# Magnetic Fields and Back-emf?

Discussion in 'Physics' started by wes, Nov 6, 2010.

1. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
Ok so the 1st question is how can a EM create a magnetic field which I would assume spreads out at Light speed while greatly diminishing in strength with distance create the Back-emf when the Coil is turned off? Because the field was spreading out at light speed then the information that the coil is off could never reach the end of it and tell the field it needs to go back to create the Back-emf.

In other words exactly how can the magnetic field reverse and casue the back-emf on the coil?

I have one idea and it has to do with the magnetic field energizing space (creating virtual photons, which also would explain the strength decrease)
and when the coil is shut off the field never comes back but the virtual photons collapse back to the vacuum and in doing so create a field moving back towards the coil and then creating the back-emf.

So yeah, I am really confused how exactly can the magnetic field create a back-emf if it is expanding outward at light speed, which if you think about it, it has to, it can't just expand out and then just stop. and if it does then how can it ever collapse back and generate the back-emf?

2. ### Wendy Moderator

Mar 24, 2008
20,772
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You are focused on the spreading magnetic field. Focus instead on the local field, the same field you see with iron filings with a conventional magnet. The field has a set strength, and is static around the coil. At this point it does not create back EMF.

When the electrical current is cut off this field collapses inward and as a side effect the inductor tries to maintain the same current. This is the back EMF. It is what buck boost power supplies use to create high voltage. The high voltage is created because there is no resistance to maintain that constant current.

CMOS 555 Long Duration LED Flyback Flasher

3. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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The distance it has expanded versus the field strength and size of the magnet that created it will stop a magnetic field from extending outward forever at light speed. "?" where is that from

Reluctance, or whatever the word is now used for resistance in a magnetic circuit. THAT increases with distance from the energizing source as well.

So the further away it goes the higher the 'resistance' to following that path to the other pole. At infinite distance there would be infinite 'resistance' to magnetic field flux. The flux disappears after a certain distance and becomes lost in the background flux from other objects (noise)

4. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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What ever "it" is, we use it advantageously all the time, yet don't fully understand what it is, or how it is manifested by electron movement (what interaction it is). Lots of theories abound, the good ones work VERY well for predicting circuit outcomes. So we use them. There is nothing written in stone that says these working theories are TRUE.

I know this is where the scammers and free energy people make there entrance, but still, we Don't know what it really is yet. Room is there for discoveries to be made. Maybe the Big Bang machine will show us more about our universe and how these forces come about.

5. ### DonQ Active Member

May 6, 2009
320
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Look at R-L time constant curves. They start quickly, and then level off, but they never reach the 'end'. This effect happens both on charge (building magnetic lines) and on discharge (the lines collapsing). The magnetic lines will reach 'infinity' when this graph levels out, in other words, never. But eventually, they reach a 'good enough' level. What 'good enough' is depends on the application. That is what we work with.

And, in the end, the only magnetic lines that matter are the ones contained within the area defined by the outer perimeter of the coils of your inductor. As that number goes up and down, the energy is added and subtracted from the wires. The course of the exterior lines has no direct effect on the inductance, except for the indirect influence they have on the 'internal' lines.

Even if the lines external to the coil mattered (other than the fact that they are the 'other end' of the ones that do matter), the energy of those perpendicular to the axis of the coil diminish as the cube of the distance. This is relatively rare in the natural world as most things diminish as the square of distance. This means that the energy contained in these lines diminishes to below what you can measure in a relatively short distance. Energy in other directions is roughly similar. Net effect, the vast majority of the energy is close to the inductor (and still, only that part that passes through the internal area of the inductor coils counts in the calculations.)

6. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
I guess I am trying to see how works from a quantum level or the most basic. I see what you are saying "Bill_Marsden" about look at it from the local point of view but I am trying to look at it from the whole picture and how it all works. I also understand how the field deminishes extremely quick but that field is still there is some form, Obviously it will never expand to infinity but it will expand to some huge distance after which like "Kermit2 said"

"The flux disappears after a certain distance and becomes lost in the background flux from other objects (noise)"

What I am trying to get around is how can the field collapse if it is expanding outward at Lightspeed because the information to I guess collapse could never reach the end of it. Also the reason for the field staying close to constant close to the source would be because it is always being replaced by the field being produced by the EM and as you get furthur out, it get's weaker. Infact actually thinking out that way, the information to collapse could never reach even the closet field to the EM becuase it is traveling outward at light speed, OMG I am so confused, lol. I just can't get my head around the thinking that the field is stationary, that just does not make any sense to me at all.

Also I can see using this way of thinking why the the vast majority of the energy is close to the inductor but there is still energy beyond that.

I Also understand that when doing the calculations, the only part of the field that matters is the ones close to and within the inductor because after a certain distance from the inductor the field are just so weak that they don't really matter, Plus for an inductor, you really only need to know the fields strengths inside the inductor.

But yeah just kinda want to get a really good understanding of how it works together, lol, Just like so many other people to, lol

7. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
I guess a quick question is are the fields attached to the EM or are they seperate?

8. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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Like I said, there is room for discovery to be made in the field of magnetic fields. Pun intended.

What makes you think that this field has any mass, or inertia that would prevent it from reversing direction? Without mass, it would be able to change directions instantaneously.

What makes you think there is no other 'force' that wants those displaced flux lines back where they were before they expanded? If such a field existed then electron movement is displacing it and creating distortions in its smooth homogenous structure that we perceive as line of magnetic force. So naturally these lines would return to thier 'ground' state the moment the displacing influence ceases. Why would that interpretation of the data not be correct?

9. ### Wendy Moderator

Mar 24, 2008
20,772
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It is possible to over-think things to the point where the obvious is obscured. A fixed magnetic field is just that, it doesn't move. When you turn off the source the magnetic field collapses into itself. Trying to force what happens into another model that doesn't fit will not help your understanding, but it could impede it. Basically you are looking at the edges, not the core field. Want to guess where the important stuff is happening? The other stuff exists, but is negligible in effect and doesn't affect the primary results.

10. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
Well first the reason I would think it has mass is in the form of energy and from what know, anything with mass or energy or even without still cannot exceed light speed.

And I do have one theory or idea of how all this could work together and still explains all these effect's, So go ahead and tell me what you think.

I sorta of think of the field as separate from the EM and the field as it spreads out over time energizes space, in this space are Virtual Particles that pop in and out and the closer to the source the more dense the Virtual particles are so the more energy. This explains the fielding spreading out and causing detectable effect's at some distant object if sensitive enough. When the field collapses, it does not really collapse but instead the EM stops Creating the field and so as it stops energizing space and so the Virtual particles that were created fall back to the vacuum and in doing so create a a field that again spreads out but this time it spreads out back toward the EM and as more and more Virtual particle collapse back to the vacuum, the field becomes just as strong as it was when EM was on. This effect now creates the back-EMF. It is more complicated then that but for me that effectively describes what happens in both cases.

I guess it is sorta like what "Kermit2 said"

"What makes you think there is no other 'force' that wants those displaced flux lines back where they were before they expanded? If such a field existed then electron movement is displacing it and creating distortions in its smooth homogenous structure that we perceive as line of magnetic force. So naturally these lines would return to thier 'ground' state the moment the displacing influence ceases. Why would that interpretation of the data not be correct? "

Plus even if what Kermit2 said was correct

" If such a field existed then electron movement is displacing it and creating distortions in its smooth homogenous structure that we perceive as line of magnetic force. So naturally these lines would return to thier 'ground' state the moment the displacing influence ceases"

That force would still never travel faster then light and the returning to ground state would basicaly be like the Virtual Particle collapsing back to the vacuum and the faster it happens the bigger the field it creates which then just means a bigger Back-EMF.

So does my thinking sound at all correct or even make any sense.
Because for me, it makes a whole lot more sense then thinking that the field is static and never expands past a certain point or thinking that the field somehow can move faster then light. Please Disprove this if you can so I don't spend more time thinking in way that is just not true.

11. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
to "Bill_Marsden"

I'm not trying to force a way of thinking how it might work for me, I just want to find one that makes since to me and everyone else, the reason for sharing these ideas is so that I can get everyone's input on what makes sense and what doesn't, If I don't understand it then I would surely like to try and figure out what I don't understand. I just want to figure out how it works not at the macro level but at the basic level. We have all seen the effect's of magnetism but why does it work like that and how does it work like that.

I have spent so many hours just thinking about how the fields would interact and how to explain all the different things that happen and so that's why I came to the Virtual Particle's and the Field as separate Idea,

12. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
Inverse square root law, it is what makes what you are agonizing over negligible. The magnetic field expands, probably intermixed with an electric field, thus creating a really low energy photon.

But again, the field next to the magnet doesn't move unless the electric current does. If you remove the current it collapses, and is converted back to electricity (hence back EMF) with incredibly small losses to other factors

By focusing on the insignificant only, you are loosing track of the bigger picture. The theory isn't as complex as you are making it out to be. Nor is it flawed.

Asking questions about the edges of anything can be important, but it doesn't affect the design of a flyback transformer if that is the goal.

13. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
I guess I have a simpler question someone may know that basicaly describes my problem. A EM is turned on and allowed to reach steady state and then it is shut off, but the tricky part is what if it turns off faster then it would take for the field to collapse.

so field spreads out to say 1 foot in 1 nanosecond

EM is then shut off within .2 nanoseconds, huge voltage spike , I know.

so how could all of the field collapsed back within that time, impossible because the field (information ) would have to travel back faster then it spread out to completly collapse. and this scenerio is basicaly my whole problem, I can't explain it any other way then the previous Idea I posted.

14. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
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wes, have you derived the formulas for how capacitors and inductors work from Maxwell's equations?

Watch The coolest MIT prof explain it. I'd suggest watching all videos of the professor, Walter Lewin's Lectures.

He explains advanced physical concepts in a way that nearly anybody can understand if they have a knowledge of calculus.

15. ### Tesla23 Active Member

May 10, 2009
323
67
This is actually a really good question, it shows someone who is actually listening in both circuit theory and physics classes and can see when they disagree.

In circuit theory we are taught that an inductor stores energy $\frac{1}{2}LI^2$. If you start a current flowing through an inductor and then switch a resistor R across the inductor, the current collapses with time constant L/R, so the energy is dissipated in R with this time constant.

In physics we are taught that this energy is stored in the magnetic field with energy density $\frac{1}{2}\mu H^2$. You know that the magnetic field spreads out away from the inductor, so this energy is actually stored away from the wires, and depending on the inductor design, it could be quite a long way away. We also know from physics that nothing travels faster than the speed of light, so there it will take a finite amount of time for the energy to get back to the wire.

The conundrum is, circuit theory says we can get the energy back as quickly as we like simply by making R large, but the physics says that it can't come back faster than the speed of light.

The golden rule in any conflict between circuit theory and physics is that physics rules. Circuit theory is a useful approximation to complex electromagnetic phenonemon, but it has strict limits of applicability (accuracy). One useful rule is that circuit theory tends to break down (or at least need special care) when the circuit time constants approach the time it takes light travel across the circuit dimensions. This is usually phrased differently: When using sinusoidal excitation, circuit theory breaks down when the circuit dimensions become an appreciable fraction of a wavelength.

That's what is happening here, you can't get your energy out of your inductor quicker than the physics says you can, and when you put in values that suggest that you can, your circuit theory model is being used outside its range of validity.

In terms of circuit theory, you would find that your inductor didn't look like a perfect inductor at frequencies around c/d, where c is the speed of light and d is an indicative inductor dimension.

There are a bunch of similar problems, charging the inductor is one, charging and discharging a capacitor are others.

Look at discharging a disk capacitor by connecting a resistor through at the center. Again, by making the resistor very small circuit theory says you can discharge it as quickly as you like, but clearly there is a limit as to how quickly you can get the energy from the outside of the disk to the resistor in the centre. If you have done transmission lines you will see that this starts to look like some sort of parallel plate transmission line, and you can get a picture of what is happening. It is relatively easy to see what is happening here, but make your capacitor of two spheres and it is nowhere as easy to understand how to model it.

Don't take the criticism here to heart, you are asking exactly the right questions, by understanding the limits to circuit theory you will be able to tackle a much wider range of problems than audio frequency type electronics. More importantly you will know when you can apply circuit theory and when it starts to break down, and that's actually quite rare, even in qualified engineers.

16. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
thanks for the back up , lol, jk, everyone's input helps in one-way or another. I knew there was problem somewhere and I just couldn't get my head around the traditional way I have heard about how they work, just didn't make sense to me. Thanks for post and if anyone else has any input then please post aswell.

also thanks for video link "thatoneguy"

17. ### GetDeviceInfo Senior Member

Jun 7, 2009
1,571
230
This could be compared to current in a wire, where one could say, how does the last electron know when to flow, or when we interrupt the circuit, does current continue to flow due to inertia.

In a quantum viewpoint, the levels of energy are stepped and supported by the previous step level. Removing a supporting level causes the outer level to step down to replace it. There will be as much force causing a collapse as there is expansion. Similar to the electron comparison, removing the supporting force at the inner most field, is immediately felt at the outer most field, causing simaltaneous collapse. The rate of collapse is determined by the rate at which the force of field generation is removed.

18. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
In Quantum Physics, I know there are certain levels of energy and that is sorta why it is called quantized for smallest amount of a physical quantity.

What I didn't know was that according to you "GetDeviceInfo"

"In a quantum viewpoint, the levels of energy are stepped and supported by the previous step level. Removing a supporting level causes the outer level to step down to replace it. There will be as much force causing a collapse as there is expansion. Similar to the electron comparison, removing the supporting force at the inner most field, is immediately felt at the outer most field, causing simultaneous collapse. The rate of collapse is determined by the rate at which the force of field generation is removed. "

I never knew that by removing the supporting level the outer most would then fall back. I don't know if you made a mistake at " is immediately felt at the outer most field, causing simultaneous collapse" because from everything I know about physics, including quantum physics is that no information can move faster then light. Even in the most Bizarre cases in Quantum physics like Entanglement where one effect on one particle causes a immediate effect on another. But even in that example, Information could never travel faster then light. (that it self would require an entire thread or maybe a hundred, lol). So when you said immediately, I just don't see how that could ever happen, the more and more I think about it and try visualizing different ways, I think to myself that there is no way the field that was emitted could ever collapse back and the whole reason for this is the one simple law of physics,(Information can never move faster then light), from what I know at-least.

If that law was broken then basically all of physics would need to be Rewritten or at least changed so that it allowed it.

Like I said in a previous post, I had one idea, the idea explained both the spreading out of the field and the collapse, it basically consists of a field and Virtual Photons. The Field spreads out and energizes space, lol, when the field stops, as in the EM stops producing the field, the Virtual photons collapse back to the vacuum and in doing so, produce a field of the same strength as the original and eventually caused the Back-emf. Now for the question on why do differn't resistor values change how fast and slow the EM Discharges and Charges, well that is all because of the circuit, the Field (magnetic field) collapsing back never collapses back faster or slower, it collapses back and the same speed it spread out, so as you can see it is the circuit parameters that determine fall times and rise times.

So Does this make sense to anyone, no one really answered before, I just want to know if it is sound or I am crazy, either one is ok, lol

I am going to stop here as this is already to long ,lol

19. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
I just wanted to anwser this one question or example you had. I could be wrong but I will never know if I don't anwser.

"This could be compared to current in a wire, where one could say, how does the last electron know when to flow, or when we interrupt the circuit, does current continue to flow due to inertia"

First off,
"This could be compared to current in a wire, where one could say, how does the last electron know when to flow"

Well the reason that last electron knows to flow at least from what I understand is because as you apply a Voltage difference across the wire and you have a conductive path, the electrons will start to move becuase of the Voltage Difference and the amount of is determined by resistance. It is basicaly the pin ball idea, one electron leaves a atom, this atom is now defeicient one electron and has a charge pulling the one next to it, more complicated then this but I don't want to get to off the main topic of the thread.

2nd,
from what I know, the reason would have to be inductance as even a straight wire has inductance. The magnetic field that was created is the reason the electron keeps moving or wanting to. The magnetic field collapse sets up a voltage and this is the real reason for it wanting to keep moving.