Magnetism in Matter

domainAni.gif (49123 bytes)
In this crude model of magnetic domains, each domain has a different magnetic orientation, so the substance as a whole is non-magnetic. When the material is placed in a magnetic field, the domains align in the direction of the field, thereby magnetizing the substance.
spinAni.gif (10256 bytes) As an electron revolves around the nucleus of an atom it also rotates on its own axis, giving it a magnetic spin
In this experiment a magnetic wire, connected in a circuit, is attached to a magnet above the circuit.  When the switch is thrown the wire heats up.  This causes the wire to and lose its magnetic properties so it falls away from the magnet.  When the switch is opened and the wire has cooled, it can again be attached to the magnet experimentAni.gif (6230 bytes)

Previously Asked Questions

Q:     Why does hitting a magnet with a hammer cause the magnetism to be reduced?

A:    Because hitting produces heat energy which would increase the mobility of atoms, increasing in this way the randomness of orientation of magnetic moments.  Macroscopic net magnetism is due to predominant orientation of magnetic moments along a certain direction.   Destruction of macroscopic net magnetism does not mean that individual atoms do not have a magnetic moment, but that the orientation of atomic magnetic moments is random.

Q:     If the North pole of a magnet points toward the North pole of the earth, doesn't this mean that poles of the same sign, the magnet's North pole and the earth's North pole, are attracting each other?

A:    The end of a magnetic needle of a compass that points toward the magnetic North pole of the earth is the South pole of the needle.  Usually it is marked with the letter N not because it represents the North pole of the needle but because it points toward the North pole of the earth.

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Diamagnetic materials:  materials which do not exhibit magnetism until they are place in an external magnetic field Bext.   Then they develop a magnetic dipole moment directed opposite Bext.   If the field is nonuniform, the diamagnetic material is repelled from regions of greater magnetic field.  The property is called diamagnetism.

Paramagnetic materials:  materials in which each atom has a permanent magnetic dipole moment mu2b.gif (57 bytes), but the dipole moments are randomly oriented and the material as a whole lacks a magnetic field.  However, an external magnetic field Bext can partially align the atomic dipole moments to give the material a net magnetic dipole moment in the direction of Bext.  If Bext is non uniform, the material is attracted to regions of greater magnetic field.   Theses properties are call paramagnetism.

Ferromagnetic materials: materials, in the absence of an external magnetic field, in which some of the electrons have their magnetic dipole moments aligned by means of a quantum physical interaction called exchange coupling, producing regions (domains) within the material with strong magnetic dipole moments.   An external field Bext can align the magnetic dipole moments of those regions, producing a strong net magnetic dipole moment for the material as a whole, in the direction Bext.  This net magnetic dipole moment can partially persist when Bext is removed.   If Bext is nonuniform, the ferromagnetic material is attracted to regions of greater magnetic field.  These properties are called ferromagnetism.   Exchange coupling disappears when a sample's temperature exceeds its Curie temperature, and then the sample has only paramagnetism.


Gauss' Law for Magnetic Fields:  The net magnetic flux through any (closed) Gaussian surface is zero.
        32-1.gif (229 bytes)

Bohr magneton constant:  32-5.gif (353 bytes)


Gauss' Law for Magnetic Fields 32-1.gif (229 bytes)
Spin magnetic dipole moment 32-2.gif (163 bytes)
Component (Sz) of S being measured along the z axis 32-3.gif (317 bytes)
Component of ( mu2.gif (832 bytes)s,z ) of mu2b.gif (57 bytes)s being measured along the z axis 32-4.gif (263 bytes)
Potential energy (U) associated with the orientation of the spin magnetic dipole moment in an external magnetic field 32-7.gif (232 bytes)
Association of the electron's orbital angular momentum (Lorb), with the orbital magnetic dipole momentum (mu2b.gif (57 bytes)orb) 32-8.gif (218 bytes)
Orbital angular momentum is quantized and can have only values given by 32-9.gif (495 bytes)
Magnitude of the orbital angular momentum 32-10.gif (316 bytes)
Potential energy (U) associated with the orientation of the orbital magnetic dipole moment in an external magnetic field Bext 32-12.gif (272 bytes)
Magnitude of the magnetization (M) of a paramagnetic material 32-18.gif (410 bytes)
Curie's Law 32-19.gif (195 bytes)
Maxwell's Law of Induction 32-28.gif (317 bytes)
Ampere-Maxwell Law 32-30.gif (394 bytes)
Displacement current due to a changing electric field 32-32.gif (215 bytes)
Ampere-Maxwell Law (rewritten) 32-33.gif (294 bytes)

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List of Topics

Measurements Electric Potential Magnetism Electrical Circuits (AC) Optical Instruments: Mirrors and Lenses
Electrostatics Capacitance Sources of Magnetic Fields Maxwell's Equations Interference
Electric Fields Current and Resistance Magnetism in Matter Electromagnetic Waves Diffraction
Electric Flux Electrical Circuits (DC) Electromagnetic Induction Interaction of Radiation with Matter: Reflection, Refraction, Polarization