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Lecture 46 – Magnetism of matter
•Review
•Magnets
•Magnetism and electrons
•Magnetic materials
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Lecture 46: Magnetism of Matter - Understanding Magnetic Materials and their Properties, Study notes of Physics
A part of a lecture series on magnetism of matter. It covers the sources of magnetism, different types of magnetic materials, and their responses to applied magnetic fields. The concept of magnetic domains and their alignment in 'soft' and 'hard' ferromagnets, which are commonly used in various technologies such as video tapes, hard drives, and credit card strips.
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Pre 2010
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Lecture 46 – Magnetism of matter
Review
Magnets
Magnetism and electrons
Magnetic materials
Review
Gauss’ Law for the Magnetic Field
Ampere - Maxwell law
Displacement Current
Maxwell’s Equations
A
d
B
B
r
r
enc
E
i t d d s d B
0
0
0
μ
ε
μ
r
r
t
d
d
i
E
d
0
Source of Magnetism?
What is the source of magnetism, if not magnetic charge?
Answer: electric charge in motion!
- e.g., current in wire surrounding cylinder (solenoid)
produces very similar field to that of bar magnet.
Therefore, understanding source of field generated by barmagnet lies in understanding currents at atomic levelwithin bulk matter.
Orbits of electrons about nuclei
Intrinsic “spin” ofelectrons (moreimportant effect)
Ferromagnets
Even in the absence of an applied B, the dipoles tend tostrongly align over small patches – “domains”.Applying an external field, the domains align to producea large net magnetization.
“Soft” ferromagnets
The domains re-randomize when the field is removed
“Hard” ferromagnets
The domains persist even when the field is removed
“Permanent” magnets
Domains may be aligned in a different direction by applyinga new field
Domains may be re-randomized by sudden physical shock
If the temperature is raised above the “Curie point” (
˚
for
iron), the domains will also randomize
Æ
paramagnet
MagneticDomains
1A
Which kind of material would you use in a video tape?
(a)
diamagnetic
(b)
paramagnetic
(c)
“soft” ferromagnetic
Example
(d)
“hard” ferromagnetic
Diamagnetism and paramagnetism are far too weak to beused for a video tape. Since we want the information toremain on the tape
after
recording it, we need a “hard”
ferromagnet. These are the key to the information age—cassette tapes, hard drives, ZIP disks, credit card strips,…
1B
How does a magnet attract screws, paper clips, refrigerators, etc., when they are not “magnetic”?
Example
The materials are all “soft” ferromagnets. The externalfield temporarily aligns the domains so there is a netdipole, which is then attracted to the bar magnet.
The effect vanishes with no applied
B
field
- It does
not
matter which pole is used.
End of paper clip
S
N