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Author Topic: Magnet Myths and Misconceptions  (Read 605808 times)

TinselKoala

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Re: Magnet Myths and Misconceptions
« Reply #75 on: October 05, 2014, 07:06:36 PM »
And what is a charge?

That which is moved by the field.

Charge, field, motion, one thing. One.


Now you might as well ask "what is matter" or "what does God really look like".

TinselKoala

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Re: Magnet Myths and Misconceptions
« Reply #76 on: October 05, 2014, 07:10:16 PM »
And what is a charge?
Seriously? It is a fundamental property of matter, one of the "quantum numbers" that describes certain kinds of subatomic particles. It is a conserved quantity, just like momentum or energy. It is that property of matter that causes the associated particle to move in certain ways under certain conditions. It comes in two "polarities" that we arbitrarily call positive and negative. Charge is quantized, which means it comes in discrete amounts, the smallest of which is the Unit charge, and the electron is the particle which carries the Unit Negative Charge.

What is "wetness"?

Pirate88179

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Re: Magnet Myths and Misconceptions
« Reply #77 on: October 05, 2014, 07:19:19 PM »
Bill,

Metals are good conductors because they exhibit free (loosely bound) electrons in their structure. Insulators don't have free electrons, so they are poor conductors of electricity.

See this pdf and this Hyperphysics page.

Just because the free electrons in a wire are constantly scattered and colliding as they make their way from one end to the other, doesn't mean the wire becomes worn out. For the electrons that do leave the wire, there are an equal number entering.

.99:

Thank you.  So, does this mean that the "charge" is like the example that MH gave of the Newton's ball arrangement then?  An electron enters the conductor and collides with (repels) one of those loose free electrons in the conductor and bumps it down along the conductor into other free electrons, etc, etc.  The Hyperphysics link described this as a chain reaction.  Your links, combined with MH's example now make this a lot more clear to me. 

Thank you very much both of you.  I am a lot closer to understanding what is really going on which, of course, could be dangerous.

Bill

TinselKoala

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Re: Magnet Myths and Misconceptions
« Reply #78 on: October 05, 2014, 07:51:09 PM »
Bill,

Metals are good conductors because they exhibit free (loosely bound) electrons in their structure. Insulators don't have free electrons, so they are poor conductors of electricity.

See this pdf and this Hyperphysics page.

Just because the free electrons in a wire are constantly scattered and colliding as they make their way from one end to the other, doesn't mean the wire becomes worn out. For the electrons that do leave the wire, there are an equal number entering.

True enough... but....

http://en.wikipedia.org/wiki/Electromigration

MileHigh

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Re: Magnet Myths and Misconceptions
« Reply #79 on: October 05, 2014, 08:07:33 PM »
Bill:

I can actually offer up a pet theory about your old wire story from the electricians.  I have not done any searching on this at all.  My pet theory is that it may be possible that the metal mixtures in some very old wire may be 'off' or the wire itself was poorly manufactured.  For example, when the metal was made into wire it was not a homogeneous mixture, or they used cheap and inferior copper with too many impurities, etc.

So over lots of time, say 50 years, some of the copper or aluminum atoms started to crystallize.  Or the impurities started to crystallize.  Atoms can actually slowly migrate through the metal over time to gather together.  That physical rearrangement of some of the atoms in the wire would cause micro-fissures or perhaps even very fine cracks visible under a microscope.  Or, the fact that the wire was partially crystallized means that the moment you handled it then fissures or cracks could happen.  The net result would be that the wire would be more resistive because the cracks would reduce the cross-sectional area in various places along the wire.  It's just a guess.

Bill/TK:

No, it was TK (I think) and others that suggested the "Newton's ball" model for current flow were "entering electrons" "push" against neighboring electrons to give you current flow because of the electrical repulsion between electrons.

I don't believe that model is correct.  What I stated is that all of the electrons in a wire will be induced to move at the same time because of the presence of an electric field in the wire,  The electric field snakes down the wire and induces all of the free electrons in the wire to move in unison.

So the electrons in the case of current flowing in a wire are not pushing against each other.  They are all just haplessly being pushed around at the same time by the presence of an electric field.

MileHigh

Pirate88179

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Re: Magnet Myths and Misconceptions
« Reply #80 on: October 05, 2014, 08:49:00 PM »
True enough... but....

http://en.wikipedia.org/wiki/Electromigration

"Electromigration reliability of a wire (Black's equation) Main article: Black's equation At the end of the 1960s J. R. Black developed an empirical model to estimate the MTTF (mean time to failure) of a wire, taking electromigration into consideration. Since then, the formula has gained popularity in the semiconductor industry. :[3][5]

"The temperature of the conductor appears in the exponent, i.e. it strongly affects the MTTF of the interconnect. For an interconnect to remain reliable as the temperature rises, the maximum tolerable current density of the conductor must necessarily decrease. However, as interconnect technology advances at the nanometer scale, the validity of Black's equation becomes increasingly questionable."


TK:


This is exactly what I was talking about in theory except, it does not appear to be a large problem until you get down to a very small scale.  Although, it appears that Black developed this equation to determine the reliability of a wire over time so that seems to indicate that some changes are taking place inside the conductor.

Possibly MH's post about the impurities in the copper (or other conductor) affecting its properties over time is the basis for this maybe?  As he posted, when these micro cracks appear this would raise the resistance of the conductor and therefore possibly cause some heating which might further degrade the conductivity of the wire, etc. (Domino effect)  Very interesting stuff here.

I know I am going a little far afield of the topic but,  I really want to know exactly (if possible) what is happening inside a conductor when electrons vibrate within it or move through it.  After all, it is this activity that creates our magnetic field around these conductors right?

Sorry that I credited MH with the Newton's ball example that you posted.  (Thanks MH for pointing that out)

This is a great topic here fellows.

Bill

minnie

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Re: Magnet Myths and Misconceptions
« Reply #81 on: October 05, 2014, 09:20:47 PM »



 Is there such a thing as "skin effect" where there is more conduction on
the surface of a conductor? Multi-strand wire ought to have good performance.

Pirate88179

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Re: Magnet Myths and Misconceptions
« Reply #82 on: October 05, 2014, 09:54:59 PM »


 Is there such a thing as "skin effect" where there is more conduction on
the surface of a conductor? Multi-strand wire ought to have good performance.

I believe that multi-strand wire has less resistance than a solid wire of the same diameter.  (I think)  Does this mean that the strands are insulated individually themselves from each other like magnet wire?  Or, is it just a bunch of small, bare wires (all shorted together) having more additive surface area?  I guess I am being lazy here as I could just look this up.

Bill

ETA  I just read that due to the skin effect, multi-strand wire will have less resistance as most of the current flows along the surface of the individual wires.  I have no idea if this is true or not but googled it and read this in several places.

bboj

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Re: Magnet Myths and Misconceptions
« Reply #83 on: October 05, 2014, 10:51:27 PM »
Seriously? It is a fundamental property of matter, one of the "quantum numbers" that describes certain kinds of subatomic particles. It is a conserved quantity, just like momentum or energy. It is that property of matter that causes the associated particle to move in certain ways under certain conditions. It comes in two "polarities" that we arbitrarily call positive and negative. Charge is quantized, which means it comes in discrete amounts, the smallest of which is the Unit charge, and the electron is the particle which carries the Unit Negative Charge.

What is "wetness"?




I got that more or less. But than as Point. asked - Why is magnetic field around a conductor with a dc current static?

TinselKoala

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Re: Magnet Myths and Misconceptions
« Reply #84 on: October 06, 2014, 01:50:07 AM »



I got that more or less. But than as Point. asked - Why is magnetic field around a conductor with a dc current static?
An unchanging DC current, perhaps you mean? I can take my DC current of one amp, increase it to two amps, and while I am increasing the current, the magnetic field also increases.
You have got to get past these "why" questions, though. Why is there air? To fill up basketballs with, of course.

Charge, motion and field are related by a rigorously defined set of relationships that are mathematically precise. These relationships are contained in Maxwell's Equations, the Biot-Savart force law, and the Lorentz force law. Why do these relationships exist? So that there will be air to fill up basketballs with, of course.

Here, knock yourself out. (MH, you  might also be interested in taking a look at this document. Warning: math ahead.)

http://www.physics.uwo.ca/~mgc/EM1sec5.pdf


TinselKoala

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Re: Magnet Myths and Misconceptions
« Reply #85 on: October 06, 2014, 01:51:54 AM »
I believe that multi-strand wire has less resistance than a solid wire of the same diameter.  (I think)  Does this mean that the strands are insulated individually themselves from each other like magnet wire?  Or, is it just a bunch of small, bare wires (all shorted together) having more additive surface area?  I guess I am being lazy here as I could just look this up.

Bill

ETA  I just read that due to the skin effect, multi-strand wire will have less resistance as most of the current flows along the surface of the individual wires.  I have no idea if this is true or not but googled it and read this in several places.

Litz wire.

http://en.wikipedia.org/wiki/Litz_wire

TinselKoala

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Re: Magnet Myths and Misconceptions
« Reply #86 on: October 06, 2014, 01:57:09 AM »


 Is there such a thing as "skin effect" where there is more conduction on
the surface of a conductor? Multi-strand wire ought to have good performance.
Yes, and "skin depth" is related to the frequency of the AC current being conducted. The higher the frequency the shallower this depth, in most materials. This is, in part, why people can get away with taking the discharge of a high voltage Tesla coil to their body. The current may be quite high... I have lit up incandescent light bulbs with body-conducted Tesla coil current -- but it travels on the surface of the skin rather than through the body. As long as the points of entry and exit are protected from the direct spark, one feels nothing.
And this is indeed why Litz wire is often used in coils that are to operate at high frequencies.

poynt99

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Re: Magnet Myths and Misconceptions
« Reply #87 on: October 06, 2014, 03:55:13 AM »
You have got to get past these "why" questions, though.
I disagree. The how is the most interesting part in my opinion.

Once the field is there, how to calculate its strength and everything else about it is "old hat" so to speak.

Every text and web page I've looked at seems to skirt around the actual mechanism of how the field is created. I'm sure however that there is a simple explanation. ;)

TinselKoala

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Re: Magnet Myths and Misconceptions
« Reply #88 on: October 06, 2014, 05:21:00 AM »
Quote from: TinselKoala on Today at 01:50:07 AM<blockquote>You have got to get past these "why" questions, though.</blockquote>
I disagree. The how is the most interesting part in my opinion.

Once the field is there, how to calculate its strength and everything else about it is "old hat" so to speak.

Every text and web page I've looked at seems to skirt around the actual mechanism of how the field is created. I'm sure however that there is a simple explanation. ;)

See what you did there? "How" and "why" are not really the same question.
As I understand it, there are a few "free parameters"  in the modern physical description of reality. Not many, only like sixteen or something like that. Everything else takes its calculated or derived value from the seemingly arbitrary values we see of these free parameters. Nobody knows just "why" these parameters take on the values they do, but what is known is that if any of them were even slightly different, the world would be very much different. For example, let the fine structure constant be only a few percent different... and no stars can form. The situation is kind of like Euclid's Axioms. Nobody knows "why", on a perfect plane, two parallel lines never intersect. But it's easy to see _how_ that happens (or rather doesn't happen) and to use that fact to prove, with mathematical certainty, other theorems about geometry.

Sure, the explanation you seek is simple. If it were otherwise... we would not be around to see it. Eventually, when we ask "why" something in Physics is the way it is, we bump up against the Anthropic Principle, weak or strong. And as you know these explanations are less than satisfactory.

So, humans invented Religion. God did it, because He wanted it to be that way. See, it says so right here in the (insert favorite holy book here).

Newton II

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Re: Magnet Myths and Misconceptions
« Reply #89 on: October 06, 2014, 06:12:37 AM »
Sure, the explanation you seek is simple. If it were otherwise... we would not be around to see it. Eventually, when we ask "why" something in Physics is the way it is, we bump up against the Anthropic Principle, weak or strong. And as you know these explanations are less than satisfactory.

You are absolutely right.   Sun always rises from the East and not from the West -   "why?" -  because God feels that East is better than West!!  (Waste!!)