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Old 2013-02-13, 22:25   #1
cheesehead
 
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Default Rebuttal to false claims about Earth's magnetic field

If, like me, you're somewhat familiar with magnetism, but last studied it in detail decades ago, and/or are a bit fuzzy about details of Earth's magnetic field and its reversals in particular, you may like a recent article on the www.skeptic.com site that rebuts four common false claims:

1. Earth’s field is about to reverse!
2.
When the field reverses and vanishes, we’ll all be bombarded by cosmic radiation!
3. What about recent studies that showed much more rapid field changes?
4. What about the evidence that the magnetic pole is rapidly changing direction?

"Magnetic Myths"

http://www.skeptic.com/eskeptic/13-02-13/#feature

(Note: the subtitle "Why magnets and magnetic fields attract New Age flimflam and flapdoodle" is misleading, because the author doesn't really explain why that happens! :-)
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Old 2013-02-13, 22:35   #2
Dubslow
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Quote:
Originally Posted by cheesehead View Post
If, like me, you're somewhat familiar with magnetism, but last studied it in detail decades ago, and/or are a bit fuzzy about details of Earth's magnetic field and its reversals in particular, you may like a recent article on the www.skeptic.com site that rebuts four common false claims:

1. Earth’s field is about to reverse!
2.
When the field reverses and vanishes, we’ll all be bombarded by cosmic radiation!
3. What about recent studies that showed much more rapid field changes?
4. What about the evidence that the magnetic pole is rapidly changing direction?

"Magnetic Myths"

http://www.skeptic.com/eskeptic/13-02-13/#feature

(Note: the subtitle "Why magnets and magnetic fields attract New Age flimflam and flapdoodle" is misleading, because the author doesn't really explain why that happens! :-)
What in the devil is a "non-dipole" magnetic field? Does he mean the contributions from n-pole terms, n>2?
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Old 2013-02-13, 22:44   #3
chalsall
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Originally Posted by Dubslow View Post
What in the devil is a "non-dipole" magnetic field? Does he mean the contributions from n-pole terms, n>2?
There are a few theories that a magnet (or, more accurately, a particle) might be found with only one magnetic pole.

Highly unlikely. But not yet proven impossible.
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Old 2013-02-13, 22:46   #4
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Quote:
Originally Posted by Dubslow View Post
What in the devil is a "non-dipole" magnetic field? Does he mean the contributions from n-pole terms, n>2?
Magnetic monopoles have figured in science fiction. Larry Niven's work comes to mind. How this relates to reality, or even scientific theory, I don't know.
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Old 2013-02-13, 22:53   #5
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Quote:
Originally Posted by chalsall View Post
There are a few theories that a magnet (or, more accurately, a particle) might be found with only one magnetic pole.

Highly unlikely. But not yet proven impossible.
Quote:
Originally Posted by kladner View Post
Magnetic monopoles have figured in science fiction. Larry Niven's work comes to mind. How this relates to reality, or even scientific theory, I don't know.
Right, the problem of course being that no one has found any yet, and that implies that there the monopole term of every magnetic field, including Earth's, is zero. However, for both electric and magnetic fields, there are an infinite number of terms in the multipole expansion, though generally the lowest terms are by far the largest, which would mean the dipole term in the magnetic case (and in some electric charge configurations). Saying "non-dipole" is very ambiguous though -- he should say something like "the dipole component is much stronger than the other multipole components".

(I learned this all last semester, and in fact just this morning took the first exam in my E&M II class. Griffiths, anyone?)

Last fiddled with by Dubslow on 2013-02-13 at 22:53 Reason: s/had/took
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Old 2013-02-13, 22:59   #6
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Right, the problem of course being that no one has found any yet, and that implies that there the monopole term of every magnetic field, including Earth's, is zero.)
Incorrect.

It is left as a problem to the student to figure out why.
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Old 2013-02-13, 23:12   #7
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... Griffiths, anyone?)
Selling the textbook so fast?
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Old 2013-02-13, 23:14   #8
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Originally Posted by cheesehead View Post
(Note: the subtitle "Why magnets and magnetic fields attract New Age flimflam and flapdoodle" is misleading, because the author doesn't really explain why that happens! :-)
Maybe the author implies that New Age flimflam and flapdoodle are ironeous?
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Old 2013-02-13, 23:27   #9
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Maybe the author implies that New Age flimflam and flapdoodle are ironeous?
Nawww... That would only be Terry Pratchett channeling a Dwarf character who really loves metals....
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Old 2013-02-14, 00:11   #10
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Incorrect.

It is left as a problem to the student to figure out why.
No, that's correct. As far as we know, \nabla \cdot B = 0 at every location in space; this (specifically, the vanishing divergence) allows us to write B as the derivative of a vector potential B = \nabla \times A, and when you expand 1/(r-r') as powers of (1/r), the term with exponent 1 (i.e. the monopole term) is zero.

I was wrong perhaps in that I was talking about n-pole terms of the field, not the potential, but you can say that the monopole term of the field is the term which corresponds to derivative of the monopole term in the potential.
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Old 2013-02-14, 00:14   #11
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What in the devil is a "non-dipole" magnetic field? Does he mean the contributions from n-pole terms, n>2?
Apparently, in geophysical parlance, the "non-dipole" field is simply the residual when the dipole field is subtracted from the observed field, without regard to attribution to particular n-pole terms because geophysicists don't care about that attribution.

Quote:
Originally Posted by http://www.britannica.com/EBchecked/topic/417452/non-dipole-field
  • TITLE: geomagnetic field (geophysics)
    SECTION: Dipolar field
    ...manner, it is found that the dipole term accounts for more than 90 percent of the field. If the contribution from a centred dipole is subtracted from the observed field, the residual is called the non-dipole field, or regional geomagnetic anomaly.
So, just call it the "regional geomagnetic anomaly" if that suits you better.
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