MH's ideal coil and voltage question

[tinman]

One last time:

Before the _fixed_ 4volt EMF is applied to the 5H inductor, there is no CEMF being generated so CEMF=0

At T=0, the 4volt EMF is applied to the inductor and current rapidly rises:


1. As di reaches .8A/s,  CEMF = EMF

2  If CEMF = EMF,  di < .8A/s

3. If di < .8A/s,  CEMF < EMF

4. If CEMF < EMF,  di increases (return to #1, loop forever)


Read thru the loop (1-4) ten times or so, perhaps it will come in to focus...  This is an example of a negative feedback loop.

(A negative feedback loop is a common concept in electronics and there are probably a hundred, if not many more, negative feedback loops operating in the electronic circuits you use daily.  Although many people think of gain setting when they think of negative feedback, the most common and numerous circuits throughout your house that use negative feedback are most likely current sources.)

As far as how much current flows at T=0, it does not really matter.  If one electron flows at T=0, then 1 picosecond later, a current flow of something just shy of 5 million electrons would be equivalent to .8A/s and that would cause the CEMF to be equal to 4 volts. (5 million electrons flowing being equal to .8 picoamperes or thereabouts, I'm tired so...)

It is the rate of change, not the absolute amount of current, that determines the CEMF  Start out with as many electrons in as narrow a slice of time as desired, but when from time slice to time slice the current rises (changes) at the equivalent rate of .8A/s, the generated CEMF will be 4 volts.



PW

PW

If the coil is ideal,then the CEMF should always equal that of the EMF,by way of the induced currents by both-correct.
So this being the case, at T=0,there would have to be a current rise,and that rise in current value must start from 0. Through the transition to 800ma?second,it must first produce a current value much less than that--it must start from a value of 0. As soon as a current rise started taking place,due to the EMF(before the maximum value of 800ma/second),it would induce a CEMF that results in producing a current that is in opposition to that of the EMF induced current.
Why dose this feed back loop of yours only start at a value of 800mA,when the starting value is only slightly above 0 shortly after T=0.

My hope with the scope shots i showed,was to show that we need to look at things a lot further down in the time domain,and in this case,with the ideal coil,we must look at speeds near the speed of light.
The coil is ideal,and so the time reference must also be ideal,in that we look at what happens at exactly T=0,and shortly after.

At T=0,we could just as easly say that the CEMF induced current equaled that of the EMF induced current as soon as 1mA of current started to flow,and your negative feed back started from there. Now your current is limited to 1mA/second right from the start.

The voltage really has nothing to do with the current flow in an ideal coil,as that rise must start from a value of 0,regardless of the voltage placed across it.

Remember, we are discussing inductance, ideal inductance, so please don't add the effects of a resistor and capacitor to the circuit being discussed.  That comes later.

Those scope shot's were indeed important at this point in time,as it shows what we will not have with an ideal inductor. We will have no charging of capacitance,or any associated losses due to parasitic capacitance or resistance.
What we have with an ideal coil,is instant action/reaction,as there are no losses,as an ideal coil dose not dissipate energy--we have your ideal inductance,at ideal speed.

Why is it we are starting at 800mA/second current flow,when we know that the EMF induced current must start from a value of 0,and rise to a value of 800mA ?. There is no such thing as going from 0 to 800mA instantly,there is a time limit to that rise to 800mA,and regardless of how short that time period is,it starts from a value of 0.

If our coil is indeed ideal,and we have ideal inductance,then as soon as a minute amount of current starts to flow,a magnetic field will rise with it,and as soon as a magnetic field starts to build,so dose the self induced current that apposes that which created it.
So it is just as correct to say that as soon as 1mA of current flows as a result of the applied EMF,then a CEMF induced current of 1mA will start to appose that which created it.
Voltage has no meaning here when dealing with an ideal coil,as even with the smallest value of voltage place across that coil,the current will still start from a 0 value,and rise to an infinite value-but it must transition from a value of 0 amps.
Even the speed of light has a limit,and as far as i am aware,this is the speed at which current can flow through any wire,and so we just cant go from 0 amps of current flow,straight up to 800mA/second--your negative feed back loop would start as soon as current started to flow,and so your current would be limited to that of the initial value at T=0,which would be minute,and no where near 800mA.


Brad

[poynt99]

Good grief Brad. I just read your post above, and you have completely missed the boat here mate!

800mA/s is a rate of rise! It is not an absolute value as you are saying. A rate of rise is SLOPE! From left at t=0, A=0, to the right at t=3, A=2.4A. What is the slope of the trace from left to right? THAT is what 0.8A/s means.

Therefore, even at t=1ms and say there is 1mA of current flowing, STILL IT IS RISING AT A RATE OF 800mA/s!!!!!!!!

So once again, as soon as current begins to flow, it is rising at a rate of 800mA/s.

[tinman]

Good grief Brad. I just read your post above, and you have completely missed the boat here mate!

800mA/s is a rate of rise! It is not an absolute value as you are saying. A rate of rise is SLOPE! From left at t=0, A=0, to the right at t=3, A=2.4A. What is the slope of the trace from left to right? THAT is what 0.8A/s means.

Therefore, even at t=1ms and say there is 1mA of current flowing, STILL IT IS RISING AT A RATE OF 800mA/s!!!!!!!!

So once again, as soon as current begins to flow, it is rising at a rate of 800mA/s.

No Poynt,i dont think i have missed the boat at all.

Let me ask you this,and in an ideal way.
We have two identical rocket engines strapped to two identical carts. Each rocket engine can produce 1000LBs of thrust. The two are put nose to nose,and they are fired up. Every time rocket engine 1 is throttled up to produce an extra 100 LBs of thrust,rocket engine number 2 also matches that increase in thrust. At no point in time will rocket engine number 1 move rocket engine number 2,as long as rocket engine number 2 matches the thrust of rocket engine number 1--it is a stale mate,and no motion takes place.

We have two current sources here ,not just one. The ideal coils CEMFs induced current will match the current that is induced by the EMF,as there are no losses.
This can only mean that every time the EMF induced current tried to make a change in value,the CEMF induced current would counteract that very current that induced it in the first place,and it would do this ideally.

I am still yet to see it explained as to how rocket engine number 1 exceeds the thrust of rocket engine number tow,so as there is a net gain by rocket engine number 1--this is of course in regards to the EMFs induced current being greater than the CEMFs induced counter courrent,so as a net gain of current flow can be had.
You and PW keep saying that because the current will rise at 800mA/second-->but who says it will. This is what this whole discussion is about--who says the current will rise at that rate?
You and PW seem to keep referring to something that is being refuted,to validate your claim.
If the EMFs induced current is x amount,then the CEMFs reverse induced current is also x amount. This happens as soon as current starts to flow as a result of the voltage being placed across the coil. Your self and PW come up with 800mA a second,even though there is an equal and opposite reverse current being produced by the CEMF.
What i see ,is a very very high frequency oscillation happening here,where the net result is 0--that is how i see the negative feed back thing happening.

This is what i see in an ideal situation-->below
This is how i see the feed back system,only billions of times faster--no net gain in either direction.

https://www.youtube.com/watch?v=0LnbyjOyEQ8


Brad

[picowatt]

PW

If the coil is ideal,then the CEMF should always equal that of the EMF,by way of the induced currents by both-correct.

The CEMF will equal to the the EMF (4 volts)  when di=.8A/s

So this being the case, at T=0,there would have to be a current rise,and that rise in current value must start from 0. Through the transition to 800ma?second,it must first produce a current value much less than that--it must start from a value of 0. As soon as a current rise started taking place,due to the EMF(before the maximum value of 800ma/second),it would induce a CEMF that results in producing a current that is in opposition to that of the EMF induced current.
Why dose this feed back loop of yours only start at a value of 800mA,when the starting value is only slightly above 0 shortly after T=0.

The inductor does start producing a CEMF as soon as any current flows, but "this feed back loop" does not begin to limit the rise in current until the rate of change reaches .8A/s.  This can, however, be considered a rather instantaneous process that happens as soon as current flows.

My hope with the scope shots i showed,was to show that we need to look at things a lot further down in the time domain,and in this case,with the ideal coil,we must look at speeds near the speed of light.
The coil is ideal,and so the time reference must also be ideal,in that we look at what happens at exactly T=0,and shortly after.

At T=0,we could just as easly say that the CEMF induced current equaled that of the EMF induced current as soon as 1mA of current started to flow,and your negative feed back started from there. Now your current is limited to 1mA/second right from the start.

The CEMF has nothing to do with the _amount_ of current flowing thru the inductor.  The CEMF is determined only by the current's _rate of change_.

The 5H inductor's CEMF equals the applied 4 volt EMF when the rate of change of the current flowing thru the inductor is equal to .8A/s.  It has absolutely nothing to do with the _amount_ of current flowing thru the inductor, and it does not matter how small your time slices are, the rate of change will be limited to .8A/s.

Using ideal components, the flow of current would be considered instantaneous and therefore achieving a rate of change of .8A/s would also be instantaneous.

The voltage really has nothing to do with the current flow in an ideal coil,as that rise must start from a value of 0,regardless of the voltage placed across it.

That is correct, however, the voltage has everything to do with the rate of change of the current flowing thru the inductor.  If 5 volts were placed across the 5H inductor, it would take a rate of change of 1A/s to achieve a CEMF equal to 5 volts, so the rate of change would be limited to 1A/s.

Those scope shot's were indeed important at this point in time,as it shows what we will not have with an ideal inductor. We will have no charging of capacitance,or any associated losses due to parasitic capacitance or resistance.
What we have with an ideal coil,is instant action/reaction,as there are no losses,as an ideal coil dose not dissipate energy--we have your ideal inductance,at ideal speed.

Why is it we are starting at 800mA/second current flow,when we know that the EMF induced current must start from a value of 0,and rise to a value of 800mA ?. There is no such thing as going from 0 to 800mA instantly,there is a time limit to that rise to 800mA,and regardless of how short that time period is,it starts from a value of 0.

You are beginning to lose me over these repeated "starting at 800ma/s" comments.  That number is the rate of change that the current will be limited to.  800 femtoamps per picosecond is the same as .8A/s.

Remember, although I have described it in a rather step wise fashion, it is a smooth and continuous process.  Consider the time intervals between the steps as being infinitely small or the entire loop as happening infinitely fast.  When described in the step-wise fashion, it is like like looking at the events in extreme slow motion.

If our coil is indeed ideal,and we have ideal inductance,then as soon as a minute amount of current starts to flow,a magnetic field will rise with it,and as soon as a magnetic field starts to build,so dose the self induced current that apposes that which created it.

Correct, and that current will be limited to a rate of change of .8A/s.

So it is just as correct to say that as soon as 1mA of current flows as a result of the applied EMF,then a CEMF induced current of 1mA will start to appose that which created it.

Again, the amount of current you wish to consider is of no matter.  Only the current's rate of change induces a CEMF and the rate of change will be limited to .8A/s because that is when the CEMF equals 4volts.

Voltage has no meaning here when dealing with an ideal coil,as even with the smallest value of voltage place across that coil,the current will still start from a 0 value,and rise to an infinite value-but it must transition from a value of 0 amps.
Even the speed of light has a limit,and as far as i am aware,this is the speed at which current can flow through any wire,and so we just cant go from 0 amps of current flow,straight up to 800mA/second--your negative feed back loop would start as soon as current started to flow,and so your current would be limited to that of the initial value at T=0,which would be minute,and no where near 800mA.

The voltage applied to the inductor, and the inductance value of the inductor, determine the rate of change the current will achieve.

Again, you seem to be confusing a rate of .8A/s as having to do with some particular amount of current, which it does not.  800ma/s is a rate of change and says nothing about the actual amount of current flowing.

PW

[poynt99]

No Poynt,i dont think i have missed the boat at all.

The concept of "rate of rise" seems to be a difficult one to grasp. Clearly it is not understood, based on your post.

As long as the rate of current rise is 0.8A/s, the cemf will equal the emf, and the rate of rise of 0.8A/s will happen the instant current begins to flow. Therefore, cemf=emf starting from t=0.