Discussion in 'Physics' started by wes, Nov 19, 2010.

1. ### wes Thread Starter Active Member

Aug 24, 2007
242
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So here is something I have been trying to figure out. I don't care if it is a stupid question, lol. I don't know it, so I want to find out.

I understand that to induce a voltage into a inductor, (let's assume a transformer ) you need to have a changing magnetic field.
A steady state magnetic field will not induce a voltage.

I know the equations and everything explain this.
but why?

why is it that a steady state field will not induce a voltage yet a changing field will?

If you could please explain it from a quantum point of view, the basics.

2. ### Wendy Moderator

Mar 24, 2008
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Because electricity is created as the lines of force cut across the conductor. Doesn't matter which ones are moving, one of them has to.

A static magnetic field is not moving, so you get a pulse as it is formed, and this is it.

The transformer core is designed to keep the magnetic field tightly contained, otherwise it can wreck havoc with other electronics. It is also why transformers are one of mans most efficient mechanisms, approaching 99% efficiency. Dr. Tesla was one smart fella.

3. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
but don't the magnetic field lines cut across the inductor even with a steady state. What I mean is even after the pulse, the changing, the field lines are still cutting across the conductor, they are just not changing, why is it that it still won't create a voltage, I know someone is going to think, "because the field is no longer changing", lol. But why is it at the most basic level.

4. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
and actually, if you think of it from a pure physics point of view. even at steady state, the field is still moving, just not changing in strength.

so why is it that from this view the field no longer induces a current at steady state, even though it is still technically moving.

5. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
what i mean from this "even at steady state, the field is still moving, just not changing in strength."

I mean that it is moving outwards into space and obviously is being emitted from the coil and continuously. it doesn't just move out and then stop, that would be impossible. A simple read over of a good physics book will show that.

6. ### retched AAC Fanatic!

Dec 5, 2009
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no. If that were the case and you put a magnet near metal filings, the filings would move around continuously.

AND an electric guitar would ALWAYS make a noise. The strings breaking the flux lines is what causes the sound. When the strings are not moving, no sound from the pick-ups.

If there were steady-state motion, the pick-ups would move the strings.

7. ### wes Thread Starter Active Member

Aug 24, 2007
242
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then how do explain the continuous repulsion of two EMs

or how a detector far away and sensitive enough would continuously pick up on the field?

8. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
I guess what I am trying to get at is how do explain all this from a quantum mechanics point of view?

why is it that a changing field creates induction whereas a steady state does not? explain all this from the view of quantum mechanics.

9. ### BillO Distinguished Member

Nov 24, 2008
993
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Quantum mechanics? Well maybe QED ... on second thought, maybe not.

QM does not deal well with fields.

No energy expenditure is required to sustain a field. The energy in a steady state field is not changing. When the field strength grows or weakens the energy in the field changes, work, such as inducing a current in a loop, can be done.

10. ### BillO Distinguished Member

Nov 24, 2008
993
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There really aren't any 'flux lines'. There is a field flux, but the lines are imaginary.

The guitar string locally focuses the ambient magnetic field. When it's plucked and it vibrates the field varies because the point of focus changes. This tiny fluctuation of the field induces a current in the pick-up coil.

11. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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As I've mentioned once before in a topic similar to this. There is still room for discovery to be made in the field of magnetic fields. Pun intended.

We DO NOT know what mechanism operates on what medium to create the 'field'. There are THEORIES on what is going on, and most of them are probably really close to what takes place in reality, but there are still many unanswered questions.

Why can a disc and magnet that spin in synchronization produce a voltage, even though there is no relative motion between the two, and when the disc is stationary and the magnet spins, no voltage is made?(homopolar generators). Theories try to explain it, but ..... does the field remain stationary while the magnet rotates? No one knows for sure, but if it moves with the magnet then why no voltage produced when the magnet spins?

See? Lots of room for discovery.

12. ### BillO Distinguished Member

Nov 24, 2008
993
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@Kermit2

I think it may just be you on this one. How homopolar generators work is fairly well understood.

“The charge separation results from the Lorentz force on the free charges in the disk. The motion is azimuthal and the field is axial, so the electromotive force is radial."

On the question of why this would work if the magnet rotated with the disc or was stationary, but would not work if the magnet rotated. Why would one think that the field between the poles would rotate with the magnet? There is no mechanical connection between the magnet and the field. And if it did or could, what possible difference could that make? The field is not a mechanical phenomena. It’s ‘rotation’ in this respect, or not, is totally immaterial as it is something it is not capable of.

13. ### wes Thread Starter Active Member

Aug 24, 2007
242
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well the biggest reason I see it as a field and from what I have read of QM.

The reason for thinking it is this.

If you have EM and a detector, When the the EM is turned on, A field is set up and after a certain time, the detector will detect the field from the EM. Once the EM is shut off, It will again take a certain amount of time for the detector to detect this.

and that right there is why I see it as it has to move and the energy in the field can't stay at steady state, but is always moving and at the same time being replenised by the EM. I see all this linking back to the question, why is it that a changing field induces a voltage but not a steady state?

Nov 24, 2008
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15. ### Wendy Moderator

Mar 24, 2008
21,012
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This is wrong.

This relates to the other thread about magnetic fields. A static field is not moving. Why overcomplicate it from there? Theory works, you can't modify it because you don't want to work with it as is. You are trying to twist ideas that have worked for over 100 years into something else. Maybe they are wrong, but they work.

Perhaps you would be more comfortable with the idea of rate of change. As with a RC time constant curve, the LC curve never truly stops changing. However, there hits a point where the rate of change is so small as to be immeasurable (ie, undetectable). At this point it doesn't matter if it is still changing, you can treat it as static.

The concepts are extremely simple (or can be simplified). DC creates a static field, a permanent magnet. AC creates a magnetic field always in a state of flux, it is collapsing, reforming an opposite polarity field, only to collapse and so on. The concepts are well understood, the math is relatively simple from the point of view of inductance.

Does DC truly make a permanent magnet? Nope, but when I was in the 3rd grade I demonstrated it to myself that it was so close as to make no difference. I "saw" the lines of force with iron filings and they didn't move. In college I saw the exponential natural log curve that is the LR curve, taught it never reached max, and then was taught 5 LR time constants were close enough, same as with capacitors.

Lines of force shown by iron filings are a myth, but they have a basis in reality, and as an analogy it works. If you casually dismiss them you will loose your eye on the core concepts.

Last edited: Nov 19, 2010
16. ### BillO Distinguished Member

Nov 24, 2008
993
137
As a way of modeling the local 'shape' of a field they are perhaps useful, but unfortunately too many people take them literally. This causes problems. People imagine theses 'lines' spreading out from a magnetic source. In the case of this thread, I feel the OP sees these lines moving away from an electro magnet after it is turned on, and reasons that, since they are moving, they are cutting across the loop and should induce a current. Kermit2 appears to feel they are somehow 'fixed' to the source, so that if you rotate the source, the lines, and hence the field will rotate. Retched sees guitar strings breaking these lines to somehow induce current in the pickups. For some reason the fact that they are to be used as a visualization aid only is not getting across. To many people attribute physical poperties to them that do not exist. The net results are erroneous conclusions. They do need to be dismissed at some (early) point in the thought process.

Apr 20, 2004
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18. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
Well here I guess I might as well just tell you my view in full and see what you guys think

here is the crazy idea I have to visulize all this, lol. I think this getting kinda of off topic though.

I sorta of think of the magnetic 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 field 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 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.

Now for the question on why do different 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 at the same speed as it spread out, so as you can see it is the circuit parameters that determine fall times and rise times.

?????

All this came from my readings and understanding of Quantum Mechanics and QED. I just can't seem to understand why it is that a changing field creates a voltage in conductor and a steady state does not, still trying to figure that out. You can probably see from my Example that I really like to visualize things, lol

19. ### Wendy Moderator

Mar 24, 2008
21,012
2,744
You can not separate electro magnetic and magnetic. This is because exactly 50% of an EM field is magnetic, pure and simple.

Current determines field strength, which determines how far the measurable field reaches, and resistance determines the current. You can not separate them so neatly, they are part and parcel of the same thing. Time constants are not related directly to how far a field reaches, this is more a matter of geometry of the coil.

Magnetism is a field, not a particle. Never heard of a particle associated with a magnetic field (unless you count an electron and its spin). If you have I would be interested in reading about it. Neither is there a particle associated with the electric field. So trying to create a particle that isn't covered by theory (especially a theory that is as researched as this one) doesn't make a whole heck of a lot of sense.

On the other hand there is a particle associated with EM fields (which is just another way of saying RF in many cases). That is the photon. There is probably something significant that both fields in perfect sync create a particle, but I don't know what it is.

The very oldest of theories is consistent on this though, it takes a conductor moving through the magnetic field to generate EMF. Key word, movement. The magnetic field can be moving, or the wire can, but if there is no movement then there is no EMF. A static magnetic field is not moving, period.

20. ### BillO Distinguished Member

Nov 24, 2008
993
137
Did someone say something different?