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Author Topic: Why can we not detect the magnetic moment of most elements?  (Read 872 times)

Offline acsinuk

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Almost all the atomic table elements/isotopes have a magnetic moment but we cannot sense that magnetism with a magnetrometer.  Why is that? and would not the same problem apply in outer space
« Last Edit: 17/05/2016 07:16:04 by chris »


 

Offline evan_au

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Re: Magnetism and Molecules
« Reply #1 on: 16/05/2016 11:53:35 »
Quote from: acsinuk
Almost all the atomic table elements/isotopes have a magnetic moment
Magnetic Resonance Imaging is able to obtain a signal from any material with an odd number of protons (or an odd number of neutrons), as these nuclei have a net magnetic field.
  • Many materials will produce a magnetic field which opposes an external magnetic field, but this diamagnetic effect is very slight (except for superconductors). 
  • Some materials produce a paramagnetic response, which swamps any diamagnetic effect.
  • Familar magnets are ferromagnetic, which arises from unpaired inner electrons which give the atom a net magnetic moment.
  • Ferromagnetism is so strong that most people would consider paramagnetic and diamagnetic materials as "non-magnetic".

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we cannot sense that magnetism with a magnetrometer.  Why is that?

A Magnetometer will detect a material which produces an external magnetic field, or one which significantly distorts the Earth's magnetic field.
  • A permanent Ferromagnet produces a strong external field, and is easily detected by a magnetometer.
  • Diamagnetic and Paramagnetic materials don't produce an external field of their own, and so are very hard to detect with a magnetometer.
  • Ferromagnetic materials after being "degaussed" have a magnetic structure where tiny domains have alternating North/South directions which cancel each other. This means that they don't produce an external magnetic field of their own.
  • However, an external magnetic field will pass through these unmagnetized ferromagnets far more easily than through air (or vacuum), so they distort the Earth's magnetic field. This makes magnetometers useful for detecting large metal objects underwater (like submarines). This led to some submarines being built with a (very expensive) titanium hull.
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would not the same problem apply in outer space
I'm not sure which problem you mean.

Many space probes carry a magnetometer to measure the magnetic field of the Sun, planets, and the Voyager spacecraft have even seen some hints of a magnetic field in interstellar space. Normally the magnetometer is held on a long boom far away from the spacecraft, so ferromagnetic materials in the spacecraft, or magnetic fields originating from electric currents in the spacecraft don't disrupt the magnetometer readings.

It would make sense to make the boom out of a diamagnetic material, and to degauss the spacecraft at the start of its mission.
 

Offline acsinuk

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Thank you Evan for your detailed explanation.  You quote "Magnetometer will detect a material which produces an external magnetic field, or one which significantly distorts the Earth's magnetic field."
I agree with that definition as it means that molecules and isotopes have a magnetic moment produced by an internal magnet which does not radiate flux outwards, because its insides are fully magnetically balanced. If we shake the molecule/isotope violently we can get a magnetic reasonance at certain frequencies which MRI machines are capable of producing.
In space however, just like the inside of a molecule/isotope the magnetic field is balanced so the magnetometer reads nothing; so in fact all space could be magnetised and we would not know!
 

Offline evan_au

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Quote from: acsinuk
in fact all space could be magnetised and we would not know!
If you read the Wikipedia article on Voyager, you will see that every few months they use the gyroscopes to rotate the vehicle so that the 3 magnetometers (each pointing in a different axis) now line up where another magnetometer was previously measuring.

This allows them to recalibrate the magnetometers against each other, and more accurately detect the direction of magnetic fields in space. And they did detect changes in the direction of the magnetic field as they left the Solar Wind and entered interstellar space.

So if all space were magnetized, we would certainly know about it.
 

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