Author Topic: MH's ideal coil and voltage question (Read 490580 times)

MileHigh · « **Reply #1215 on:** June 23, 2016, 03:13:24 PM »

Quote from: hoptoad on June 23, 2016, 11:38:22 AM

When an inductor is first connected to a Voltage Source at time=0 and counting, the EMF and CEMF are EQUAL and their is NO current flow.

At end of TC 1 (TC = time constant of the inductor) the CEMF will be .37 of The EMF and supply current is flowing.
At end of TC 2 the CEMF will be .14 of The EMF and supply current is flowing.
At end of TC 3 the CEMF will be .5 of the EMF and supply current is flowing.
At end of TC 4 the CEMF will be .2 of the EMF and supply current is flowing.
At end of TC 5 the CEMF will be .0 of the EMF = non existent and supply current is flowing.

Any moment after that, the current through the inductor will be steady, the EMF will be steady and there will be NO CEMF.

MH, for someone who insists that others use the correct nomenclature when referring to circuitry, you seem to be pretty liberal with how you apply it yourself. Voltage drop across a component is not CEMF. EMF is not CEMF. Constant EMF is sustainable, constant CEMF is not.

Here's a link showing the relationship of CEMF in a series LR circuit, with the resistance external to the inductor and the presumption of an ideal inductor. Step through the exercise and answer the questions.

https://www.wisc-online.com/learn/career-clusters/stem/ace5903/an-inductor-opposing-a-current-change

Cheers

Yes Hoptoad, you are just describing the standard inverse exponential curve for the energizing of an ideal inductor in series with a resistor. However you make a big fail in your reasoning and I will explain.

As the current increases through the (inductor + resistor) the net EMF presented across the inductor will be the original EMF minus the (resistor x current).

What that means is that over time the net EMF applied to the inductor decreases, and the CEMF generated by the inductor in response to the decreasing applied EMF is equal and opposite.

Quote

When an inductor is first connected to a Voltage Source at time=0 and counting, the EMF and CEMF are EQUAL and their is NO current flow.

And if the inductor was ideal and there was no resistance then after time=0 the CEMF will remain equal to the EMF and current will start to flow.

Quote

MH, for someone who insists that others use the correct nomenclature when referring to circuitry, you seem to be pretty liberal with how you apply it yourself. Voltage drop across a component is not CEMF. EMF is not CEMF. Constant EMF is sustainable, constant CEMF is not.

CEMF = counter electromotive force. All that really means is a voltage that is opposite to the applied voltage. Anything that generates an opposite voltage to the applied voltage can be considered a source of CEMF. The good old "backwards battery in the circuit for charging" is a CEMF source. Even a resistor can be considered a CEMF source but like I already posted, for clarity it is fine to call it a "potential difference" for a resistor.

I will repeat to you, a voltage drop across any component is actually a CEMF. However, for sure it makes sense to try and keep the nomenclature simple and understandable for all.

MileHigh

MileHigh · « **Reply #1216 on:** June 23, 2016, 03:23:45 PM »

Quote from: hoptoad on June 23, 2016, 03:13:18 PM

Indeed. Anybody got an ideal inductor?

Every time you use the equation R = V/I, that assumes an ideal resistor.

Every time you use the equation v = L di/dt that assumes an ideal inductor.

Every time you use the equation i = C dv/dt that assumes an ideal capacitor.

All hobbyists and experimenters use the same equations day in and day out that are based on ideal components.

"Anybody got an ideal inductor?" is just bullshit talking. I know that many of you play this "it ain't real" game but at the same time you unwittingly use equations that assume ideal components.

Don't resist the application of knowledge and reason when it makes you "uncomfortable." Just get with the program because you are already assuming ideal components without even thinking about it.

poynt99 · « **Reply #1217 on:** June 23, 2016, 03:28:39 PM »

I didn't think I would have to spell it out in more detail, but looks as though I do.

Yes of course a real voltage source is not ideal, but compared to the output impedance of the coil, the voltage source is always going to appear as a heavy load, which is going to cause the cemf to go to practically 0V, if you were able to measure it, which of course you are not able to.

The point being, the cemf, no matter it's value, will effectively be shorted by the load. But as I explained there is no real consequence, because the resulting induced current does the job of limiting the current.

MileHigh · « **Reply #1218 on:** June 23, 2016, 03:31:41 PM »

Quote from: tinman on June 23, 2016, 12:47:04 PM

author=hoptoad link=topic=16589.msg486958#msg486958 date=1466674702]

It is good to see there is some one else that is not falling for MHs gobble doc.

But now 2 questions for you Hoptoad.
1-why dose the magnetic field that is inducing the CEMF,slowly decrease in change over time in an inductor from T=0-->moment of the applied voltage across the coil.
2-why is the CEMF equal to the applied EMF at the moment the voltage is placed across the coil?.

As i have said many times,MH changes things when it suit's his needs.
Do as i say--not as i do.

Indeed.

I doubt that will happen. MH is on a !!MH!! is right saga,and nothing gets in the way.

Brad

No, MileHigh responded to Hoptoad and when you read the reply you will see that the inductor's CEMF is equal and opposite to the decreasing EMF. Hoptoad was wrong and it was clearly and unambiguously explained to him.

I have not changed a single thing to suit my needs. This is all just collective mass hysteria, Night of the Living Anti-Kirchhoff Zombie Pod People.

poynt99 · « **Reply #1219 on:** June 23, 2016, 03:39:53 PM »

Quote from: MileHigh on June 23, 2016, 03:13:24 PM

CEMF = counter electromotive force. All that really means is a voltage that is opposite to the applied voltage. Anything that generates an opposite voltage to the applied voltage can be considered a source of CEMF. The good old "backwards battery in the circuit for charging" is a CEMF source. Even a resistor can be considered a CEMF source but like I already posted, for clarity it is fine to call it a "potential difference" for a resistor.

I will repeat to you, a voltage drop across any component is actually a CEMF. However, for sure it makes sense to try and keep the nomenclature simple and understandable for all.

MileHigh

I strongly disagree. A voltage drop across a resistor is simply that. It can not be considered a "source" of emf or cemf. A resistor dissipates energy supplied by an emf. See the attached definition by Hyperphysics.

An emf is a source of energy. The voltage measured across a resistor is an indication of the dissipation of energy. Let's not confuse the two.

hoptoad · « **Reply #1220 on:** June 23, 2016, 03:47:47 PM »

Quote from: MileHigh on June 23, 2016, 03:13:24 PM

snip...
What that means is that over time the net EMF applied to the inductor decreases, and the CEMF generated by the inductor in response to the decreasing applied EMF is equal and opposite.
snipp...
I will repeat to you, a voltage drop across any component is actually a CEMF. However, for sure it makes sense to try and keep the nomenclature simple and understandable for all.
MileHigh

There is never a decrease or increase in the APPLIED EMF (voltage/current) source after connection. Remember - its an ideal voltage/current source to feed the ideal inductor.
Any variables like the varying resulting current through the inductor and its magnetic field strength are due to the variability of the emergent cemf that arises from the ideal Applied EMF. If the cemf was a steady value, all other factors would also be steady.

Of course in the real world unless your supply is very low impedance, when you connect an unloaded voltage source to a load, be it resistor or inductor the applied voltage source itself will fall slightly. How much it falls depends on the capacity of the source to deliver the required current into the load at a given maintained voltage.

Messing with the nomenclature seems to be the source of most of the tension in the discussion. There are times when it fails to communicate the concept or nuance of a particular issue, but that's when its time to look for a unique new descriptor to avoid confusion, not apply a well understood and widely used description of a phenomenon to another in which agreement and usage of another term is already common and well established.

Yes we use ideal equations to design circuits, work out currents, voltages etc.. We also know that our circuits and calculations contain error margins due to the fact that none of our components are ideal. We don't build perfect circuits but we still build good ones that are accurate enough for the tasks they are designed for.
Cheers

citfta · « **Reply #1221 on:** June 23, 2016, 04:07:13 PM »

I am going to repost this until you reply to it. You have clearly gotten confused between voltage drop and CEMF.

Quote from: citfta on June 23, 2016, 01:00:14 PM

MileHigh,

You are apparently using the voltage drop across the coil as YOUR definition for CEMF. That is not what the rest of the electronics world uses as that definition. For the rest of us CEMF is the generated voltage that opposes the applied voltage. The CEMF is generated by the increasing magnetic field of the coil as the current rises. If your claim that the CEMF equals the EMF were true then no current would flow and that means the could NOT BE any CEMF. Sorry, but your argument makes no sense at all. I haven't read all the posts in this thread but it appears you are the only one that believes CEMF can equal EMF. I really don't think all the rest of us are wrong and you are the only one correct about this.

Respectfully,
Carroll

MileHigh · « **Reply #1222 on:** June 23, 2016, 04:09:26 PM »

Quote from: poynt99 on June 23, 2016, 03:39:53 PM

I strongly disagree. A voltage drop across a resistor is simply that. It can not be considered a "source" of emf or cemf. A resistor dissipates energy supplied by an emf. See the attached definition by Hyperphysics.

An emf is a source of energy. The voltage measured across a resistor is an indication of the dissipation of energy. Let's not confuse the two.

I am not going to disagree with you strongly but I will add one caveat. For starters, I already mentioned that several searches used the term "potential difference" for a resistor in a current loop with an EMF source. So, indeed, a resistor is not a source of direct energy, but there is a tangible voltage associated with it. "A source or perhaps instance of potential difference (when current is flowing through the device)" is a reasonable thing to say.

Let me explore the caveat. Let's look at MOS-type semiconductors for a second. When you get into the details of how they are constructed and how they function, it quickly becomes apparent that it's all about electrons and the moving about of electronics. It starts to become an impediment to talk about current flow in these devices because it just gets too damn cumbersome. So that's why they use the terms "source" and "drain" in that realm. That refers of course to a source of electrons and a drain for electrons.

If you are in the realm of academic electrical engineering research, and looking at things on a deeper level, you may indeed use a different nomenclature. I am talking purely hypothetically here. When you start looking at voltage in detail, and you are snaking your way around a loop and observing the electric field, what do you see? You typically observe very weak electric fields in wires, strong electric fields in capacitors, and as you snake your way through a resistor, depending on the value and the current flow, you can observe say a moderate or a very strong electric field. In that realm of academic electrical engineering research, it may indeed be very convenient to refer to a resistor as a CEMF source because when you pass through it you can "go downhill" in terms of the electric field.

The realm of electronics and electrical engineering is so wide and so huge that different sectors will use their own rationalized units and use their own preferred nomenclature for devices and variables, etc.

So I am not "pushing" to say a resistor is a CEMF source, I am just saying that it may be valid to say that even if in this realm it's not an appropriate term.

MileHigh

hoptoad · « **Reply #1223 on:** June 23, 2016, 04:10:16 PM »

Quote from: poynt99 on June 23, 2016, 03:39:53 PM

I strongly disagree. A voltage drop across a resistor is simply that. It can not be considered a "source" of emf or cemf. A resistor dissipates energy supplied by an emf. See the attached definition by Hyperphysics.

An emf is a source of energy. The voltage measured across a resistor is an indication of the dissipation of energy. Let's not confuse the two.

Thanks for posting that. In referring to nomenclature, that screenshot raises an issue which I had to deal with some years ago in trying to convey the nuance of difference between generating cemf and using it. As the picture says, EMF (CEMF)is not a force it is a potential.

For the purpose of explaining a concept I instead referred to the accepted CEMF as CEMP (Counter electromotive potential) to convey the difference between an unloaded flyback voltage, and CEMF (Counter electromotive force) comprising a loaded CEMP resulting in flyback current. Outside the particular examples I was trying to explain, I would not use the term. But as I said, occasionally the nomenclature we use lacks nuance without added descriptors. For example, BEMF or CEMF is a term that is widely used to describe a voltage that can be induced in a number of different ways that doesn't give clear insight into the specific method of its creation. Is it self induced back emf, rotor magnet induced back emf, collapsing field induced back emf. It is all the same thing but the mechanisms vary, while the name gives little nuance to any specific method of production.

The confusion can really become quite chaotic in pules motor operation descriptions because you have cemf that arises from the applied emf simply due to the inductor characteristics. Then you have cemf induced from the rotor magnets, and also cemf from the collapse of the magnetic field when the emf is removed again. If we had accepted unique names for the cemf according the method of creation, it would simply make it easier to understand and negate the need for additional descriptors to avoid confusion. Underlying all the methods is one commonality - changing current/magnetic fields. But a broader vocabulary would help the description process.

Cheers

MileHigh · « **Reply #1224 on:** June 23, 2016, 04:28:30 PM »

Quote from: citfta on June 23, 2016, 01:00:14 PM

MileHigh,

You are apparently using the voltage drop across the coil as YOUR definition for CEMF. That is not what the rest of the electronics world uses as that definition. For the rest of us CEMF is the generated voltage that opposes the applied voltage. The CEMF is generated by the increasing magnetic field of the coil as the current rises. If your claim that the CEMF equals the EMF were true then no current would flow and that means the could NOT BE any CEMF. Sorry, but your argument makes no sense at all. I haven't read all the posts in this thread but it appears you are the only one that believes CEMF can equal EMF. I really don't think all the rest of us are wrong and you are the only one correct about this.

Respectfully,
Carroll

If you just read my long posting about the term "CEMF" applied to resistors then you will see there is a similar discussion that can be had about your posting above.

A voltage drop across a coil is not MY definition of CEMF. The simple fact is that they are synonymous. In the realm of electronics often enough there are multiple ways to state the same thing. There is nothing wrong with that, often a givien identical situation can be viewed from differing perspectives using different terminology that in the long run all mean exactly the same thing.

Quote

For the rest of us CEMF is the generated voltage that opposes the applied voltage.

Yes, and from the battery's perspective that generated voltage is causing the battery's voltage to drop through the coil. So the CEMF and the generated voltage that opposes the applied voltage and the voltage drop across the coil are all exactly the same thing.

Quote

If your claim that the CEMF equals the EMF were true then no current would flow and that means the could NOT BE any CEMF.

Yes, I know, we are still in zombie territory. Just like you can have multiple terms that mean exactly the same thing, you can have a device that behaves in a way where it is like two things happen at the same time.

In a coil, the CEMF is being generated as a result of the changing current flow through the coil (v = L di/dt) and this is all talking place because the battery is imposing EMF across the coil. Everything is awesome and it's all happening at the same time and it's all perfectly normal. It's NORMAL and nobody should be even flinching or questioning it. I suspect the root cause of the hysteria is "Brad's disease." Brad says something completely wrong and a bunch of zombie sycophants follow along because for some strange reason you latch onto what he says. THINK FOR YOURSELF, and wake up. You connect a battery to a coil and you measure 12 volts at the junction of the battery and the coil. By definition the EMF is 12 volts and by definition there is changing current in the coil generating a counter EMF of 12 volts. This is not rocket science, this is basic electronics.

The EMF and the CEMF are the same damn thing! The battery says, "I am imposing 12 volts across you." The coil says, "Oh shit, then I have to let changing current flow through me at a rate where I muster up the same 12 volts." They are the SAME THING. They both measure 12 volts with a volt meter and have the same polarity if you use the same ground reference. They HAVE to be the same potential because they are CONNECTED to each other.

If they are the same damn thing then why is one called CEMF? It's because you "travel though the loop" in ONE DIRECTION only. So if you go clockwise and you go UP in potential because of the EMF, then as you continue on your journey through the coil you go DOWN in potential. Hence the "counter." You go up in potential and then you counter that by going down in potential. But when you are not "in the loop" the EMF and the CEMF are EXACTLY THE SAME with the SAME polarity.

I will say again, there is nothing really to debate here. We are all zombies trapped in a Monty Python sketch. We need to bust out.

MileHigh

tinman · « **Reply #1225 on:** June 23, 2016, 04:31:34 PM »

Quote from: hoptoad on June 23, 2016, 02:00:57 PM

The magnetic field of the inductor is still increasing through each time constant until it is 100% maximum value derived from steady current at the end of TC5. Only the RATE of increase in the source current/magnetic field is getting less with each TC.

The CEMF does not arise from the source current/magnetic field of the inductor per se, it arises from Changes in the source current/magnetic field of the inductor. The level of cemf is most dependent on the Rate of change of the source current/magnetic field not necessarily the strength of the magnetic field or amount of source current. At actual time =0, in a real world inductor, nothing happens really. But at 0+ picoseconds to microseconds the RATE of change is highest causing the maximum cemf to arise.

But cemf is not self sustaining because it is an emergent phenomena with a value based on rate of change of current/magnetic field, and there can be no more change if emf = cemf, so the cemf begins to drop, and as it does, more source current flows. The diminishing cemf still opposes the Rate Of Change of the source, but not the actual flow of current per se. In opposing the rate of change it diminishes the cause of its own existence. So the cemf diminishes in the familiar TC curve we see in all inductors.

Cheers

OK-good.
Now i want you to think about this very carefully Hoptoad-very carefully.

We have an ideal coil,and that is one free from any winding resistance. It is also void of a time constant--has none.
So from T=0,a voltage is applied across this ideal coil from an ideal voltage source-remember,no time constant,due to no winding resistance resistance.
At T=0,the current will continue to rise at a steady rate,and never reach a peak--the current rises to an infinite amount over an infinite amount of time. The CEMF as you said,is governed by the change in current flow induced by the applied voltage over time. But with our ideal coil,there is no change in current,as the current rises at a steady state for an infinite amount of time. So the current flow is the same as it was at T=0(-the moment a voltage was placed across the coil)for an infinite time.

Will the CEMF change from it's starting value(T=0),if the induced current from the applied voltage always rises at the same rate for an infinite amount of time?

P.S
To add your statement
Quote: If the cemf was a steady value, all other factors would also be steady.

Brad

tinman · « **Reply #1226 on:** June 23, 2016, 04:40:00 PM »

Quote from: poynt99 on June 23, 2016, 03:39:53 PM

I strongly disagree. A voltage drop across a resistor is simply that. It can not be considered a "source" of emf or cemf. A resistor dissipates energy supplied by an emf. See the attached definition by Hyperphysics.

An emf is a source of energy. The voltage measured across a resistor is an indication of the dissipation of energy. Let's not confuse the two.

Thank you Poynt.
It is good to see some sanity still exist here.

Brad

hoptoad · « **Reply #1227 on:** June 23, 2016, 04:48:42 PM »

Quote from: tinman on June 23, 2016, 04:31:34 PM

OK-good.
Now i want you to think about this very carefully Hoptoad-very carefully.

We have an ideal coil,and that is one free from any winding resistance. It is also void of a time constant--has none.
So from T=0,a voltage is applied across this ideal coil from an ideal voltage source-remember,no time constant,due to no winding resistance resistance.
At T=0,the current will continue to rise at a steady rate,and never reach a peak--the current rises to an infinite amount over an infinite amount of time. The CEMF as you said,is governed by the change in current flow induced by the applied voltage over time. But with our ideal coil,there is no change in current,as the current rises at a steady state for an infinite amount of time. So the current flow is the same as it was at T=0(-the moment a voltage was placed across the coil)for an infinite time.

Will the CEMF change from it's starting value(T=0),if the induced current from the applied voltage always rises at the same rate for an infinite amount of time?

Brad

Good question - I don't know. The scenario you paint seems a bit like those dastardly 'which came first, the chicken or the egg' situations
Will have to sleep on that.
Cheers

tinman · « **Reply #1228 on:** June 23, 2016, 04:49:51 PM »

Quote from: MileHigh on June 23, 2016, 04:09:26 PM

I am not going to disagree with you strongly but I will add one caveat. For starters, I already mentioned that several searches used the term "potential difference" for a resistor in a current loop with an EMF source. So, indeed, a resistor is not a source of direct energy, but there is a tangible voltage associated with it. "A source or perhaps instance of potential difference (when current is flowing through the device)" is a reasonable thing to say.

Let me explore the caveat. Let's look at MOS-type semiconductors for a second. When you get into the details of how they are constructed and how they function, it quickly becomes apparent that it's all about electrons and the moving about of electronics. It starts to become an impediment to talk about current flow in these devices because it just gets too damn cumbersome. So that's why they use the terms "source" and "drain" in that realm. That refers of course to a source of electrons and a drain for electrons.

If you are in the realm of academic electrical engineering research, and looking at things on a deeper level, you may indeed use a different nomenclature. I am talking purely hypothetically here. When you start looking at voltage in detail, and you are snaking your way around a loop and observing the electric field, what do you see? You typically observe very weak electric fields in wires, strong electric fields in capacitors, and as you snake your way through a resistor, depending on the value and the current flow, you can observe say a moderate or a very strong electric field. In that realm of academic electrical engineering research, it may indeed be very convenient to refer to a resistor as a CEMF source because when you pass through it you can "go downhill" in terms of the electric field.

The realm of electronics and electrical engineering is so wide and so huge that different sectors will use their own rationalized units and use their own preferred nomenclature for devices and variables, etc.

So I am not "pushing" to say a resistor is a CEMF source, I am just saying that it may be valid to say that even if in this realm it's not an appropriate term.

MileHigh

Oh i see lol.

I say the very same thing as Poynt did,but i get a totally different reply.

To gutless to treat Poynt like you treat me?
Oh,and here is the kicker--the guy(me) that you think is such an amateur,and knows so little,gave the very same answer as Poynt,who is very well versed in EE--only i gave it first.

This speaks volumes about your true nature MH--your pathetic.

Brad

MileHigh · « **Reply #1229 on:** June 23, 2016, 04:56:22 PM »

Quote from: hoptoad on June 23, 2016, 03:47:47 PM

There is never a decrease or increase in the APPLIED EMF (voltage/current) source after connection. Remember - its an ideal voltage/current source to feed the ideal inductor.

In your standard example of an EMF source driving a resistor in series with an ideal coil then what you MUST look at is the APPLIED EMF ACROSS THE IDEAL COIL, and not the unchanging EMF source before the resistor.

Why is this?

Because as more current flows through the resistor the resistor causes an EMF DROP.

The EMF drop results in a new lower EMF across the ideal coil.

The ideal coil responds to the lower EMF imposed across it with an equal CEMF.

The lower EMF and the lower CEMF MUST BE EQUAL because they are CONNECTED to each other.

I know that I am really just repeating myself, try to understand it this second time.

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