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infinitely long magnet and a single coil leg?
By kitno455, Section Mechanical Posted on Fri May 06, 2005 at 06:04:01 PM MST
Trying to get a handle on the root cause of electron motion in a coil...

ok, i know this is hard to test, but i am looking for a way to determine just what is the root cause of electron motion in a coil, as a magnetic field passes by.
i have an oscilliscope, and i have definately seen a decent looking sine wave using a single mag and a single coil. the bumps in the wave correspond with the mag passing directly over the two coil legs (sides) at that moment. there is no iron core or backing, i used a piece of wood as a handle for the coil.
so my dilema is thus: is it the lines of flux cutting the coil leg, or the change in flux density that causes the electron motion? i realize that the lines of flux are sort of 'virtual' way of describing density, so these two things are very nearly the same. but they are not exactly the same, as one is more the derivative of the other.
if you took an infinitely long magnet and passed it under one coil leg only, would you get a flat-topped bump in your wave, or would it continue to rise, or dip in the middle of the bump? i realize infinitely long is impossible, but what i am trying to say is: if the density of the flux above a mag is constant along its length, and it passes a bit of wire, is electricity produced?
allan

infinitely long magnet and a single coil leg? | 24 comments (24 topical)

Re: infinitely long magnet and a single coil leg? (3.00 / 0) (#1)
by ghurd on Fri May 06, 2005 at 12:22:32 PM MST

I will have a stab at it.

The conductor will have voltage induced as the flux changes around it.
The voltage will cause current to flow.
The flowing current produces flux. (like an electromagnet)
The 2 will balance out somewhere.

But an infinitly long magnet passing over a coil, assuming you mean it is infinitly passing the coil, will not produce electrons to flow.
The magnetic field is stabile, not changing, like a stationary magnet.
No change in flux, no induced voltage, no current.

It would not even produce voltage as it started moving because it is starting somewhere in the middle of the magnet where the flux is the stabile. Infinatly long has no ends, right?

Thats why you can't take a pair of microwave magnet, mount one on each rotor and produce voltage. (unless you cut them up and flip every other piece)

Anyone can point out what is wrong and expand on it.
No offense taken, as always.
G-
Ghurd.info

Re: infinitely long magnet and a single coil leg? (3.00 / 0) (#2)
by BeenzMeenzWind on Fri May 06, 2005 at 01:17:42 PM MST

I suppose you could get an output if you rotated a coil in a camlike fashion inside the field of your infinitely long magnet, since the rotational velocity and changing distance would create a variable flux. Just for the purposes of this thought experiment, of course.
'....If I even knew that I know nothing, that'd be something. But I don't!'
[ Parent ]

Re: infinitely long magnet and a single coil leg? (3.00 / 0) (#3)
by ghurd on Fri May 06, 2005 at 03:07:32 PM MST

That would be like moving the magnet.
G-
Ghurd.info
[ Parent ]

Re: infinitely long magnet and a single coil leg? (3.00 / 0) (#4)
by kitno455 on Fri May 06, 2005 at 08:10:21 PM MST

ok, so i did an experiment. i made an 'infinite' magnet by putting a bunch of curved ceramics on an armature, all the same pole up. took a little coercion with some tape.

i put the motor back together with two blocks of wood separating the end plates, instead of the stator housing.

hand-held coil on a scrap of wood, with the coil lying 'on' the face of the armature, no output at all. scope shows just the tinyest wiggle in the line. nothing the trigger will even pick up.

same coil, held sticking 'out' from face of armature, so that one coil leg is near the mags, and one is away. now we have a definite 'signal' on the scope. zero volts in long, flat segments, except where there is a crack between the mags, then we get a little bump. the wider cracks have bigger bumps.

same setup, but with alternating mags makes big, beautiful sine wave, almost no matter how you hold the coil. biggest when coil lies flat on surface of armature.

that pretty much settles it for me, and jives with my understanding of how transformers work too.

change in flux density/direction around the wire is the cause of electron movement in that wire. if the thing that makes the flux (and hence the fog of flux too) moves past the wire, but the density of the flux at the wire does not change, then nothing happens.

i suppose that this means that the superiority of larger surface area mags is only due to the increased flux density near their centers?

allan

[ Parent ]

Re: infinitely long magnet and a single coil leg? (3.00 / 0) (#5)
by ghurd on Sat May 07, 2005 at 06:26:36 AM MST

Big mags, and there is more or longer wire being 'fluxed'.

Say you get 1 volt per inch of wire(wow- great magnets).
With a 1/2" square magnet, you get 1/2V per turn.
With 1" square mags, 1V per turn.
And 2" square mags get 2V per turn.

And with a 2" x 1/2" mag, the dead Cu at the top and bottom is only 1/2",
when with a 2" square mag, it is 2". 4X more wire just adding resistance.

Thats why with the 1 x 1/2 mags, etc, are set in with the 1" parallel to the active legs.
The other way only has 1/2" of Cu being 'fluxed', and the top and bottom of the coils are 1" of dead wire.

Don't take it too extreeme, like a 3" x 1/8" is probably not much good.

None of that is exactaly right, but you get the idea.
Geeze. I hope it is right.

Left out some stuff. Probably not very important for 'understanding' windmills.
My thought is the mags long side is parallel with the active part of the coils,
and use the biggest strongest mags that fit.
But I am always working on something tiny.

G-
Ghurd.info
[ Parent ]

Re: infinitely long magnet and a single coil leg? (3.00 / 0) (#6)
by ghurd on Sat May 07, 2005 at 06:42:35 AM MST

Are you familiar with the "Right-Hand Rule"?
Might want to google it.
G-
Ghurd.info
[ Parent ]

Re: infinitely long magnet and a single coil leg? (3.00 / 0) (#24)
by finnsawyer on Fri May 13, 2005 at 11:08:11 AM MST

Your results don't negate what I've said if the effects are small. The equation involves the velocity of the charge, not the magnetic flux, so, if moving the magnet does not cause a time varying change in flux across the coil (or loop - you must create a loop to measure any voltage) there is no voltage induced. On the other hand, moving the wire or loop causes a voltage to be induced in each leg of the loop. It is possible for these voltages to cancel so you must design your experiment carefully and the velocity must be high enough. I suggest you try the copper disk over a circular magnet. These are are well known and accepted effects. You might try reading up on it. Spinning a copper washer or disk over a magnet will generate a voltage that may be too small for practical use. On the other hand the current can be quite large.

You saw the effects of a time varying flux change when the vertically oriented coil crossed the gaps. Over the gaps the flux will drop.
GeoM
[ Parent ]

Re: infinitely long magnet and a single coil leg? (3.00 / 0) (#14)
by finnsawyer on Sun May 08, 2005 at 10:12:05 AM MST

Only comment is that the coil could care less about the flux around or outside of it. It's the change (or specifically the time rate of change) of flux inside the coil that creates the voltage. Over a long bar magnet the induced voltage will dip as the coil travels it's length, rising again near the end (a double hump). If you replace the coil by a single wire across the magnet, however the the induced voltage will rise to a maximum, stay there until you get near the end and then drop to zero (a single pulse with a flat top).
GeoM
[ Parent ]

Re: infinitely long magnet and a single coil leg? (3.00 / 0) (#15)
by kitno455 on Sun May 08, 2005 at 01:01:05 PM MST

g- what you just described was my original thought, but it does not jive with my experiment.

you state that 'flux change' is what creates power, and then you say that a really long mag with a single wire passing by will have a broad, flat topped power peak. by definition, that is not flux change, and hence, no power should be made. both things you state cannot be true.

i had a long coil of wire, with just one leg near the 'long bar mag' and i most definately did not see a 'a single pulse with a flat top'. in fact, i saw diddly squat in the middle. 0 v. only a bump on each 'end' (crack in the mags). where the flux changed.

so, i think my original though (and your current one) is wrong.

allan

[ Parent ]

Re: infinitely long magnet and a single coil leg? (3.00 / 0) (#17)
by ghurd on Mon May 09, 2005 at 04:18:15 AM MST

That last one ('current one) was not mine.
G-
Ghurd.info
[ Parent ]

Re: infinitely long magnet and a single coil leg? (3.00 / 0) (#18)
by ghurd on Mon May 09, 2005 at 04:24:53 AM MST

But now I see what he is saying.
Finsawyer talking about a 'very' long magnet, instead of infinately long.
When the coils is nearer one end, it is nearer the N or S.
As it moves nearer the other end, it is nearer the opposite pole, so the flux changes, and that causes voltage.
G-
Ghurd.info
[ Parent ]

Re: infinitely long magnet and a single coil leg? (3.00 / 0) (#19)
by kitno455 on Mon May 09, 2005 at 07:32:33 AM MST

no, i meant geoM when i said g, sorry :)

if you read carefully, he (fin/geom) says two different things, and both of them cannot be true at the same time.

allan

[ Parent ]

Re: infinitely long magnet and a single coil leg? (3.00 / 0) (#20)
by finnsawyer on Mon May 09, 2005 at 09:15:46 AM MST

Actually they can. To begin with I assumed the magnet surface was one pole N or S. As far as the two cases they are flip sides of the same coin. The way I look at it (check Maxwell's Equations) is that a moving charge creates it's own magnetic field that then reacts with the magnet's field to put a force on the charge (the qxvxB thing), so the free electrons move along the wire and build up a charge at the end of the wire. In the case of the loop the time varying magnetic field creates an electric field that then reacts with the electrons in the wire loop causing them to again pile up at one end of the coil (the Ax(dB/dt) thing). While the two formulations certainly appear different they deal with the same phenomena (electro-magnetism) and hence cannot contradict each other. Each is useful in solving certain types of problems or explaining certain observations. A hall effect device, for example is based on the first equation; a transformer on the second. Both work.
GeoM
[ Parent ]

Re: infinitely long magnet and a single coil leg? (3.00 / 0) (#22)
by kitno455 on Mon May 09, 2005 at 12:22:04 PM MST

ok, can i quote your original mail?

"Over a long bar magnet the induced voltage will dip as the coil travels it's length, rising again near the end (a double hump)."

ok, this makes sense to me. assuming the coil is much narrower that the mag is long, there will be points in the motion where both coil legs are over the mag. they will cancel out. at the ends, when one coil is near or past the edge of the mag, the field it cuts will be weaker, and the other leg of the coil will dominate. right?

"If you replace the coil by a single wire across the magnet, however the the induced voltage will rise to a maximum, stay there until you get near the end and then drop to zero (a single pulse with a flat top)."

this does not jive with my results, hence my confusion. how is it that a single wire, moving along a really long mag, will have any output at all near the center? i mean, the flux strength is not changing much, why does the output not drop? that would be what i saw with my experiment, the output was low over the center of the mags, but high at the cracks between mags.

was i testing something else, and not realizing it?

allan

[ Parent ]

Re: infinitely long magnet and a single coil leg? (3.00 / 0) (#23)
by finnsawyer on Thu May 12, 2005 at 10:42:19 AM MST

Basically, when a charge moves in a magnetic field it is subjected to a force that is at right angles to both it's direction of motion (velocity v) and the direction of the magnetic flux (B). The magnitude of the force is given by qxvxB. You can determine the direction of the force (for a positive charge) by putting the fingers of your right hand in the direction of the velocity of the charge and turning the hand into the direction of the magnetic field (North pole toward South pole). The thumb then points in the direction of the force. This is the force that redirects particles in the solar wind toward the Earth's poles and gives rise to the Auroras. It makes the particles spiral around the lines of magnetic force, which flow to the magnetic poles.

In the case of the wire, only the electrons can move, so they pile up on one end of the wire and cause a voltage difference across the wire. When you connect a voltmeter, however, you add more wires (form a loop) that also can move and have induced voltages. You can eliminate that problem by placing a copper disk centered over the north or south pole of a circular magnet. By spinning the copper disk you will induce a voltage between the center of the disk and the rim. You can measure this voltage without having any wires moving even though you have created a loop. No voltages are induced in any of the wires.
GeoM
[ Parent ]

Re: infinitely long magnet and a single coil leg? (3.00 / 0) (#21)
by ghurd on Mon May 09, 2005 at 10:43:50 AM MST

I agree with geoM/ Finsawyer. They can be true at the same time.
But I think one is more related to a 'very long' magnet, the other for an infinatly long magnet.

Anyway. If the magnet IS infinatly long...
It can be argued equally it MUST or CAN'T be closer to one end.

But even if it is closer to one end (say N), it then is still infinatly far from the center (or end), so moving it toward the center (or end) will NEVER get it any closer to the center (or end). The coil Never get closer or farther from either 'head of the coin'.

Therefore, the flux will NEVER change and no voltage will be induced.

So there is no moving charge creating flux to act with the magnets flux. If there was it would still balance out eventually.
(from around my first post, something like "balance out... to stabile"

And if the math is done between the 2 equations, 'overunity' is possible!
Therefore 1 or both must have somekind of error, somewhere.
(don't remember how, but we did it in school... the math- not the actual overunity)

My brain hurts. Not used to using it much lately. :/
Man! This is fun!

G-
Ghurd.info
[ Parent ]

Re: infinitely long magnet and a single coil leg? (3.00 / 0) (#7)
by electrondady1 on Sat May 07, 2005 at 10:31:29 AM MST

this is interesting, and important to understand if your building from scratch. obviously long skinny coils /mags are the most effective . i am curious as to the ideal proportions.

Re: something useful out of little neos? (3.00 / 0) (#8)
by BeenzMeenzWind on Sat May 07, 2005 at 02:15:24 PM MST

Just wondering if anyone has any thoughts on a use for 10 1/2 x 1/8 disc neos? I got them off Ebay for £1, free postage. Looked at the right time, no bids - 52 seconds to go. lol

They came today, along with a cheque from a land sale for £5700 which I've been waiting on for months. Been a good day.

The magnets are bloody great, pardon my French. I took them out of the bubble wrap and the two stacks of 5 immediately snapped together, with my pinky in the middle. It hurt too! I nearly cried! Well, almost.

Any suggestions would be appreciated. Got to be some interesting little PMA I could build?
'....If I even knew that I know nothing, that'd be something. But I don't!'
[ Parent ]

Re: something useful out of little neos? (3.00 / 0) (#10)
by nanotech on Sat May 07, 2005 at 03:49:14 PM MST

Heh, I won't ask for the check, but I will ask just how much of a profit would you be looking for if you sold the magnets?
__________________________________________
My ship finally came in, but it was the Kobayashi Maru!
[ Parent ]

Re: infinitely long magnet and a single coil leg? (3.00 / 0) (#12)
by kitno455 on Sat May 07, 2005 at 05:44:28 PM MST

i am curious too. i have been playing some with linear alternators, and it seems that most of those have very short mags, something like a few millimeters wide in the direction of motion. i guess thats mostly due to their short strokes.

i may have to buy a mixed batch of mags from otherpower and try some experiments. at some point, the mag being shorter in the direction of motion is going to make so much lower output that its width wont matter. the point at which this occurs is going to have more to do with coil leg width, i think.

allan

[ Parent ]

Re: infinitely long magnet and a single coil leg? (3.00 / 0) (#9)
by electrondady1 on Sat May 07, 2005 at 03:17:15 PM MST

you can keep the mags but mail the check !!!

Re: infinitely long magnet and a single coil leg? (3.00 / 0) (#11)
by BeenzMeenzWind on Sat May 07, 2005 at 03:55:03 PM MST

Fraid I'm keeping both for the moment. The cash will go a long way to finishing this turkey so we can sell it and the magnets are useful for pinning notes to the fridge until I think of a proper use for 'em. lol
'....If I even knew that I know nothing, that'd be something. But I don't!'
[ Parent ]

Re: infinitely long magnet and a single coil leg? (3.00 / 0) (#13)
by BeenzMeenzWind on Sun May 08, 2005 at 02:10:59 AM MST

I found a diagram / photo of a 10 magnet PMA rotor by Googling. Unfortunately it doesn't show the stator windings very clearly. I'm sure I can work that out, though.

But that can keep until next week, cos we're off on vacation. I've cunningly arranged that we go in the van, so I can pick up some metal from a supplier near where we're staying. Two birds with one stone and all that! Suppose I better go and kick the tyres and check all the lights work.

Catch ya later.
'....If I even knew that I know nothing, that'd be something. But I don't!'
[ Parent ]

Re: infinitely long magnet and a single coil leg? (3.00 / 0) (#16)
by nothing to lose on Mon May 09, 2005 at 01:09:37 AM MST

I wonder what would happen if we took a sphere magnet and spun it in the middle of a bunch of wires, or spun the wires around it? I geuss in a way what I am thinking would be the same as a PM motor with only 1 of each pole? Just made different.
.
nothing to lose

Spelin and tpying are my strong points, not electronics.

infinitely long magnet and a single coil leg? | 24 comments (24 topical)

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