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### Network # Magnetic flux - is directional? How?

#### NuLED

Jan 7, 2012
294
For the "right hand rule" where the thumb is conventional current flow, and your fingers are the direction of magnetic flux... how do they know that? The flux lines grow instantaneously, no? Or do they actually grow outwards from the wire? Seems a bit hard for me to get my head around.

#### BobK

Jan 5, 2010
7,682
Flux lines are not a real thing, they are a mathematical simplification of the interactions between photons and charges that are fully explained by quantum electrodynamics.

How do they know that? By observation.

Do the flux lines grow instantaneously? No, the effects of turning on a current in a wire will propagate outwards at the speed of light, not instantaneously.

Bob

#### NuLED

Jan 7, 2012
294
Thanks Bob.

Does the "direction" of them growing (the "rotational" direction) affect inductance?

I don't mean the growing "sideways" of the lines (expanding outward). As that is what I understood to be the fingers on the hands (pointing to the rotational direction).

#### john monks

Mar 9, 2012
693
The short answer is yes.
You are asking very good questions that can only be explained by using 4-dimensional physics, the three physical dimensions and time. I might be able to explain some of this but unless you studied relativity in a physics class you would have a very tough time comprehending this. I suggest that you start by reading Einsteins general andd special theory of relativity where he hints at some of this and then repost your questions and I will be happy to go into it.
Let me give you a hint: Magnetic forces are electrostatic forces in 4-dimensional physics. That is magnetism comes as a result of electrons traveling at relativistic velocities.

Jan 7, 2012
294

#### john monks

Mar 9, 2012
693
Quantum physics comes into play here but let me see if I can explain some of this without it. Let's just stick with relativity. So let's just forget about the right hand rule and focus in on the fact that like charges repel each other and unlike charged attract. This is what we discover in nature and we don't know why this is so. Now let's look at the end of a bar magnet. It has atoms with an equal number of electrons and protons so the net electrostatic force on a charged particle is zero. That means that no force exists between the magnet and some nearby stationary magnet. Now this is where the magic occures. Now in the magnetif the electrons are orbiting clockwise around the positively charged nucleous and if you force the electron to move purpendicular close to the end of the magnet then the electrons orbiting the nucleous on the right side of the atom is traveling faster than the electrons on the left side of the nucleous. Here is the tricky part. Occording to Einstein, when something is traveling relative to on object then the something is going through a spacial contraction and the density will increase. Therefore the stream of electrons coming at you as you travel across the pole will be greater than the electrons that are traveling with you. So the electrons electric field on the right of the atoms have a greater force that the electrons traveling with you. Now remember like charges repel. Therefore the electron traveling close to the magnet is going to have a force to the left. And that is how magnets affect electrons flowing in a wire. And this is how solenoids work and this is the basis for answering your questions.

#### NuLED

Jan 7, 2012
294
I don't understand this bit:

"Now in the magnetif the electrons are orbiting clockwise around the positively charged nucleous and if you force the electron to move purpendicular close to the end of the magnet then the electrons orbiting the nucleous on the right side of the atom is traveling faster than the electrons on the left side of the nucleous"

How do we know what direction they are orbiting at? I thought they just orbit like a sphere (e.g., Heisenberg). I don't get how they can move perpendicular or what the left and right hand side of the nucleus are.

Really appreciate your trying to shed light on this. Maybe if I understand this bit I will get the rest of it.

#### john monks

Mar 9, 2012
693
We derive the direction of travel of the electrons from Einstein's theory.
The explanation I use in regard to the direction of travel is the easiest one to explain and the one I think makes the most sense.
We can force electrons to go in a circle in a coil of wire and we develop a north and south pole, exactly like that found in a permanent magnet.
So let's look at this a different way. Let's say there are two rows of electrons equally spaced apart. And one rows is traveling in one direction and the other the opposite direction but at the same speed. If you are standing in the middle of the two rows the electric affect from one row is exactly the same as the other row. And if you were holding a negative charge such as an electron there would be no net force from the two rows. Now if you started running straight ahead between the two rows and if the rows on your right was coming at you from the front and the row on your left is traveling in the same direction as you the row on the right would contract according to Einstein's theory of relativity and the row on your left would expand. Now the charge you are holding will be more greatly affected by the row on your right than on the left because the electrons are more greatly packed because of the space contraction and the charge you are holding will be pushed to your left.
Einstein wrote a lot about spacial dilation and contraction in his papers and book and this effect seems to be universal whether you are talking about trains or rows of electrons. And I believe this concept is what causes the most confusion.
So keep this in mind. When something is traveling in space it contract in the direction of travel and its density increases with respect to a stationary observer.

#### NuLED

Jan 7, 2012
294
Hey thanks! I like your two rows analogy. Now I get why it has to do with relativity.

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