# Back-EMF/Induced Voltage question?

Discussion in 'General Electronics Chat' started by wes, Dec 12, 2010.

1. ### wes Thread Starter Active Member

Aug 24, 2007
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I have been wondering about magnetic fields and the back-emfs they can create when they collapse.

My question is how does they magnetic field creates this back-emf?

I have thought before that magnetic fields could overlap/intersect (well actually not sure, had my doubts).

Example. A transformer, I thought that the secondary, when it's field built up from the current would then travel across the core and induce a back-emf.
hence the overlap/intersect idea.

But now I have researched some more and have found this cannot be the case.

Instead all the secondary's Field can do is effect the primary's field, so when the primary collapses back, it generates the back-emf, just smaller because the secondary effect on the primarys flux.

Is this correct?

This leads me to one more question, lol
I want to test it but Radio shack is really far, lol

Two coils, Coil A and Coil B , both are very close, so the effect is greatest

Coil A is kept at steady state (A normal DC Electromagnet )
Coil B is then powered up very quick.

result=
1.Coil A will feel the Induced voltage from Coil B,
Coil A's voltage will decrease then rise back up after Coil B is at steady state
This would be the MAGNETIC FIELDS CAN OVERLAP/INTERSECT.

2. Coil A will feel nothing since it is not changing. Since it is not changing, even if it's flux decreases, the field is not collapsing back with less flux which would induce less voltage and so the net voltage would be greater causing greater power pull. NO CHANGE AT ALL ON COIL A

Before I thought it might be 1. But after more research, I am leaning more towards 2.

Help on this would be greatly appreciated as my mind is going crazy with this idea.

2. ### R!f@@ AAC Fanatic!

Apr 2, 2009
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You are going crazy.........

3. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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The back EMF will always arrange itself to OPPOSE the change of current. If you increase current flow, a current will be introduced that OPPOSES that increase. If you decrease current flow, a current is created that OPPOSES the decrease.

That is it in a nutshell. Ever have a question what back EMF will do? Find out what the current flow direction is and the resulting magnetic polarity, and always know the back EMF will manifest whatever polarity it must, in order to OPPOSE it.

4. ### marshallf3 Well-Known Member

Jul 26, 2010
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When the field collapses on the primary (driven) inductance the magnetic field wants to collapse into something, and since you've cut off the primary it tries to go anywhere it can, often rising to as much as 10x the original driving voltage. If there's a secondary winding it will try to escape there.

Poor explanation, but I think this may be of interest: http://blog.secretscience.org/wp-co...ant-Energy-and-Over-Unity-Power-Explained.pdf

5. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
To Kermit2, I understand that the back-emf will always try to oppose change any way it can like in the coils in my example. But the way the back-emf is created in those coils is simple (I am 99% sure) is that as the EMF travels around the coil, the current will follow at just about the same rate as voltage until the Magnetic field from the first turn reaches the second turn and causing a back-EMF on that turn and so on. At this point the Current is now lagging the Voltage by usually 90 degrees, because of the back-EMF.

If that was a little complicated, (not because the idea is hard, just the way I explained it. )

then here is a easier explanation, the reason for the Back-EMF in a coil Is because of the Magnetic Fields effect on the surrounding turns.
Obviously there is always a little Inductance on even a straight wire. but the coil just amplifies the inductance and hence the Back-EMF as well due to the coils being very close together.

Now I have another Idea, maybe this will be easier to figure out what happens.

EXAMPLE.
this will ask the question of do Magnetic field overlap/intersect or not

I am leaving out the Detailed info to make it simple

1. Coil has say 10 turns
2. the turns are spaced about 1 Nanosecond apart (aka 1 foot)
3. The Voltage/EMF is Applied to the coils
4. The EMF will travel the length of the coil, through each turn.
5. Let's assume this is a special coil.
6. Since the EMF will travel around the coil and the inductance of each turn is very low, the current is nearly the same phase as the voltage.
7. The reason is because the Magnetic field from each turn has not reached the next turn yet, but the voltage has already reached it and so has the current.
8. The Coil now reaches full power before the Magnetic field from turn 1 reaches turn 2.

Question now is, What happens when the Magnetic field from turn 1 reaches turn 2, or will it ever reach it, will the field instead just decrease the flux of turn 2 and turn 2 will only feel the back-emf when it turns off like in the transformer Explanation

Last edited: Dec 12, 2010
6. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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I think you still have some conceptual errors in your summary.

EMF is a force not something that flows.

Voltage is applied to a coil. Nothing happens initially except the appearance of a potential electrical difference between the opposite ends of the wire. IN RESPONSE TO THE APPEARANCE of this EMF, electrons MOVE toward the electrical potential that indicates a lack of electrons (+). If it moves in the other direction the electron encounters a net larger number of electrons moving the opposite direction and therefor travels a shorter distance before being impacted and sent back in the 'proper' direction.

Resulting in a one way 'flow' of electrons, if, and ONLY if, the wire from which they leave, has a connection to a supply of negative charges(electrons). It must be connected to result in a 'flow', otherwise EMF remains merely a static 'potential' difference of charge.

7. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
I am sorry for the misuse of words, lol. I know it is a force, not something that flows like current does, I meant by flow that it travels through the wire causing a force on the electrons to flow. If you reread what I posted and replace flow with what I meant then it might sound better and easier to understand

8. ### wes Thread Starter Active Member

Aug 24, 2007
242
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by the way a simple yes it is 1 or 2 and then maybe a explanation why would be great

9. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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It is not a function of MY understanding. I was trying to help you understand. If your understanding is complete, what is the post here asking for then? Perhaps you are asking a different question that I do not understand. Understand?

10. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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The circular magnetic field around a conductor expands as current flow increases. The expansion of the field cuts across adjacent turns of wire, and induces a current flow which attempts to reduce the current that is increasing.

The current in a conductor is decreasing and the circular magnetic field surrounding the conductor is contracting. As it contracts it cuts across the adjacent coils of wire and induces a current flow that attempts to increase the decreasing current.

11. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
Exactly Kermit2, you are correct.

Now what I am asking is what happens if the current and thus the field is built up in the turns of the coils before the field from the first turn reaches the 2nd turn, so the Magnetic field from the 2nd turn will encounter the first field.

QUESTION IS: WILL THE FIST TURNS FIELD STILL INDUCE A VOLTAGE ON THE 2ND TURN

All this arises because magnetic fields cannot be at the same place at the time
they can only add or subtract from the existing field.

(as far as I know)

12. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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The expansion and contraction of the field begins with the movement of the electrons. There cannot be the situation you describe because of the timing of the events. When the current flow is established it has to 'build' up. The first increase of current causes counter EMF that opposes the increase, so there is a definite time lagging component to establishing full conduction through a solenoid. The current flow if frozen in time would show that the number of electrons at any point in coil would be related to amount of time which has passed since the connection was made and the distance from the negative end or positive end. The conductor would be full of electrons at one end and their number would decrease as you approach the opposite end of the conductor. Each turn of wire is affected by the magnetic field of the previous turn and produces induced current which opposes the growing current flow.

The second turn of wire will have a smaller net field flux than the first turn because fewer electrons are moving through it. Therefor the magnetic field of the established current flow will continue to grow, but with negative feedback, from the turns it hasn't flowed into yet.

13. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
Ok I knew somebody was going to say that.

The coil is special as in instead of the current flow needing to move through the whole coil, the new coil allows the current to only need to flow through one turn instead of many.

I know the current and the magnetic field would never be exactly in phase with the voltage because of small inductance's caused by the wire itself among other effects. But for this, the current should act as if it is moving through a straight wire and not a coil.

why?
Because the rise time of the current and thus the Field is faster then the time it takes for the Field to reach the second turn. Simple

Now as I said earlier, what happens when that field reaches the second.

does it not reach it and instead encounters the field from the second and thus decreases the flux.

OR

does it pass right through the second field and directly induce a voltage on the second. This I do not believe could happen after my research on magnetic fields.

I am trying to make this as simple as possible.

14. ### blueroomelectronics AAC Fanatic!

Jul 22, 2007
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May I ask the obsession with magnetic fields in general? What are you looking to discover that hasn't been discovered before? Perpetual motion? Overunity?

15. ### wes Thread Starter Active Member

Aug 24, 2007
242
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just because in my research on them, I found a lot of things that I find really interesting and would like to know more about them.

Such as this question, lol

16. ### blueroomelectronics AAC Fanatic!

Jul 22, 2007
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Based on your old posts over the last three years you're trying to build something that puts out more energy than you put into it.

Well you can never have overunity, it's an enormous waste of time and thankfully the threads get locked after awhile.

There is a site called overunity.com that enjoys this sort of thing.

17. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
well those sites suck, lol

A lot of what I find on those sites is people that don't even understand the physics that well. I mean there are people that actually think that by just measuring the voltage output, they can say they are outputting more power, lmao

Or there is the magnetic engine Idea, which can actually work, as long as you don't load it, lol (there was one I remembered that could actually be loaded, but anymore and it would stop within seconds)

There are also the one's about capturing the Back-emf. Sure you may get back a high voltage but what about the current, lol also how long is the pulse from the back-emf captured, lol

I like this forum because well, all your answers are based on REAL physics.

18. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
but besides that I don't want to get off topic.

Message me if you want to talk about it

19. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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There are things about magnetic fields interactions which are not fully understood. Theories are abundant and most have merit, but there is still room for curious people to discover more in this portion of physics.

20. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
lol I keep getting this same anwser, lol.

Scr## it, I am going to go to radios shack and try to get the stuff to do this and see what happens, lol