1. ## Magnetic mystery

Following on from that interesting electricity discussion, I thought I'd drop in a real poser on magnetism.

Level of difficulty 10.10... you've been warned! <img src=/S/evilgrin.gif border=0 alt=evilgrin width=15 height=15>

It's a fairly standard topic in (advanced) physics texts, to explain magnetism as a relativistic manifestation of an electric field. Basically, an existing charge density will be seen as being higher by an observer who is moving relative to the charges. This is a direct consequence of the relativistic Lorentz contraction. It's rather hard locating decent relevant material on the net, but this page gives the general idea.

The books then go on to show how a current-carrying conductor exhibits no magnetic force on a charged particle, if the particle is not moving relative to the conductor; but a magnetic force will "appear" if the test charge is moving along the line of the current.

The great downfall in the explanations I've seen in (all) the books, is that they assume a "current" is the result of both positive and negative charges, moving with equal mobility, in equal numbers, in both directions. This makes for a nice explanation of the observed behaviour in a conducting liquid (e.g. electrolyte solution) or gas (e.g. fluorescent tube). BUT... in a solid conductor, like a copper wire, the + charges aren't mobile - they are the copper atoms (ions if you like) themselves. This means that they will always be at rest in the reference frame of the conductor.

And this is where the textbook argument falls to the ground. In fact, it predicts that a charged particle in the reference frame of the wire will feel a magnetic force perpendicular to the wire, but will feel no such force if it travels in the direction of the current, at half the effective speed of said current. Clearly cobblers, and quite at odds with observed behaviour - that there is no magnetic force in the rest frame of the wire.

Your mission then, is to explain the observed behaviour for such a solid conductor. It can be done in the same manner, but requires coming up with a better model of a "real life" conductor. The downfall of the textbook approach is that their model (of a couple of rows of +/- charged billiard balls) is too simplistic, and inadequate to account for all the effects occurring in a current-carrying solid conductor. When they start coming unstuck, they either sweep it under the rug, try to crib their way around it, or avoid it altogether. Regardless of the method of evasion, the authors clearly don't know the answer, or don't really understand the subject (or both).

bon chance

Alan

2. ## Re: Magnetic mystery

<span style="background-color: #FFFF00; color: #FFFF00; font-weight: bold">Am I right to start moving towards researching the Hall Effect?</span hide>

3. ## Re: Magnetic mystery

<img src=/S/dizzy.gif border=0 alt=dizzy width=15 height=15>
Say whaa.....???? Can you repeat the question??? <img src=/S/laugh.gif border=0 alt=laugh width=15 height=15>
You must be kidding Alan.... <img src=/S/drop.gif border=0 alt=drop width=23 height=23>

4. ## Re: Magnetic mystery

Rudi have a look <!post=here,561741>here<!/post> this should help.

5. ## Re: Magnetic mystery

<img src=/S/bash.gif border=0 alt=bash width=35 height=39>

6. ## Re: Magnetic mystery

The situation can be better modelled, using only those concepts of the Lorentz contraction and the electric field produced by a charge density. At this stage, I will say that I did the calculations rigorously as a "proof", but it's possible to demonstrate the required result (or more accurately, the effects which occur and could give rise to the observed behaviour) just by analysing the situation qualitatively.

Alan

7. ## Re: Magnetic mystery

Oh right got you, so we can visualise this using the Lorentz-Fitzgerald contraction hypothesis to explain the negative result of the Michelson-Morley experiment. maybe we could get Rudi in here with his conjecture.

8. ## Re: Magnetic mystery

But I thought the Heisenberg compensators would take care of that?

Steve

9. ## Re: Magnetic mystery

You're not taking the <img src=/w3timages/censored.gif alt=censored border=0> by chance, are you Jezza? When is your first performance review due? <img src=/S/thinks.gif border=0 alt=thinks width=15 height=15> They did tell you about the probationary period I hope.

Alan

10. ## Re: Magnetic mystery

Ahh yes the Heisenberg compensators <img src=/S/grin.gif border=0 alt=grin width=15 height=15>

So tell me Steve "How do the Heisenberg compensators work?" (careful trekky question here <img src=/S/evilgrin.gif border=0 alt=evilgrin width=15 height=15>)

11. ## Re: Magnetic mystery

They work "very well". <img src=/S/evilgrin.gif border=0 alt=evilgrin width=15 height=15>

Steve

12. ## Re: Magnetic mystery

Still is a state of <img src=/S/dizzy.gif border=0 alt=dizzy width=15 height=15> <img src=/S/drop.gif border=0 alt=drop width=23 height=23>......sorry...no conjecture from me!!!!

13. ## Re: Magnetic mystery

OK. I don't think you lot are really trying here. I know there are several physics/ electricity/ magnetism types out there (you know who you are) and I think my poser deserves more than a dismissive WTF?! So let me start in (hopefully) more familiar territory.

Going back to basic relativity, if I have a 1m ruler in my hands, I see it as 1m long. If I place it down and move parallel to it at speed v, then I see its length as 1m x (1 - v

14. ## Re: Magnetic mystery

Well, I shall have a stab but has <span style="background-color: #FFFF00; color: #FFFF00; font-weight: bold">your signature</span hide> hold a clue to the way the experimented is carried out.

15. ## Re: Magnetic mystery

<img src=/S/nope.gif border=0 alt=nope width=15 height=15> Any temperature will do.

Alan (no red herring sig)

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