MH's ideal coil and voltage question

[tinman]

Brad:

How about this:  Take the identical question and add a 0.00000000001 ohm resistor in series with the 5 Henry ideal coil.  Now you have a real-world coil.

Can you answer the question now?  If you answer that properly then as far as I am concerned your answer will be perfectly legitimate.

MileHigh

Yes MH,now that question can be answered,as we have the needed value to calculate the L/R time constant.

But first i want to resolve your original question-the one this thread is about,as i have done nothing but think about the situation that could exist in this ideal case,and the outcome of such an event.

I am hoping that Poynt will keep working on this with the rest of us here,as i have a feeling that even he may not be sure of the outcome,due to the result of the attempted simulation.

At this point in time,i will stick with my theories and answers given,but it is not yet resolved,and so will continue research toward the answer.



Brad

[tinman]

<<< Poynt has just stated that your circuit as defined cannot be simulated,and so your circuit dose not represent a real world device. >>>

Of course it can be simulated.  To my surprise in this case he has to add a resistor.  We have seen him add little phantom resistors many times in the past to get the simulator to run so ultimately there is no real surprise.

The place to simulate this setup is in your head, like I explained before.  Then there are no issues.  I am attaching the "short answer that solves everything" again to this posting.

Your short answer dose not solve your question.

Below you can see what i mean that a small difference in resistance value can have an infinite difference in the end resulting value. One has a finite value,and the other has an infinite value,even though the two resistance values are very close. This is the difference between a real world inductor and an ideal inductor.


Brad

[tinman]

Brad:

How about this:  Take the identical question and add a 0.00000000001 ohm resistor in series with the 5 Henry ideal coil.  Now you have a real-world coil.

Can you answer the question now?  If you answer that properly then as far as I am concerned your answer will be perfectly legitimate.

MileHigh

So using your resistance value MH,i have calculated the L/R time constant to be 500,000,000,000.00 seconds or 1388888888.88 hours.
So we can safely say(as poynt did),that next to nothing will happen when the ideal voltage is placed across the  inductor and series resistor-->!OR!,the inductor having a very low resistance value,will causes the current to rise to 63% of 400000000 kiloamps,keeping in mind that we still have our ideal voltage that will drop to a value of 0 across the inductor,but remain at 4 volts acros the resistor /inductor ,now that we have the series resistor,as it dose when using real world circuits.


Brad

[partzman]

I've attached an LtSpice sim of a 5 henry inductor with a dc resistance of 1e-11 connected to a 4 volt ideal voltage source for 3 secs.

At the end of three secs we see the inductor current has reached 2.4 amps.  This is as close to a perfect inductor as one could want.

Using the formula that MH supplied that is, delta I = Et/L, this results in delta I = 4*3/5 = 2.4 amps.

What happens if we increase the dc resistance to a higher resistance? The peak current reached will simply be lower than the above depending on the value of the resistance under otherwise the same conditions.

partzman

[poynt99]

Brad,

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.)

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?