MH's ideal coil and voltage question

[tinman]

Brad,

To be clear, I am in agreement with MH. It makes sense to me now, and apparently when I answered the question years back on OUR, I also got the answer correct.

Regarding my simulation, when using such small resistance values without changing some settings in SPICE (LT Spice must already be set to handle this), the simulation engine runs out of computational precision, which is why it "flatlines" with very low values. When the value is too low, the sim runs out of gas and starts making gross approximations, which is evident below with R=1f Ohm. But you do see that it is honing in on the 2.4A value? Any smaller in value and the trace just flatlines.

For an ideal inductor, yes Tau is infinite, but this has does not preclude current flow through the inductor. If however the inductance was an unrealistically large value like 1 million Henries, then yes the current would essentially be zero for a relatively long duration of time. That was my confusion. Tau simply determines the rise time, and since it is infinitely long, the trace becomes a nice straight line rather than the curve we normally see.

I was wrong in my analysis, but it is clear to me now.

I dont think it is clear Poynt,and your original thought (current will not flow)is correct.

If your sim can replicate any real electrical event in a circuit,why dose it crash when you use an ideal inductor that no resistance?. Why is your sim unable to carry out a simple analysis such as you think MH has successfully done?.

The answer is simple,and as i stated. MH is using math that is based on the premise that the inductor will reach a maximum current level. !!Can you calculate what the maximum current level will be of an ideal inductor using MHs calculations? Will this answer (When t = 3 seconds that's 12/5 = 2.4 amps) then be correct?.

When modeling a circuit ,an ideal inductor is used--along with a series resistor that mimics the resistance that would exist in a real world inductor. This is why you have to place some resistance in series with your ideal inductor in your sim to stop it crashing.

Why dose your sim crash without it?
Because your using an ideal inductor. An ideal inductor has ideal inductance,no resistance,no capacitance,and dose not dissipate power. This means that the CEMF is also ideal,and so is equal to the EMF ,and so an equal current will flow in the opposite direction to that of the current produced by the EMF.

Here is verpies statement.
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.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.

Remember-it is only the resistance and parasitic capacitance that allows the EMF to be greater than the CEMF,and allow the flow of current,something that an ideal inductor is void of.


Brad

[Magneticitist]

An EMF measured across an open inductor is proportional to the rate of change of flux penetrating that inductor (dΦ/dt).
EMF measured across an inductor is not proportional to the current flowing through it and in an open inductor the current cannot flow at all.
Precursor of what?

well, look.. I hadn't thought about this kind of stuff in a while and in this debate tried to quickly brush up on my terminology online, and read how the EMF is actually the potential. I was thinking the electric field was the voltage when apparently that's not a metaphor. Now I don't really know what to call what at the moment because I am getting more confused at the terminology other people are using that seems to contradict what I just read. I need to spend some more time reading this in detail before I know exactly what do call what here but I think of it like electric- voltage, magnetic - current. maybe that is a complete falsehood that has emerged from too much misinterpreted tinkering and not enough reading. at any rate the point I was trying to make is that any magnetic force that is present is present when there is current flowing and if not we just have voltage from the source.
there's no di/dt to even consider, no magnetic field to consider, no lenz to consider, because it's all perfectly working against itself in theory and pretty much just not existing at all. like any inductor the voltage is the precursor unlike the opposite in a capacitor.

[allcanadian]

@Tinman

Remember-it is only the resistance and parasitic capacitance that allows the EMF to be greater than the CEMF,and allow the flow of current,something that an ideal inductor is void of.

I would agree and if the resistance is defined as zero in the question then Ohms law has no application... let's move on. No resistance and no capacitance which leaves an Electro-Motive Force from the source which are Coulomb forces due to the Ideal voltage source electric field. The source Emf acts forward however the moment something tries to move a magnetic field evolves producing an equal and opposite Counter-Force, our Cemf, which opposes the charges motion.


Logic suggest that if nothing can dissipate and energy is perfectly conserved then the Cemf must balance perfectly with the source Emf... remember these are perfectly conserved forces. If an ideal superconductor produces a perpetual loss-less current closed loop then an ideal superconducting coil must also produce a perpetual loss-less Cemf countering our source Emf. One cannot say the rules always apply then change the rules simply because they do not like the answer. Either the forces balance perfectly and energy is conserved or energy is not conserved in a loss-less system and we have problems. Ideally it must be ideal because we have already defined it as such.


AC

[tinman]

@Tinman

I would agree and if the resistance is defined as zero in the question then Ohms law has no application... let's move on. No resistance and no capacitance which leaves an Electro-Motive Force from the source which are Coulomb forces due to the Ideal voltage source electric field. The source Emf acts forward however the moment something tries to move a magnetic field evolves producing an equal and opposite Counter-Force, our Cemf, which opposes the charges motion.


Logic suggest that if nothing can dissipate and energy is perfectly conserved then the Cemf must balance perfectly with the source Emf... remember these are perfectly conserved forces. If an ideal superconductor produces a perpetual loss-less current closed loop then an ideal superconducting coil must also produce a perpetual loss-less Cemf countering our source Emf. One cannot say the rules always apply then change the rules simply because they do not like the answer. Either the forces balance perfectly and energy is conserved or energy is not conserved in a loss-less system and we have problems. Ideally it must be ideal because we have already defined it as such.


AC

Absolutely  AC
Ideal means perfect-a losless conversion between EMF-forward current-counterEMF-reverse current.of the same amount.

So that would mean a dead short when an ideal voltage from an ideal source is placed across the ideal inductor,as as much current would be trying to flow back into the ideal voltage source,as the ideal voltage source is trying to deliver.

My answer stands--you cannot place an ideal voltage across an ideal inductor.

Brad

[Magneticitist]

@Tinman

I would agree and if the resistance is defined as zero in the question then Ohms law has no application... let's move on. No resistance and no capacitance which leaves an Electro-Motive Force from the source which are Coulomb forces due to the Ideal voltage source electric field. The source Emf acts forward however the moment something tries to move a magnetic field evolves producing an equal and opposite Counter-Force, our Cemf, which opposes the charges motion.


Logic suggest that if nothing can dissipate and energy is perfectly conserved then the Cemf must balance perfectly with the source Emf... remember these are perfectly conserved forces. If an ideal superconductor produces a perpetual loss-less current closed loop then an ideal superconducting coil must also produce a perpetual loss-less Cemf countering our source Emf. One cannot say the rules always apply then change the rules simply because they do not like the answer. Either the forces balance perfectly and energy is conserved or energy is not conserved in a loss-less system and we have problems. Ideally it must be ideal because we have already defined it as such.


AC

I don't see why we should have to take it further than "perfectly and absolutely resists current change".  This automatically means no EMF or counter EMF doesn't it?