Electrical current and direction of magnetic field

[SAZAR]

Magnetic field has direction.
It has one side and the other side. (we called them south and north...)
If we imagine that magnetic field is actualy a string of directed particles of "mass of space" that go one behind the other to form magnetic field lines, then: WHAT THE DIRECTION OF FLOW OF ELECTRICITY HAS TO DO WITH THE DIRECTION OF MAGNETIC FIELD?

Actually - my question here is: why electrical current passing thorough (e.g. a wire) makes the magnetic field that it creates "swirl" and always in the same direction?

(current runs forward(!) - I mean: what inside conductor's material (atoms) makes field go around that forward direction of current, and why is it ALWAYS the same direction of "swirl"? what mechanism, what logic is implied there??)

 

[masudr]

Choosing which direction the magnetic field is, is merely a matter of choice. Once chosen, we then define motion of charged particles either along or against that field vector depending on whether the charge is positive or negative.

In any case, if the current ran "backward," then the particle in the magnetic field would feel a force in the "other" direction.

 

[SAZAR]

What I was asking on the other hand is: what direction of current has to do with how magnetic field forms?

Illustration:

----O-----O--------O--->

Explanation: that up there is a wire; '>' shows direction of flow of electrons thorough wire; 'O' is magnetic field encircleing the wire.

Next example:
>
O

Here: 'O' is the same as above; and '>' shows how compass orients in that magnetic field.

--------

So: flow of electricity is perpendicular to crossection of magnetic field it created ('O') - how does that "perpendicular motion" of electrons affect atoms of a wire to create magnetic filed "going around" (and always the same way according to the direction of electron flow thorough the wire)?

(Where I epmhasize "perpendicular motion" and "going around" (those two aren't logical together))

---------------
Analogy:
It's like as if a wire is a tube and inside of tube there is spral (coiled) "sub-tube", so when you pass water thorough that "sub-tube" from one direction it spirals one way and vice versa...
NOW THE POINT OF WHAT I'M TALKING ABOUT HERE:
Wire does not have a "sub-tube" (from this analogy with hydraulics) - IT'S SOLID!
SO: what inside of a wire makes that "surface-wise perpendicularily-aligned going-around" of the magnetic field? (which has its laws! - magnetic field "goes" (orientates) that specific way when electicity flows from left to right (and vice versa))?

------
Shortly:

Electrons flow straight, magnetic field is perpendicular; how come?

 

[zoobyshoe]

Electrons flow straight, magnetic field is perpendicular; how come?

I am not sure that anyone has proven that electrons flow straight, just that their overall direction of motion ends up being along the length of the conductor from negative to positive. Regardless, I think if I wanted to try and unravel the perpendicular direction of the magnetic field in a current carrying conductor I'd sit and do some pondering about the Lorentz force as well, since they surely must cast some light on each other.

 

[SAZAR]

"I am not sure that anyone has proven that electrons flow straight"

How do they propose electrons move then?

Does it has to do with orientation of atoms/molecules of conductors, and how come they are always orinentated in such a way that the magnetic field is always orientated the same - by that rule?

(If I wanted to pounder things I wouldn't be asking a question here (that's what this forum is about - to give instant answers to those who don't know things by those who know things (I mean - I'm just interested, free mind... I on my own without any enforced need want to know stuff (how things work in nature (things so common - viewable every day; but can't really explain their origins (strange things - like: what inside matter makes the actual refraction of light, and this magnetic field paradox...))))))

I looked at wikipedia:
http://en.wikipedia.org/wiki/Lorentz_force
and (consequently)
http://en.wikipedia.org/wiki/Magnetism#Magnetic_monopoles

I don't understand it. It doesn't answer my question.
I seek an instant answer.

 

[zoobyshoe]

"I am not sure that anyone has proven that electrons flow straight"

How do they propose electrons move then?

A crooked sort of path, but one which has a general average direction. This site shows the concept I've seen proposed most often:

http://www.qrg.northwestern.edu/projects/vss/docs/Power/2-whats-electron-flow.html

Does it has to do with orientation of atoms/molecules of conductors, and how come they are always orinentated in such a way that the magnetic field is always orientated the same - by that rule?

I'm not sure what you're referring to here.

(If I wanted to pounder things I wouldn't be asking a question here (that's what this forum is about - to give instant answers to those who don't know things by those who know things (I mean - I'm just interested, free mind... I on my own without any enforced need want to know stuff (how things work in nature (things so common - viewable every day; but can't really explain their origins (strange things - like: what inside matter makes the actual refraction of light, and this magnetic field paradox...))))))

I looked at wikipedia:
http://en.wikipedia.org/wiki/Lorentz_force
and (consequently)
http://en.wikipedia.org/wiki/Magnetism#Magnetic_monopoles

I don't understand it. It doesn't answer my question.
I seek an instant answer.

I haven't ever encountered a physical model that explains why the electron movement in a conductor causes a magnetic field at right angles. I think this is probably because no one has proposed a physical model that could be definitively tested.

That is highly unsatisfying, but when it comes down to it, you don't need such a model to work with the fact that the field behaves this way. Simply knowing that it does gives you useful information for designing circuits and devices.

Physics is packed with situations like this. Because things like electrons are not directly observable much of what they do can't be reliably explained in terms of straightforward mechanics. It took a long time before people figured out very, very clever, but very indirect, ways of gathering evidence that such a thing as an electron actually existed.

What I meant when I said you'd have to ponder the Lorentz force as well, was aimed at you or anyone who wanted to buckle down and try to work out a reasonable physical explanation for the same force at right angles we see in both cases. It wasn't clear to me that you didn't realize that no one has probably ever satisfactorily worked out the answer to your question.

 

[ZapperZ]

I am going to make a simplifying assumption to your question. As best as I can understand it, you're asking for why the geometry of the magnetic field due to a long, straight current-carrying wire is the way it is. My short answer to this is the symmetry of the situation.

Here's the long answer. First, let's consider a long, infinite LINE CHARGE. For clarity, let's say it is along the z-axis. If you walk out of the room and I translate the line charge by a certain amount along the z-axis, when you walk back in, you won't notice any differnce. If you again walk out of the room and I rotate the wire around the z-axis, and you come back in, you also won't notice any difference. Do the same thing but this time, I transpose z to -z (i.e. make a mirror reflection), you also won't notice any difference to the line charge. To you, each one of these symmetry operations doesn't change anything with respect to how you see the charge is arranged.

Thus, the RESULTING FIELD must also have the same symmetry. It must have the same translation, rotational, and mirror-image (or parity) as the source. The only possible geometry of the E field for this charge distribution is then the radial field. You'll notice that this field geometry obeys the same symmetry as the source.

Now let's tackle the line current. Again, for simplicity, let's have the current flowing in the +z direction. Repeat all of the symmetry operations I have mentioned above. You'll notice that it is invariant to all of the symmetry operations EXCEPT for one. If I flip the wire from z to -z, the direction of the current is now different! You will walk back into the room and would notice that the current is now flowing in the -z direction instead of +z. So the field must also reflect this. The curled field reflect this because flipping the direction of current changes the rotational SENSE of the field. This field geometry still obeys the invariance in rotation and translation along the z-axis.

That's why geometry of the magnetic field is the way it is.

Zz.

 

[zoobyshoe]

I understood him to be asking about the mysterious fact that a magnet placed near this wire will be deflected at right angles at all, instead of being pushed along in the same direction as the current flow like a car hit by a train, or a cork dropped into a flowing river.

 

[ZapperZ]

I understood him to be asking about the mysterious fact that a magnet placed near this wire will be deflected at right angles at all, instead of being pushed along in the same direction as the current flow like a car hit by a train, or a cork dropped into a flowing river.

But that "magnet" is only behaving due to the field geometry. It is the same way if you put either a test charge, or a electric dipole, in an electric field. It is only reacting to the geometry of the field.

Zz.

 

[jtbell]

Again, for simplicity, let's have the current flowing in the +z direction. Repeat all of the symmetry operations I have mentioned above. You'll notice that it is invariant to all of the symmetry operations EXCEPT for one. If I flip the wire from z to -z, the direction of the current is now different! You will walk back into the room and would notice that the current is now flowing in the -z direction instead of +z. So the field must also reflect this. The curled field reflect this because flipping the direction of current changes the rotational SENSE of the field. This field geometry still obeys the invariance in rotation and translation along the z-axis.

But suppose the field were parallel to the wire, in the same direction as the current, with a magnitude that depends only on the distance from the wire. Wouldn't that have the same symmetry?

(hmmm... I just remembered that the magnetic vector potential does run parallel to the wire, in the Coulomb gauge!)

 

[ZapperZ]

But suppose the field were parallel to the wire, in the same direction as the current, with a magnitude that depends only on the distance from the wire. Wouldn't that have the same symmetry?

(hmmm... I just remembered that the magnetic vector potential does run parallel to the wire, in the Coulomb gauge!)

I actually did ask that question to my prof. one time when this was explained to me, because I brought up the infinite, long solenoid case to him. His explanation was that we already need to know maxwell equation and that the magnetic field B or H (not A, which is the magnetic vector potential that you mentioned), cannot be parallel to the direction of motion of the electrons. So you have to rule that one out a priori.

Still, I would love to hear if someone has an answer to this based simply on symmtry principle.

Zz.

 

[SAZAR]

zoobyshoe:

"I'm not sure what you're referring to here."
(Does it has to do with orientation of atoms/molecules of conductors, and how come they are always orinentated in such a way that the magnetic field is always orientated the same - by that rule?)

------
Never mind; I guess it has to do with elctrons themselves, not conductor. (I guess even electrons passing thorough vacuum of Space create magnetic fields... right?)

 

[zoobyshoe]

But that "magnet" is only behaving due to the field geometry. It is the same way if you put either a test charge, or a electric dipole, in an electric field. It is only reacting to the geometry of the field.

You explained above that the only possible geometry was the radial field. It's not clear to me, though, how that radial field exerts torque on the magnet such that it always takes a position perpendicular to the direction of current flow. For instance, I don't see that the radial field has a north pole on one side to attract the south of the magnet.

Also: Is the radial field physically rotating around the wire? I keep having visions of the rotating cylindrical brush in a vacuum cleaner. Is the radial field traveling along the wire at the drift velocity of the electrons giving rise to the field?

 

[ZapperZ]

You explained above that the only possible geometry was the radial field. It's not clear to me, though, how that radial field exerts torque on the magnet such that it always takes a position perpendicular to the direction of current flow. For instance, I don't see that the radial field has a north pole on one side to attract the south of the magnet.

The "radial field" was for the example of a line charge.

I was illustrating why the electric field geometry looks the way it does for that charge configuration. This is not the magnetic field.

Zz.

 

[zoobyshoe]

The "radial field" was for the example of a line charge.

I was illustrating why the electric field geometry looks the way it does for that charge configuration. This is not the magnetic field.

Understood. The magnetic field arises in the second circumstance, the line current. The only difference between the two, you pointed out, is that the line current doesn't demonstrate symmetry when flipped end to end because the current has changed direction.

So, you're saying that the radial field geometry is not present in the line current? If not, what is it's geometry and how does this lead to the deflection of the magnet perpendicular to the direction of the flow of current?

 

[ZapperZ]

Understood. The magnetic field arises in the second circumstance, the line current. The only difference between the two, you pointed out, is that the line current doesn't demonstrate symmetry when flipped end to end because the current has changed direction.

So, you're saying that the radial field geometry is not present in the line current? If not, what is it's geometry and how does this lead to the deflection of the magnet perpendicular to the direction of the flow of current?

The geometry is the circular "curl" field with a rotation sense. The sense of rotation is arbitrarily set by convention, but once it is set, then you have a particular rotational direction of the field. Flipping it via a mirror reflection doesn't change the geometry of the field, but changes the sense of rotation.

Other than the field parallel to the wire that has been discussed and ruled out via Maxwell equation, this is the only symmetry that I know of that also follows the symmetry of the "source", which is the straight, long line current.

Zz.

Zz.

 

[SAZAR]

Yeah, you just pointed-out to the probable sorce of the phenomenon (that difference arising from the fact that particles now move*), not the explanation itself.
--
* But, it is not the only difference (except geometricaly (which I think was his point)), there is another difference which is maybe even more important: electric field is caused by difference in amounts of positive/negative particles; while in magnetc field conductor is electro-neutral (there is equal amount of both + and - particles, it's that only some move...).

 

[jtbell]

there is another difference which is maybe even more important: electric field is caused by difference in amounts of positive/negative particles; while in magnetc field conductor is electro-neutral (there is equal amount of both + and - particles, it's that only some move...).

It's not necessary for the source to be neutral in order to get a magnetic field. A single moving charged particle generates a magnetic field. It's not as simple as the field produced by a steady current, because it varies (at the observer's location) as the particle approaches, passes, and recedes. Or you can have a beam of many particles traveling through space, which produces a magnetic field that is (ideally) just like the field outside a current-carrying wire.

The only difference with a neutral wire is that it doesn't produce an electric field around the wire, so it's easier to observe the effect of the magnetic field.

 

[SAZAR]

It's not necessary for the source to be neutral in order to get a magnetic field.

Nor did I claim the opposite.

I just said that the fact which is maybe more important than that geometrical ZapperZ talked about, is that in order for MAGNETIC field to exist it is not neccesary that the conductor is non-electroneutral (i.e. it can be electroneutral as a whole).

------
(by the way, electrons have same charge - they should disperse - how come they form a spark (like e.g. a thunder); wouldn't it be logical that cloud electric discharges are actually a cloud-like beam of glowing mist?)

 

[zoobyshoe]

(by the way, electrons have same charge - they should disperse - how come they form a spark (like e.g. a thunder); wouldn't it be logical that cloud electric discharges are actually a cloud-like beam of glowing mist?)

I think the answer is that lightning follows a path of least resistence through the air. Such a path never exists in a straight, neat line. The lightning has to, opportunistically, follow the first twisted, complex route of low resistance that presents itself, just like someone bushwhacking through a jungle. Many twists and turns are necessary because plants don't grow wild in neat rows. The path through always ends up being confined and narrow. That's probably why you don't see spreading out into anything like a cloud.

 

[SAZAR]

This about folowing the least-resistive path is a different story , it has nothing to do with dispersion of electrons due to same charge of every one of them.

Electrons repel each other, yet they "don't mind" being thight in a thunder - that makes no sense.

 

[jtbell]

Beams of charged particles in a vacuum do have a tendency to disperse because of their electrostatic repulsion. People who design particle accelerators have to deal with this by using suitable configurations of external magnetic fields.

 

[zoobyshoe]

This about folowing the least-resistive path is a different story , it has nothing to do with dispersion of electrons due to same charge of every one of them.

Electrons repel each other, yet they "don't mind" being thight in a thunder - that makes no sense.

They do mind! The repulsion is directed along the direction of travel, just as it is when current flow is a wire. Other directions offer too much resistance.

 

[SAZAR]

They do mind! The repulsion is directed along the direction of travel, just as it is when current flow is a wire. Other directions offer too much resistance.

But in a wire elecrons are FORCED to flow in a line (affecting only those in front/behind) because of huge difference between metal and the other stuff (e.g. air or plastic), yet completely in air difference between environments (electric-wise) is not that great - it's ignorable. (certanly electric forces should overcome that difference!)

---
(What I'm aiming at here is explanation for that thight shape (nondispersed-under-the effect-of-repulsion-of-electrons-inside-thunder) - explanation that resides in a story about a field thunder forms. (IS IT RESEARCHED?))

 

[marcusl]

The phenomenon you are referring too--electrons repelling each other because of their identical charge, thus altering current flow--is called a "space charge effect" and is a major limiting factor in getting big currents to flow in vacuums in transmitting tubes (look up Child's law), klystrons, particle accelerators (as jtbell pointed out), etc. Space charge can also be a problem in high power semiconductors and semiconductor/metal contacts. It isn't a problem for metals, though, where the overall charge is near zero because of the presence of positive atom ions in the metal crystal--basically the potential along a wire is extremely small (that's why we use wires to conduct electricity!)

Before a lightning bolt, there is a high potential drop but no current flow because air is an insulator that can conduct only when ionized. If the potential between cloud and ground is big enough and if there is some trigger event like a gamma or cosmic ray passing through the atmosphere, electrons are ripped off of the air atoms creating a low-resistance path of positive ions and free electrons. The situation in that path soon resembles that in a metal where huge currents can flow with little potential drop. Space charges therefore don't pinch off the current flow, nor do the electrons spread outwards from the conductive path into spaces filled with insulating gas.

IN FACT, space charge helps explain why the current can't spread outwards. Any electrons forced into the insulator by high potentials create such a strong space charge potential or field that they quickly repel any additional flow and maintain the conductor/insulator boundary.

 

[zoobyshoe]

But in a wire elecrons are FORCED to flow in a line (affecting only those in front/behind) because of huge difference between metal and the other stuff (e.g. air or plastic), yet completely in air difference between environments (electric-wise) is not that great - it's ignorable. (certanly electric forces should overcome that difference!)

Conducting wire represents the path of least resistance given the alternatives. It's easier to flow in the wire than by any other route, therefore current flows through the wire. In any circumstance where it becomes easier to flow outside the wire the current will do that. If you believe and accept that, then we can always assume that the path the current takes is the path of least resistance under the circumstances. Air isn't homogenous in temperature or density. The voltage takes advantage of the first twisty path it can find through that insulation. Once it starts to flow the heat generated causes turbulence in the air and the path of least resistance constantly changes, which is why the spark seem to "writhe".

 

[SAZAR]

OK... then it cannot help giving an answer to the main question here...
...
--------------

I mean - let's face it - ZapperZ's answer just points to a probable answer but certenly it isn't final... Rotational sense... I mean - the thing that creates electro-static field creates magnetic field - and that is completely different thing... it's like a cross between positive and negative el-stat. field, yet neither... completely different...
(and then after... electromagnetic radiation...)

 

[zoobyshoe]

OK... then it cannot help giving an answer to the main question here...
...
--------------

I mean - let's face it - ZapperZ's answer just points to a probable answer but certenly it isn't final... Rotational sense... I mean - the thing that creates electro-static field creates magnetic field - and that is completely different thing... it's like a cross between positive and negative el-stat. field, yet neither... completely different...
(and then after... electromagnetic radiation...)

I'm not sure what you mean. Are you saying you thing the electric spark is twisting itself into a thin strand like you might make a fluffy piece of yarn thinner by twisting it?

 

[SAZAR]

I just thought - maybe magnetic field that spark creates affects the spark itself in return - i.e. contains a thunder so it doesn't scatter. And twisting... Of course - there must be something else too (because direction of spark is (.) (crossection), mag.field is (->) and el.stat. is (*) (scattering), or (|) -that on top , so the resulting vector is (/) - and that's not what happens)... I just said that it seemed odd to me that they stick together, but:

marcusl explained it - quote "IN FACT, space charge helps explain why the current can't spread outwards. Any electrons forced into the insulator by high potentials create such a strong space charge potential or field that they quickly repel any additional flow and maintain the conductor/insulator boundary."

I just pointed towards that phenomenon (I don't know whether it has something to do with the main thing we're talking about here or not - I just asked) - anything that could give any kind of clue to completely answer the first question here (why magnetic field is directed at a side (and always the same side) perpendicular to current flow (above it)).
ZapperZ says just about geometry, but as I've said before there's more to it - so I suggested interaction between p+ and e- fields inside conducting matter while some of particles are moving and how it affects Space around, and the other assumption that magnetic field exists even without that interaction of e- with p+ (the story about electron flow in vacuum of space (I mean - do electrons create field on their own? (nothing else - just electrons and Space vacuum))).
...Maybe I just cannot understand what ZapperZ means by that rotational explanation... It is clever thing to start from but I don't know what to make of it...

 

[zoobyshoe]

I just pointed towards that phenomenon (I don't know whether it has something to do with the main thing we're talking about here or not - I just asked) - anything that could give any kind of clue to completely answer the first question here (why magnetic field is directed at a side (and always the same side) perpendicular to current flow (above it)).

This is why I brought up the Lorentz effect earlier. If a single electron in motion creates a magnetic field then many electrons in motion together are all experiencing the lorentz force from the magnetic fields of the other electrons. How does the spiral path of the charge in a magnetic field play out in this somewhat different situation? Somewhere in there must be the explanation for the field having unidirectional "rotational sense", and for why two conductors parallel to each other carrying current in the same direction attract each other.

 

[SAZAR]

Does proton create a magnetic field when it moves, and is it different (and if: how) from magnetic field electron creates?

 

[SAZAR]

Does proton create a magnetic field when it moves, and is it different (and if: how) from magnetic field electron creates?

That's a VERY good question. I've wondered about it myself and don't know the answer. Hopefully someone else will have some insight into this.

:)
Noone answered yet.

(@ zoobyshoe: In the mean time - aren't hydrogen atoms actualy protons? - I mean it can be easily studied by scientists.)

 

[SAZAR]

Come one people; what's the deal here?!
Why doesn't anybody answer the question - what is the difference between electron's and proton's magnetic field?

 

[marcusl]

Same but opposite sign (direction).

 

[SAZAR]

Same but opposite sign (direction).

Are you just guessing, or you've read about it somewhere so you know it for a fact?