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### Author Topic: MH's ideal coil and voltage question  (Read 449080 times)

#### tinman

• Hero Member
• Posts: 5363
##### Re: MH's ideal coil and voltage question
« Reply #90 on: May 09, 2016, 07:28:10 PM »

Now, replace the ideal inductor with an ideal discharged capacitor, what is the outcome at t=0?

Do you still think with the inductor scenario current instantly going to infinity is a possibility?

Quote
Yes, nothing will happen with the ideal inductor, i.e. it will have 4V (or whatever the voltage is at any point in time) across it and zero current through it for ever and ever. (You can't have both answers.)

Ok,in this point in time,i will provide my real world answer,along with my !two! theoretical answers.

My real world answer is(and has been throughout this thread)that you cannot place an ideal voltage across an ideal inductor,as an ideal inductor dose not -and never will exist.

Theoretical answer 1--> As R = 0,then the L/R time constant is infinite-as you have just stated is correct Poynt. The first thing this means is that the question asked by MH with the values given in that question,results in nothing at all happening--as you also just stated would be the situation.

What we now need to do to understand my second theoretical answer, is define the properties of an ideal inductor. An ideal inductor has no resistance or capacitance,but only inductance. This is now when we find out that the ideal inductor has no inductance at all-regardless of the inductance value of that ideal inductor. For inductance to occur,current must flow into that inductor,and as we have both just established,no current will into that inductor ,regardless of the voltage across that ideal inductor,or the time the voltage is across that inductor,as the current rise time is infinite,and there for there is an infinite time before current starts to flow-->there is no current flowing into that inductor-ever,as we know know.

As we have no capacitance value,then the voltage across the inductor is not stored across the inductor,so this is omitted. But what we do have,due to there being no resistance,is a dead short across the inductors terminals,which brings rise to my theoretical answer 2.

Theoretical answer 2-->. As it has now been concluded that there is no current flowing into the ideal inductor,then there is no inductance taking place. We are now left with an ideal voltage across a dead short,due to the zero value of resistance. This leads to an instant current rise to an infinite value due to the ideal voltage now being across a dead short. I will define instant being the ideal speed at which current can flow,that being the speed of light.

So you see now why i called this a conundrum theory.
But what these two theories prove,is my real world answer,and that is,as you know-->that you cannot place an ideal voltage across an ideal inductor,as an ideal inductor dose not -and never will exist.

There is also a second conundrum,and that being that we all agree that a voltage cannot exist across an ideal inductor that has a steady DC current flowing through it. If an ideal inductor,with an ideal voltage placed across it,never has current flowing into or through it,how did the steady state current get induced into the ideal inductor,that dose not allow a voltage to be seen across it?.

We have worked out that current will never flow into an ideal inductor,and now all we have to do is work out the rest of what i posted--or should i say Poynt,have your opinion toward what i have posted above.

#### tinman

• Hero Member
• Posts: 5363
##### Re: MH's ideal coil and voltage question
« Reply #91 on: May 09, 2016, 07:35:23 PM »

You have actually been given part of the answer, and you see that you were dead wrong.  Turn that into a learning experience.

These are the two challenges for you:

1.  Brad gets up the learning curve and understands the original question and then answers it correctly all by himself and clearly demonstrates that he understands what he is doing.
2.  Brad admits that he is wrong when he stated that my response to the harder question is wrong.

I will repost my answer to the harder version of the question on this thread.

MileHigh

MH
Did you not see Poynts post (80)
Quote: Yes, nothing will happen with the ideal inductor, i.e. it will have 4V (or whatever the voltage is at any point in time) across it and zero current through it for ever and ever.

This in itself says that the results to your question will be-nothing happens. It also shows that one of my theoretical answers was correct,and my real world answer was also correct--when you read my last pots,you may start to understand the complications with your !original! question.

To quote your post 69-- Sorry but I have to lol, when the "answer" is two completely contradictory answers.,i think once again,you have laughed at me to soon.

#### tinman

• Hero Member
• Posts: 5363
##### Re: MH's ideal coil and voltage question
« Reply #92 on: May 09, 2016, 07:42:03 PM »
Here is the harder version of the question and the answer:

You have an ideal voltage source and an ideal coil of 5 Henrys.  At time t=0 seconds the coil connects to the ideal voltage source.  The voltage source waveform is 20*t^2.  So as the time t increases, the voltage increases proportional to the square of the time.

The question is what happens starting at t = 0

The current through the ideal coil starts from zero at time t = 0 and then increases with this formula:  i = 1.33*t^3.

Time..........Voltage.........Current
0...............0.................0
1...............20...............1.33
5...............500.............166.67
10.............2000............1333.33
20.............8000............10666.67
50.............50000..........166666.7

Brad, you need to try to get up the learning curve such that you get to the point where you come back and acknowledge the answer given above is correct.

As i clearly stated MH,we are not resolving your modified version until your original question is answered.

#### tinman

• Hero Member
• Posts: 5363
##### Re: MH's ideal coil and voltage question
« Reply #93 on: May 09, 2016, 07:46:31 PM »
Here is the harder version of the question and the answer:

The question is what happens starting at t = 0

The current through the ideal coil starts from zero at time t = 0 and then increases with this formula:  i = 1.33*t^3.

Time..........Voltage.........Current
0...............0.................0
1...............20...............1.33
5...............500.............166.67
10.............2000............1333.33
20.............8000............10666.67
50.............50000..........166666.7

Brad, you need to try to get up the learning curve such that you get to the point where you come back and acknowledge the answer given above is correct.

Quote
You have an ideal voltage source and an ideal coil of 5 Henrys.  At time t=0 seconds the coil connects to the ideal voltage source.  The voltage source waveform is 20*t^2.  So as the time t increases, the voltage increases proportional to the square of the time.

If the voltage increases,then it is not an ideal voltage,as an ideal voltage dose not change in time.
Your original question clearly states an ideal voltage of 4 volts for 3 seconds.

The rest of your answer is not related to the original question,as the voltage is not 4 volts for 3 seconds.

https://en.wikipedia.org/wiki/Voltage_source
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.

#### MileHigh

• Hero Member
• Posts: 7600
##### Re: MH's ideal coil and voltage question
« Reply #94 on: May 09, 2016, 07:48:32 PM »

Re: Poynt's comment:

Quote
Yes, nothing will happen with the ideal inductor, i.e. it will have 4V (or whatever the voltage is at any point in time) across it and zero current through it for ever and ever.

I am not sure of the context for that or he may simply have been sarcastic.

What I can say to you is this:

The question was posed to you with an ideal inductor and you objected to that.  So I then reposed the question to you and added a resistor that gives you a time constant of 158,440 years and you agreed in principle that that variant of the question can be answered.  On top of that, you have already been given a partial correct answer.

So what you (or anyone else) need to do is try to answer either question, the original or the variant.  Both answers will essentially be the same.

So there is only one real answer, even if you use the variant question.  And the answer is still not forthcoming from you or from anyone else.  That means you have an opportunity to shine and demonstrate that you understand what is going on, you understand inductance, and you understand how an inductor works.

MileHigh

#### tinman

• Hero Member
• Posts: 5363
##### Re: MH's ideal coil and voltage question
« Reply #95 on: May 09, 2016, 07:54:01 PM »
Thank you partzman for running the simulation.

MileHigh

If partzman runs the same simulation with the resistance value removed,so as the circuit represents your ideal inductor,he will derive the same answer as myself and Poynt.

Quote
I am hoping that this gives Brad and others enough information to answer the full question and demonstrate that they understand the concepts and show competence in the subject matter.

The rest of us are hoping that MH learns that when you add ideals into questions,it changes everything drastically,and the situation in no way represents real worl outcomes.

Quote
In that sense please refrain from providing further answers to the complete question that defines a multi-part voltage waveform.  The hope is that the interested parties undertake to teach themselves as apposed to being spoon fed the answers.

All the information i have provided has been under my own understandings and research. There has been no spoon feeding thank you very much MH.

#### MileHigh

• Hero Member
• Posts: 7600
##### Re: MH's ideal coil and voltage question
« Reply #96 on: May 09, 2016, 07:59:43 PM »

<<< If the voltage increases,then it is not an ideal voltage,as an ideal voltage dose not change in time. >>>

Actually, that's a "rule" that you seem to have made up for yourself.  An ideal voltage source can be a function of time.  i.e.; v(t) = 7*sin(omega*t).

That's a +/-7-volt sine wave at the angular frequency omega that has a zero output impedance.

I can't fathom how you "lock" yourself sometimes.  An ideal voltage source is a voltage source that has a zero output impedance.  It has nothing to do with it possibly varying in time.

Like I said before, you can have an ideal voltage source that outputs an Ozzy Osbourne song or The Star Spangled Banner - it's still an ideal voltage source.

MileHigh

#### tinman

• Hero Member
• Posts: 5363
##### Re: MH's ideal coil and voltage question
« Reply #97 on: May 09, 2016, 08:00:53 PM »
I just want to point out this argument started over an actual inductor. like, a non imaginary, real world, coil that is part of a real circuit.
The argument began when the group was discussing just how the circuit operated. Now I'm no expert, but I'm fairly sure
this inductor has an inductance and a resistance, and I'm sure some capacitance in the windings.. I'm also pretty sure we can track down
the rest of the factors in the circuit such as source voltage.. frequency.. etc.. I have seen no one do that. Instead days have been spent
arguing over a completely fictitious scenario that basically boils down to a basic philosophical argument about duality or the chicken and the egg.

At this point I don't see how we are any closer to agreeing just how the JT circuit originally in question fires below 300mV.

Instead we have moved to the fantasy realm of an ideal hypothetical scenario because MH want's to prove that
Brad just blindly threw the circuit together and has no idea how it works.

At least I learned something.. I learned that even though one can understand the characteristics of an inductor, how to use it
in a circuit, how it behaves in comparison to other components, and how to work out its role in a circuit over time.. none of that
has absolutely anything to do with how an inductor really works unless you can answer some ambiguous question about a
hypothetical coil.

what happens after T=0? well, there's a dog somewhere that probably took a dump, someone's favorite tv show probably came on,
somewhere along the lines the inductor started charging.. oh and it also continued to display characteristics of any other inductor even though
we have to assume that entirely of faith.

I'm assuming since MH has finished his little test exercise, can we go ahead and start figuring out
what's going on in the actual circuit now?

MH ask this question to a couple of members of this forum,and he has been ranting on ever since how they could not answer his question correctly.

I decided to have a go at answering the question,which is posted in the first post of this thread.
I opened this thread to discuss this original question,and not MHs modified version that now has a non ideal inductor.

I told MH that his question is more complicated than he believes it is.
It has already been established that from T=0 to T=13 seconds,nothing will happen,as current will not flow through an ideal inductor. This opens cans of worms all over the place,as you can see in my previous 4 to 5 posts.

#### Magneticitist

• Full Member
• Posts: 131
##### Re: MH's ideal coil and voltage question
« Reply #98 on: May 09, 2016, 08:04:10 PM »
MH you have to understand, the problems I have with your 'ideal' argument and your test question are entirely based upon the context in which you are using them..

As I said I totally understand the entire use of this circuit theory. Why would I argue that? Of course, by knowing the nature of what you are playing with, you can use certain tools to approximate possible outcomes.. essentially design a functioning circuit on paper and know how it will work before even having to wire it.. I get it..  The argument I have against the 'ideal' 0 resistance situation is simple.. we don't know for sure how a circuit would act with 0 resistance. we just don't. we lie to ourselves and say once we get to a point where from our perspective things become far too 'small' to say they matter anymore, we can just go ahead and say they don't exist.

Anyway like I said it seemed as if you had presented this question like it somehow related to the circuit that was originally being discussed. I'm trying to explain how your pop quiz had entirely nothing to do with the real world circuit in question and you take that as me trying to crap on the basic circuit theory you had to learn and have grown to respect as a useful tool in EE, all talks of 0 resistance aside.

If it would make you feel any better we can dispense with the trivial motions we are sure to play out.. I'm dumb, you're smart. I have absolutely no idea what an inductor is and you're the master of the electronic universe. I resign to the fact that I am utterly incapable of designing even the simplest functional circuit so long as it contains an inductor, and you know them inside out, top to bottom, with no mysteries to uncover.

Now that we have all that out of the way, I'd really like to get back to the joule thief discussion myself.. Didn't you and Brad have a disagreement about the circuit operation?
Is there some further explanation that proves your reasoning somewhere buried underneath this pop quiz question we have spent pages and pages discussing?

#### tinman

• Hero Member
• Posts: 5363
##### Re: MH's ideal coil and voltage question
« Reply #99 on: May 09, 2016, 08:05:50 PM »
The formula for the voltage source that connects to the coil is v = 20*t^2.   Therefore when t=0 the voltage is zero.  This is like setting the voltage on your bench power supply to zero and connecting up a coil.  Then you turn the knob to increase the voltage, and you turn it faster as time progresses.  So the question is what is the current through the coil when you do this?

Do you perhaps think that is a dumb thing to do?  Millions would disagree with you.  What if you play the Star Spangled Banner across the coil?  Well, there will be a current waveform in that case also, and it would not be that hard to determine.  Notice that I say "determine" and NOT "measure."  You don't need to measure the current when the Star Spangled Banner is playing across the coil, you can determine what it is using your brain and a few tools.  That's the whole point in this discussion.  Note that sometimes you simply can't measure the current, you need to use your wits instead to determine the current.

Please check with Brad first and get his take on how to answer the question.  He should know how to respond to that.  If it is still not clear after that then I will answer you.

The way the question was posed was intentional.  You are supposed to have the electronics aptitude and experience to know what to say for the answer.  I am not saying this to give you a hard time.  Rather, I am just stating this to illustrate how far many bench experimenters have to go to get to a better and more productive place.  There is no point in playing with coils if you don't really know what you are doing.  It might be tough medicine to say that but it is good medicine.  And that brings us full circle back to the question that I asked EMJunkie being repeated here.  He had been playing with coils on his bench for ten years and he couldn't answer the question either.

MileHigh

Quote
At this point how to answer the original question should be pretty obvious to most.

Except to you MH.

The first answer we have so far is--nothing will happen.
I am awaiting Poynts review on the reasons for my second theoretical answer-and my theories on the rest of it.

#### MileHigh

• Hero Member
• Posts: 7600
##### Re: MH's ideal coil and voltage question
« Reply #100 on: May 09, 2016, 08:08:25 PM »

Quote
If partzman runs the same simulation with the resistance value removed,so as the circuit represents your ideal inductor,he will derive the same answer as myself and Poynt.

I will repeat this a second and last time.  The addition of the resistance to the simulation is to solve a constraint in the way the software works.  It has nothing whatsoever to do with what you are referring to.

You are in for an unpleasant surprise when Poynt comes back.  You think that both of you are on the same page but you are not.  Like I said, I can't explain his quote but in due time we will find out.

You literally have been given a partial correct answer from partzman in two forms.  Someone else earlier in the thread described an ideal inductor behaviour perfectly.  It looks like it's going to take a few more days before you come round and it may have to end up being a forced spoon feeding session from a guru.

MileHigh

#### Magneticitist

• Full Member
• Posts: 131
##### Re: MH's ideal coil and voltage question
« Reply #101 on: May 09, 2016, 08:13:06 PM »
MH ask this question to a couple of members of this forum,and he has been ranting on ever since how they could not answer his question correctly.

I decided to have a go at answering the question,which is posted in the first post of this thread.
I opened this thread to discuss this original question,and not MHs modified version that now has a non ideal inductor.

I told MH that his question is more complicated than he believes it is.
It has already been established that from T=0 to T=13 seconds,nothing will happen,as current will not flow through an ideal inductor. This opens cans of worms all over the place,as you can see in my previous 4 to 5 posts.

one thing that majorly confused me, and apparently you as well, was the 'ideal voltage' source also being a variable supply. so not only does this magic power supply provide infinite current with 0 vdrop it can change voltage at any time. I was under the impression an ideal vsource was a fixed source for all intents and purposes, though apparently not. this is why I was saying that kind of question is more what you would expect from your EE professor, in class. without being primed with the basics of that class, it's quite hard to know where one is supposed to start answering, or how.
IMO that does not mean a person is completely void of electronics knowledge of circuit building capability.

people still make fun of Ed Leedskalnin but could he not manipulate magnetic current to do his bidding? sure, maybe he could have done so much more had he earned a degree, but his lack of degree by no means made him a clueless idiot.

#### MileHigh

• Hero Member
• Posts: 7600
##### Re: MH's ideal coil and voltage question
« Reply #102 on: May 09, 2016, 08:18:40 PM »
Magneticitist:

The question defines a voltage source connected to an inductor.  It defines the voltage waveform.

The only unknown in the setup is the current.  You have to solve for the current.

MileHigh

#### tinman

• Hero Member
• Posts: 5363
##### Re: MH's ideal coil and voltage question
« Reply #103 on: May 09, 2016, 08:20:49 PM »

Re: Poynt's comment:

I am not sure of the context for that or he may simply have been sarcastic.

What I can say to you is this:

The question was posed to you with an ideal inductor and you objected to that.  So I then reposed the question to you and added a resistor that gives you a time constant of 158,440 years and you agreed in principle that that variant of the question can be answered.  On top of that, you have already been given a partial correct answer.

So what you (or anyone else) need to do is try to answer either question, the original or the variant.  Both answers will essentially be the same.

So there is only one real answer, even if you use the variant question.  And the answer is still not forthcoming from you or from anyone else.  That means you have an opportunity to shine and demonstrate that you understand what is going on, you understand inductance, and you understand how an inductor works.

MileHigh

As i have now stated 3 times MH,we will first answer your original question,and then we will look at your modified question,which now includes a resistance,and so is no longer related to the original question,as the inductor is no non ideal. The two answers and outcomes will be infinitely different--that you can be assured.

I do not believe Poynt is being sarcastic,as this is a very important subject as far as im concerned.

I am no longer interested in proving you wrong,as your original question has me fired up like i have not been in a long time. My brain has gone into overdrive on this one,and it has been a long time since i have had so much interest in a subject. It is now 2.15 am,and i cant sleep. I just lye in been thinking about this subject--going over and over my thoughts on it. I have to start a 9 hour shift in 4 hours,but i cant sleep.

The ramifications of my theories being correct,change everything as far as what is believed to be an ideal inductor. If Poynt was being sincere with his answer about there being no current flow through an ideal inductor,which as you know,was also !one! of my answers,then we have started to travel down a road no one has traveled yet.

I have to wait for Poynts view on my answers i provided.
Maybe even PW,Vortex1,verpies would like to join in on the discussion?.

#### poynt99

• TPU-Elite
• Hero Member
• Posts: 3582
##### Re: MH's ideal coil and voltage question
« Reply #104 on: May 09, 2016, 08:21:03 PM »