Can Moving Charges Create a Magnetic Field? A Discussion on Electromagnetism

[zoobyshoe]

Originally posted by Ambitwistor
Okay, I get it. The wire twirls because a magnetic field (in this case, of a permanent magnet) exerts a force on a current-carrying wire. The force is always perpendicular to both the field and the current, which given the geometry of the experiment, makes the wire trace out a circle in the mercury. This force is due to the ordinary Lorentz force law for a charge moving in a magnetic field: F_B = qv x B. It doesn't require a changing field.

You may not believe it, but this explanation of the results Faraday got makes perfect sense to me.

(I don't get the sub B after Force, though)

Edit:It seems to me that the explanation of the behaviour of magnetic poles in permanent magnets must also lie in this same Lorentz equation, with the magntic field of one magnet producing the same perpendicular forces on the moving charges in the other magnet (at least those whose spin is uncompensated) but I can't visualise just how at the moment since these charges would be revolving rather than traveling in a straight line. (It could be that's just whacky thinking, though.)

 

[zoobyshoe]

Back to this:

Originally posted by Ambitwistor
The field in the case of an infinite wire is completely static: it looks exactly the same at any time.

I would like to be able to pin you down about this. Looking exactly the same at any time, doesn't necessarily mean there is no motion. When you say the field in this case is completely static I can understand that there is no change in its intensity, or change in its shape. Yet it seems logical to assume the field is, in fact, flowing down the wire at the drift velocity of the current.

For a point charge, the field "moves along with the charge, but keeps the same shape".

This being the case, it seems logical to conclude that the electric field around the perfect wire, which is the sum of all the electric fields of the point charges that constitute the current, is traveling along with all the point charges as they move along the wire. However, you call this notion into question when you continue with:

(Technically, it doesn't make sense to speak of a field "moving",

You are trying to be careful about not being ambiguous here, which I appreciate, but I'm not grasping why it doesn't make technical sense to speak of a field moving.

but what I mean is that the field will have the same "shape", except that it translates with the velocity of the charge, i.e. E(x+vt,y,z;t) = E(x,y,z;0) for a charge moving at speed v in the +x direction.)

Here you bring in the word "translates". This seems to be the term you would prefer to any form of the word "move" in regard to a field. I am sure there must be important differences between these two terms which make you want to use one rather than the other. You include the equation as an illustration of this translation (I think). So, it seems obvious to me that I'm not going to grasp this situation with the perfect wire without a good understanding of the concept of "translation" as it is used in this situation. Can this be explained verbally, or do I need to be conversant with all the equations?

 

[Fairfield]

Originally posted by Ambitwistor
If you can define an experiment by which I can measure that a field is "flowing down the wire", whatever that means, then I might say that the field flows down a wire. But since any measurement of the field at any point with give the same value at any time, I don't know how you're going to do that.

Zooby and Ambitwister:

It seems to me you just have a semantic problem going on here. The shape of the field around a steady current (or otherwise) is one detectable phenomenon. What's going on under that shape is another detectable phenoenon.

 

[Fairfield]

I would like to add a comment to this thread which may or may not be helpful.

In studying magnetism, It seems clear to me that a problem arises from misconstruing what a compass needle really is indicating when it is placed near a straight current. The name magnetism was originally applied to certain objects which could display MUTUAL attraction or repulsion with a measurable FORCE. You, therefore, can't have real magnetic lines of force without two such objects. Therefore, to apply this title to something which (supposedly) circles a current carrying wire, but doesn't have two objects to refer this force to, redefines the meaning of the phrase "magnetic lines of force" in midstream. This redefinition follows from falsely assuming that any active magnetic compass needle indication always refers to true magnetic lines of force. This is absolutely not true.

To explain this inadvertent switch in the meaning of the phrase, "magnetic lines of force", as it is applied to a straight wire current, we have to consider what the true definition of magnetism originally referred to, and therefore, should continue to refer to, unless formally redefined.. The term "magnetism" was originally applied to objects which, unknown to everybody at the time, contain looped currents for there particular force manifestation, which, therefore, have, spatially speaking, no less than two opposing parallel currents in them (opposite edges of the loop or coil). Since a straight wired current in no way has a either a loop, or two opposing currents, it, does not qualify as a magnetic object which can generate any magnetism, or real magnetic lines of force.

The situation of a compass near a straight current is a different kind of relationship (easily explained) than between two genuine magnetic objects (of which the compass needle can be one of them). The compass needle therefore, near a straight current, falsely projects, in one's imagination, something to which the original meaning of the phrase, "magnetic lines of force", absolutely doesn't apply.

To have a name (phrase) kicking around in science which has two different meanings is not a helpful thing in my opinion. If the incorrect name, "magnetic lines of force", for this particular situation (compass indication near a straight current) has been incorporated into so many physics formulas that it can't be extricated, or if it is used in a monitoring/calculating reference system, then I suggest that, at least, the name of the indicated lines be changed to "Oersted's north-south lines" (OF NO FORCE), and the term "electromagnetic waves" be changed to "electro-Oersted waves". It would be even better, but a mouthful, to call these compass indicated "north-south" lines around a straight current, "Oersted's right angle current direction indicators", because they are simply derived from that by the physioelectric response of a (equivalent) loop current near a straight current, depending on the straight current's direction. This kind of physioelectric effect between current carrying wires should be called Ampereism instead of magnetism (see below).


Further, you also can't have those, "lines of magnetic force", (in diagrams) around the individual wires of a current carrying coil because you ONLY get genuine magnetism off the end of a loop or a coil as a MUTUAL resultant force from the two sets of opposing parallel currents, one set in each magnetic entity, when they are brought near each other. The vector amount of this force varies with different orientations between any two magnetic entities. Aside from that coil, (mostly) "end" effect, you don't have any other genuine magnetic lines of force present Only 'Oersted's lines of NO FORCE' "around" the single internal wires. These lines of no force, naturally cannot be added up to create a net force.

Since genuine magnetism requires at least two parallel opposing currents in each magnetic entity, it is clear that magnetism is a more complex arrangement of a more simple force system relating to the physical reactions between close parallel currents. Since it was Andre M. Ampere who first discovered this physical reaction between close parallel currents, it would seem proper to call this system, Ampereism, and the forces operating there, Ampere's lines of force.

From the above considerations, it appears to me that the overall problem of properly relating magnetism to electricity is that magnetism is a superstructure forces relation system built up of a lower order forces relation system, which latter system should properly be called, Ampereism. Therefore magnetism provides only a confusing view of electro-Ampereism.

What both the loop currents and the straight currents have in common is they both have inductive fields which the working physicist and electrical engineer need to keep track of in order to get a mathematical hold on either field's electrical and physical effects. But it is not helpful to drag around confusing names. A straight wire current's inductive field (Ampereic field) is just that. It is not a loop current's inductive field, so it is not a magnetic inductive field. A magnetic inductive field is a resultant of at least two ampereic fields (spatially speaking). If a common name is going to be used for both types of inductive fields, and the lines for their (flux) densities, it should obviously be Ampereic flux density instead of magnetic flux density.

Fairfield