MH's ideal coil and voltage question

[tinman]

Ideal inductors do exist in our society today. I am certainly no expert in the subject but examine the superconducting electromagnets used in MRI. Once below their critical superconducting temperature, certain materials exhibit zero resistance and yet maintain inductance. The inductive fields exist outside the confines of the wire but they do exist. There are qualifications for these ideal conductors to work as they do but they are in use everyday.

Resistance of a coil does not determine it's inductance, it simply hinders pure inductance.

I have attached another sim using a coil resistance of 1e-110. This parameter may have passed a preset limit in LtSpice however.

partzman

A 5 henry superconducting coil is still 5 henries even with zero resistance and they will "store" their current with zero voltage drop for extremely long periods of time.

Partzman
A super conductor that has a steady DC current flowing through it,has no voltage across it. If it did,then it would be dissipating power,and super conductors do not dissipate power.


Brad

[tinman]

So I guess the question is how much time does it take for an ideal inductor to reach a particular current over time if resistance was not an obstacle. This would mean that the ideal inductor is functional as an inductor.....

Well the ideal inductors current rise when the ideal input voltage is applied will be a straight line increase and not a curve because the absence of voltage division because of no resistance. So the current could rise indefinitely over time, directly related to time and the resistance value does not need to be in the equation L/R.   Correct?


That is if the ideal inductors bemf ends up not being equal to the input and the ideal inductor actually works.

Mags

You have nailed it on the head-or near to Mag's.

Unfortunately MH is just not getting it,and he is trying to use a math function that dose not account for the voltage and inductor on being ideal.

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.

The above is not applicable to an ideal inductor with an ideal voltage across it.
The math above is based on the premise that the inductor will eventually reach a maximum current value in Tau x 5s. We already know that by using the L/R time constant,that Tau is infinity.
We can also solve this a second way. That is to place the ideal voltage across the ideal inductor,and time how long it takes for the maximum current value to be reached. We then divide this time by 5 to obtain our Tau time constant. This also results in an infinite time,as the voltage is ideal,and the inductor has no resistance. It also means once again that there will be no current flowing through the ideal inductor .

As i said,and have all along--you cannot place an ideal voltage across an ideal inductor,because as you see,you are left with a paradox.
If an ideal voltage is placed across an ideal inductor(that has no resistance to control the flow of current),then the current would take an infinite amount of time to reach it's peak level.
This then means that the current would also take an infinite amount of time to start to flow into that inductor at T=0,as when you divide an infinite amount of time by any other amount of time,you end up with an answer that is also infinite.
So that is the paradox,but it is also correct,and once again backs up all my answers i have given in regards to the original question.

Even MH cannot deny that it would take an infinite amount of time for the current to reach maximum value,when an ideal voltage(a voltage that dose not change over time) is placed across an ideal inductor that has no resistance(dose not limit or control the flow of current)

If he understands this,then he may also understand the conundrum/paradox associated with his!so called! simple circuit,being that,from the above,we also know it would also take an infinite amount of time for the current to start to flow through the ideal inductor.
This will lead to the point where he now understands my answer--you cannot place an ideal voltage across an ideal inductor

Can i explain both of my theoretical answers?--yes i can,and i am surprised Poynt did not pick up on it-the paradox. MH laughed at my two theoretical answers being totally different.
1-being that there is no current flowing through the inductor--that Poynt did agree with--not sure what he thinks now after MH threw in his wobbly mathematics.The reason being-->as above. If it takes an infinite time for the current to reach it's maximum,then it also takes an infinite amount of time before the current starts to flow.

2-@ T=0,when the ideal voltage is placed across the ideal inductor,the current will rise instantly to an infinite value. Answer one makes answer two also correct,in that,as the current is going to rise for an infinite time to an infinite value,then at the very start of that infinite time ,T=0,the current will also be at an infinite amount. As i said,no matter how many parts you divide an infinite value by,each part in itself will also have an infinite value.

This all sounds crazy i know,and hence the reason i included the word conundrum and/or paradox with my answers.
This also shows that MHs question cannot be answered,as it cannot exist.
Changing values around,and changing from an ideal to a non ideal,and using math that is based around non ideal situations,is not going to make the original question answerable.


Brad

[Magneticitist]

electric field moves at light speed doesn't it? how would we know it lagged over distance without resistance.
yet a graph can somehow show some kind of change over time when time has already been omitted due to there
being no relative way to measure it.

one day they'll get a super conductor 100% ideal with absolute 0 resistance, and it will be when the entire universe
is one giant solid piece of silver, and there's absolutely nothing else in the universe to compare it to.
to agree with Albert Einstein, or to disagree with Albert Einstein?..
That is the question.

[wattsup]

OK, it's 2:30 AM and I am sleepy so here goes nothing.

The ideal business is just a show piece. You have this ideal voltage meaning it provides anywhere from none to infinite current. As long as your wire is fat enough and let's say 200 turns, that fixed voltage will be applied and you will have enough current in the source to heat up those fat wires. Since those wires of the coil as so fat, the resistance is almost nothing even though nothing can be at an absolute zero resistance.

Now put that same ideal voltage into a coil of thinner wire and only 20 turns. Now that wire has some resistance because it is much thinner then that fat wire coil. Now the voltage is applied and the current, even though it could go to infinity if your wire was of infinite diameter, the current will stabilize at a given level and stay there. The wire determines the current because in any case, the wire has x number of copper atoms and cannot invent or materialize any more so there are only an x amount of atoms conveying and hence x current in the coil. With only 20 turns that 4 volts has a chance to make it to the end of the coil so that whole coil with have current conveyed. I am saying conveyed because I do not believe in flow but that should not detract from this.

Now take that same thin wire coil that now has 2000 turns hence higher henries. Apply your ideal voltage of 4 volts. Even if the current can go to infinity, the coil is only hit with 4 volts and 4 volts in a 2000 turn coil is nothing, probably won't even be conveyed to the end of the coil so the current will be greater at the start of the coil and very weak by the time it gets to the end of the coil hence the current at the start will be greater then the current at the end of the coil. Yes this is counter andwould need to be tested with a multi tapped coil.

Yes this is not an ideal coil. Unfortunately yes, @MH did not need to include an ideal coil in this question and that is a mistake he needs to man up to but that's his business not mine. It would have saved 100 pages of nullisms.

Just for the record I actually do know that DC does not work like that in our coils but that's another topic and should again not detract from this subject as an EE discourse goes.

So again, what @MH just wants to explain is actually very rudimentary but by using the ideal voltage construct just makes the none to infinite current available to match the coils wire AWG and length and topology. Just apply the voltage and the current will find its own level.  Yes a DC power supply will do that for you as well. Even if the coil had wire the diameter of the sun and the length of our solar system it will still have resistance so it would be better to remove the ideal from the coil and leave it with the voltage source.

But I still think @MH needs to concede that the original question should not have employed two ideal subjects in the same question. One is enough to explain the process which is again so rudimentary that is is sort of insulting but again, maybe guys take is for granted that you really do not need to always measure currents to know what's going on going your coils.

I rarely use anything to measure watts. My LEDs or bulbs say it all already. When one of them blows, I'll know something is really good. hahahahahaha You don't need to be so obsessed with measurements as I have found it takes time, it already shows what you should already know and it especially eats away at your bench time, patience and distracts you from the most important part of testing a build and that is.................... working the variables. You make a build and test it. You learned one thing. You then test variables on the build as comparisons and now you are learning multiples more on effects. That's where the gold is.

GGGGGGHHHHHHHHHHHHHHHH.

wattsup

[verpies]

We can also solve this a second way. That is to place the ideal voltage across the ideal inductor,and time how long it takes for the maximum current value to be reached. We then divide this time by 5 to obtain our Tau time constant. This also results in an infinite time,as the voltage is ideal,and the inductor has no resistance. It also means once again that there will be no current flowing through the ideal inductor .

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.

Of course, it is debatable whether an ideal inductor must have an infinite inductance.  Some would say that it is enough for it to have zero resistance and zero parasitic capacitance.

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.

Last but not least - inductors are current devices and voltage creates no effects in them.  Voltage cannot even be measured in shorted ideal inductors (neither practically nor theoretically!). Measurement of voltage (emf) is meaningful only for non-ideal inductors (e.g. open inductors or inductors with series resistances).  Open inductors or inductors without current flowing though them are dummy inductors - they create no effects on the environment.  Voltmeter deflection notwithstanding.

P.S.
I'm just replying to Tinman's post and I have not read what others wrote in this thread.


^* (without delay and regardless of its inductance)