline of current (0A) for at least 100 seconds.

I did work this out in my head first, and the sim supports my conclusion.

#### MileHigh

• Hero Member
• Posts: 7600
##### Re: MH's ideal coil and voltage question
« Reply #129 on: May 09, 2016, 09:25:57 PM »
Well now I feel like I am in the Twilight Zone.

Poynt:  The current is one over "L" integral v dt.

That's 1/5 * integral (4) dt.

That's 1/5 * 4t.

That's 4/5*t.

When t = 3 seconds that's 12/5 = 2.4 amps.

Can we get out of the Zone now?

#### minnie

• Hero Member
• Posts: 1244
##### Re: MH's ideal coil and voltage question
« Reply #130 on: May 09, 2016, 09:27:13 PM »

What does old Lewin think?
YT  Kirchoffs rule is for the birds. Stripey shirt 16.01.

#### poynt99

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##### Re: MH's ideal coil and voltage question
« Reply #131 on: May 09, 2016, 09:47:12 PM »
Perhaps the sim skids out at these lower R settings, and defaults to 0 rather than crashing as it does with no resistor at all.

It does seem to settle at the 2.4A point when R is relatively small though. So, yeah it must be right.

It doesn't makes sense to me from the infinite tau perspective though.

#### MileHigh

• Hero Member
• Posts: 7600
##### Re: MH's ideal coil and voltage question
« Reply #132 on: May 09, 2016, 10:10:09 PM »
The Tau business is fairly simple to explain.

When you transition from a finite Tau to an infinite Tau the current waveform goes from an inverse exponential curve to a straight line.  Note that it is a straight line with a constant slope of V/L.

So Tau at infinity just means the current trace is a perfectly straight line.  Since it is a straight line the concept of "reaching 63% of the maximum value" does not apply anymore because that concept does not exist when the current waveform is a perfectly straight line.  i.e.; "There is no time constant."

So Tau being infinity does not mean stopping current flow, it means linearly increasing current flow.

Since we are discussing limits, the only possible way for the current to flat-line at zero "forever" would be for the inductance to be infinity.  Then you have a "more real" Tau = infinity because this time L/R becomes infinity/R.

So when Tau = infinity/R that gives you the horizontal current trace stuck at zero with a slope of zero (V/infinity), whereas when Tau = L/0, you get a current trace that is a straight line with with a slope of V/L.

#### poynt99

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##### Re: MH's ideal coil and voltage question
« Reply #133 on: May 09, 2016, 10:14:16 PM »
Thanks for the explanation MH.

#### minnie

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• Posts: 1244
##### Re: MH's ideal coil and voltage question
« Reply #134 on: May 09, 2016, 10:18:01 PM »

My idea was to stick in a series resistor of 1-66666666 Ohms.
John.