Why do Parallel Currents Attract?

[krab]

My response to his answer was: "The North-South pole attraction for parallel current flow explanation is exactly what I think is true.".

..and is exactly why your understanding of magnetic phenomena is limited. A wire carrying current does not have North or South pole, and yet is attracted to another such wire, by the magnetic field it generates. North-South pole attraction is a simple way of understanding mechanical interaction between bar magnets. In the case of a general magnetic field, it is not all that enlightening.

 

[pervect]

Ease up Pervect. It's just something I read. If they want to investigate it further, they will. Don't be such a science nazi.

Sorry to pick on you, your post had enough disclaimers in it to alert almost anyone that you were speculating wildly.

 

[Metallicbeing]

North-South pole attraction is a simple way of understanding mechanical interaction between bar magnets.

...and if you took a powered electro-magnet, you can determine which end is the same as North or South compared to a bar magnet. You're right, there is no permanent North or South pole on an electro-magnet (or wire), but it helps to visualize the concept. Sorry, it's not always perfect.

Why would you try to explain to somebody who knows little or nothing about this subject in a way they might only understand after a few years of college? First they need to grasp the concept. The math comes later.

 

[Metallicbeing]

Sorry to pick on you, your post had enough disclaimers in it to alert almost anyone that you were speculating wildly.

I understand the importance of math. Unfortunately, not everyone speaks "Mathematician". Don't you want to help people (layman) understand? It's one thing to descibe your variables, but you need to tell others (who haven't kept up on their math) what it all means. If you can't do that, then that is where you're lacking.

 

[what_are_electrons]

..and is exactly why your understanding of magnetic phenomena is limited. A wire carrying current does not have North or South pole, and yet is attracted to another such wire, by the magnetic field it generates. North-South pole attraction is a simple way of understanding mechanical interaction between bar magnets. In the case of a general magnetic field, it is not all that enlightening.

Hmmm. Most of the phenomena found in atomic and quantum physics at the most fundamental level appear as discrete binomial or trinomial systems (eg +/- electron-proton up-down UUD DDU particle-antiparticle N-S super-symmetry...) so I do not take offense at your inference, in fact, it is a compliment since I am focusing on the fundamental nature of particles not their macroscopic phenomena which are described by Maxwell, Faraday, Lorentz, Gauss and many others.

 

[pervect]

Based on context, your response is aimed to me, the thread author.

Yep. It appears to me that you are really in love with this idea that all magnetic interactions must be via a N-S pole interaction. I was beginning to suspect this from the pattern of your responses, your direct response has confirmed this.

There's only one small problem with this idea, as I've tried to point out several times - when you make it rigorous enough to test, it doesn't actually work.

An example: the force on a wire in a magnetic field depends on the current and the length of the wire. The force on a dipole/bar magnet in a magnetic field depends not on the value of the field, but how fast the field changes in space (it's spatial gradient).

You seem to actually prefer the theory to be vague enough not to test, rather than eager to find formulas with which to test your ideas and match the results against experiment.

 

[what_are_electrons]

An example: the force on a wire in a magnetic field depends on the current and the length of the wire. The force on a dipole/bar magnet in a magnetic field depends not on the value of the field, but how fast the field changes in space (it's spatial gradient).

Let's focus, for the moment, on static magnetic fields where there is no obvious current. In this case, is your dipole/bar magnet example meant to deal with two static magnets or a bar magnet in the presence of an electromagnet?

You seem to actually prefer the theory to be vague enough not to test, rather than eager to find formulas with which to test your ideas and match the results against experiment.

No, I'm not allergic to formula, just I'm not up to speed on them as is clear from my writing. As I wrote in another response, I'm looking for fundamental properties which are often binomial or trinomial in appearance. This means that I'm not currently focused on the macroscopic formula that result from the fundamental nature of these forces/phenomena. Does that help explain my perspective?

 

[krab]

Why would you try to explain to somebody who knows little or nothing about this subject in a way they might only understand after a few years of college? First they need to grasp the concept. The math comes later.

OK, I'll bite.

Imagine two cylindrical electromagnets arranged coaxially one above the other, each has an N and an S. If the currents flow in the same direction, each has N in the same direction (up, say), and so the S of the upper is near the N of the lower, and they attract.

Now reduce these electromagnets to single loops. They still attract. Now imagine these loops are very very large in radius compared with their separation. They still attract. Imagine their radii are so large that when you see the two close up, you cannot even tell the wires are curved. Attraction still occurs. It does not seem logical that the whole circle of wire is needed to result in an attractive force. In fact, attraction occurs even if the wires are short lengths.

Without the math, of course, I haven't proven anything, But I hope I've made it plausible.

 

[what_are_electrons]

OK, I'll bite.

Imagine two cylindrical electromagnets arranged coaxially one above the other, each has an N and an S. If the currents flow in the same direction, each has N in the same direction (up, say), and so the S of the upper is near the N of the lower, and they attract.

Now reduce these electromagnets to single loops. They still attract. Now imagine these loops are very very large in radius compared with their separation. They still attract. Imagine their radii are so large that when you see the two close up, you cannot even tell the wires are curved. Attraction still occurs. It does not seem logical that the whole circle of wire is needed to result in an attractive force. In fact, attraction occurs even if the wires are short lengths.

Without the math, of course, I haven't proven anything, But I hope I've made it plausible.

Your picture is the same as two parallel wires with current going in the same direction right? So we've made no progress yet.

Suppose however that we shrink down to the size an electron and go inside those two wires. The electrons mainly travel over the skin of the wire but the actual electrons move slowly on the order of cm/sec if memory is correct. The current of the electric field however travels at 10e7 m/sec. (Please correct any errors in phenomena or wording.) Let's assume that the electrons travel in basically a straight line as they skim the surface. Next, let's talk about a single unit of the current - a single charge. What is it about that charge that produces a force that is attracted to the other single charge in the corresponding parallel wire?
...

 

[Metallicbeing]

OK, I'll bite.

Imagine two cylindrical electromagnets arranged coaxially one above the other, each has an N and an S. If the currents flow in the same direction, each has N in the same direction (up, say), and so the S of the upper is near the N of the lower, and they attract.

Now reduce these electromagnets to single loops. They still attract. Now imagine these loops are very very large in radius compared with their separation. They still attract. Imagine their radii are so large that when you see the two close up, you cannot even tell the wires are curved. Attraction still occurs. It does not seem logical that the whole circle of wire is needed to result in an attractive force. In fact, attraction occurs even if the wires are short lengths.

Without the math, of course, I haven't proven anything, But I hope I've made it plausible.

That's a very good description. However, I never said that the wire need to be looped. As very elegantly described, the two single strands of wire with current flowing through them can still be viewed as having the equivelent of a North and South pole on either side of a wire.

If you disagree, then where in your description did the poles cease to exist?

 

[Integral]

"The North-South pole attraction for parallel current flow explanation is exactly what I think is true.".

Could someone explain to me how this is an explanaiton of anything? Just what does it mean? Could you please be specific?

 

[what_are_electrons]

Le Chatelier's principle at work?

Le Chatelier's principle is the "least-energy principle" which suggests that dynamic systems tend to seek lowest energy forms of equilibria. If we use the simple picture that North seeks South in two parallel wires with current going in the same direction, the question arises: Is this tendency due to the least-energy principle?

If this line of thought is valid, then what is it about the North and South magnetic fields that want to produce lower energy states? Are there two waves that want to blend to form a lower energy wave?

 

[Integral]

What I get out of your explanation is that because magnets attract each other there is a force between them. Wow! that's profound! Least energy is the basis for the Lagrangian, a common approach to finding solutions to dynamic systems so you are not breaking any new ground there nor does it provide additional insight as to WHY the force exists. I do not see that you are gaining anything. The only way you will ever get a handle on the force between magnets is to become familiar with the Math. Of course this will require some real effort, much more effort then what is required to read what you will find on the web.

 

[what_are_electrons]

What I get out of your explanation is that because magnets attract each other there is a force between them. Wow! that's profound! Least energy is the basis for the Lagrangian, a common approach to finding solutions to dynamic systems so you are not breaking any new ground there nor does it provide additional insight as to WHY the force exists. I do not see that you are gaining anything. The only way you will ever get a handle on the force between magnets is to become familiar with the Math. Of course this will require some real effort, much more effort then what is required to read what you will find on the web.

Dear Integral,
I did not explain anything in my last post. I did however ask several questions.

 

[Integral]

Was that a question? Tell me this, since when are 2 types of magnetic field? You speak of N and S as if they were different fields. This is your personal theory and it is this approach that just got this thread moved to Theory Development.

 

[Chronos]

Duh, what part of Krab did you not understand? Field strength is NOT frame-dependent. You put all the charges together in a background independent reference frame and all the charges total to ... drumroll... exactly zero... beating head against wall, pardon me.

 

[Chronos]

Le Chatelier? You may as well quote Setterfield. Your references lack credibility.

 

[what_are_electrons]

Le Chatelier? You may as well quote Setterfield. Your references lack credibility.

You're right. Should have just quoted "least energy priniciple". My Penguin Physics dictionary referenced Le Chatelier's principle (rule) which I read and at first read it was the same, but Le Chatelier's principle does not directly address the idea of minimization which is the key.

 

[axawire]

hi,

I am in the understanding that the magnetic field is just a relativitic effect

Simplified picture...
you take two really long parallel wires 1 and 2. These wires appear neutral.

Okay, remember that in relativity that things moving relative to one another will see each other being shorter from space contractation... I am just going to ignore time aspect.

Anyway we run an electron current through both wires same magnitude and direction. (only electrons are moving)

So the moving electrons in wire 1 sees the electrons in wire 2 as stationary (as they are both moving same speed+direction) so they would feel the normal electric repulsive force. BUT now the moving electrons in wire 1 sees the protons in wire 2 as moving and so sees the protons in wire 2 as space contracted resulting in wire 1 electrons seeing a higher proton density than electron density in wire 2 so the electric attractive force to the protons is greater than the repulsive electric force from the eletrons. SO wire 1 is attracted to wire 2 because the moving electrons in wire 1 see wire 2 as being positivily charged from the space contraction.

Now the stationary protons in wire 1 sees similar effects in wire 2 but this time the stationary protons see stationary protons so same electric repulsive force here. BUT this time the stationary protons in wire 1 sees moving electrons in wire 2 these moving electrons in wire 2 appear space contracted to wire 1 protons perspective resulting in wire 1 protons seeing a higher electron density than proton density in wire 2 so the electric attractive force to the electrons in wire 2 is greater than the repulsive force from the protons in wire 2. SO wire 1 is attracted to wire 2 because the protons in wire 1 see wire 2 as being negativily charged from the space contraction.

So looking at the electrons perspective in wire 1 it is attracted to wire 2
looking at the protons perspective in wire 1 it is attracted to wire 2
So wire 1 is attracted to wire 2

Same anaylsis can be done for wire 2 looking at wire 1 and you will find that wire 2 is attracted to wire 1

although the electrons are moving really slowly this force is really small... but there are SO MANY electrons and protons in the material it makes the effect really significant.

This is your magnetic force... its just a lot easier to use the magnetic force equations that were already hashed out before relativity than apply lorentz contractions to electric fields all the time and this is prolly why it is allways tought as a separate force.

on the flipside in http://www.iop.org/EJ/abstract/0143-0807/17/4/006 indicates that its just as reasonable to say that the electric field is the relativistic effect... but then again maybe not...
http://www.iop.org/EJ/abstract/0143-0807/18/2/013

I put my dime with relativity on this one.

sorry for any crappy spelling mistakes etc...

 

[pervect]

hi,

I am in the understanding that the magnetic field is just a relativitic effect

Yes, this is a very good way of looking at the magnetic field relativistically. It can be found in "Fenyman's lectures on physics", among other places.

The only drawback it has is that it's moderately famous for confusing people. I would encourage people to learn enough of the non-relativistic theory to be able to correctly calculate forces and fields before moving on to the relativistic version.

This approach eventually leads to the Faraday tensor, which unites the electric and magnetic fields into a single "geometric object" which transforms in a standard manner by relativity theory.

 

[krab]

I am in the understanding that the magnetic field is just a relativitic effect

...

This is your magnetic force... its just a lot easier to use the magnetic force equations that were already hashed out before relativity than apply lorentz contractions to electric fields all the time and this is prolly why it is allways tought as a separate force.

...

Good post, axaware.
I'm always a little suspicious of statements of the type "such-and-such effect is just a consequence of such-and-such", as if an alternative description makes everything simpler. In some situations it does, in others it does not. So, looking at a beam of particles traveling together in vacuum, contemplating the fact that there is a magnetic field in the lab frame but none in the particles' rest frame, can teach one a lot about relativity. OTOH, forces between neutral, current-carrying wires are not more easily described by appealing to relativty.

 

[Epsilon Pi]

Hi Vince,

This is an explanation, from the point of view of the basic unit system concept, not a why things are in a such a way.
In fact the magnetic field is clearly a dual entity because of its two inherent polarities and as such we must represent it by a complex entity such as the BUS, which has both kinds of parities:
- that one that has to do with space that has consequently both signs: positive and negative, as it were, attractive and repulsive,
- but it also has even parity, the one that is associated with sort of non relativistic effect, it remains the same in spite of change; is not this a reason why it was precisely the magnetic field in weak interactions the medium to break symmetry?

When the sense of rotation according to the right-hand rule is the same the forces are attractive, but it is not the same when that sense is inverse, as is the case. We must take into account that with that rotation is associated a frequency, so in one case we have resonance, but not in the other.
Regards
EP
PD: or in one case the prevailing one is the magnetic field, and on the other the electric field or charge of the electrons that repel each other as the magnetic field is cancelled?

I've read that when currents in two parallel wires are going in the same direction, they attract each other. The same text said that when the currents are going in opposite directions, the two wires repel each other.
Why does this happen? Is there a different behavior for AC and DC currents?

 

[Metallicbeing]

hi,

I am in the understanding that the magnetic field is just a relativitic effect

Simplified picture...
you take two really long parallel wires 1 and 2. These wires appear neutral.

Okay, remember that in relativity that things moving relative to one another will see each other being shorter from space contractation... I am just going to ignore time aspect.

Anyway we run an electron current through both wires same magnitude and direction. (only electrons are moving)

So the moving electrons in wire 1 sees the electrons in wire 2 as stationary (as they are both moving same speed+direction) so they would feel the normal electric repulsive force. BUT now the moving electrons in wire 1 sees the protons in wire 2 as moving and so sees the protons in wire 2 as space contracted resulting in wire 1 electrons seeing a higher proton density than electron density in wire 2 so the electric attractive force to the protons is greater than the repulsive electric force from the eletrons. SO wire 1 is attracted to wire 2 because the moving electrons in wire 1 see wire 2 as being positivily charged from the space contraction.

Now the stationary protons in wire 1 sees similar effects in wire 2 but this time the stationary protons see stationary protons so same electric repulsive force here. BUT this time the stationary protons in wire 1 sees moving electrons in wire 2 these moving electrons in wire 2 appear space contracted to wire 1 protons perspective resulting in wire 1 protons seeing a higher electron density than proton density in wire 2 so the electric attractive force to the electrons in wire 2 is greater than the repulsive force from the protons in wire 2. SO wire 1 is attracted to wire 2 because the protons in wire 1 see wire 2 as being negativily charged from the space contraction.

So looking at the electrons perspective in wire 1 it is attracted to wire 2
looking at the protons perspective in wire 1 it is attracted to wire 2
So wire 1 is attracted to wire 2

Same anaylsis can be done for wire 2 looking at wire 1 and you will find that wire 2 is attracted to wire 1

although the electrons are moving really slowly this force is really small... but there are SO MANY electrons and protons in the material it makes the effect really significant.

This is your magnetic force... its just a lot easier to use the magnetic force equations that were already hashed out before relativity than apply lorentz contractions to electric fields all the time and this is prolly why it is allways tought as a separate force.

on the flipside in http://www.iop.org/EJ/abstract/0143-0807/17/4/006 indicates that its just as reasonable to say that the electric field is the relativistic effect... but then again maybe not...
http://www.iop.org/EJ/abstract/0143-0807/18/2/013

I put my dime with relativity on this one.

sorry for any crappy spelling mistakes etc...

Axawire,

Your explanation was very good. I'm sure you could have said the same thing with an equation, but I'm glad that you resisted. There are many here who miss out because some only "speak" with math. I've tried to speak in layman terms to others who want to learn, only to be bashed at almost every turn for not speaking in the "proper tongue". My hat's off to you.

 

[Chronos]

Apologies. We only speak in 'math' because that is the only language of science that is known. Propose a better 'language' and go collect your nobel. In other words, I think you have mastered bull-speak... Just like the Andrew Grey. I have not even totally figured out the simplest dq equations of anything in the universe. Show me yours.

 

[what_are_electrons]

Apologies, we only speak in 'math' because that is the only language of science that is known. Propose a better 'language' and go collect your nobel.

How 'bout drawings/pictures plus math to give a 1,000 and 1 words/descriptions? That way, us How To Dummies, can keep pace as best we can despite our illiteracy. That fair?

 

[Chronos]

I would say you can't picture it.

 

[Math Is Hard]

I would say you can't picture it.

I agree with Chronos. For instance, how would you draw a picture of a five-dimensional object? Math allows us to describe things even where our visualizations fail us.

 

[Chronos]

apologies, you can't even describe it in 3 dimensions.

 

[Chronos]

What math are you supporting?

 

[Chronos]

Duh, I have no math! Just totally wrong ideas!