Category : JEE Main & Advanced
On the basis of mutual interactions or
behaviour of various materials in an external magnetic field, the materials are
divided in three main categories.
(1) Diamagnetic materials :
Diamagnetism is the intrinsic property of every material and it is generated
due to mutual interaction between the applied magnetic field and orbital motion
of electrons.
(2) Paramagnetic materials : In
these substances the inner orbits of atoms are incomplete. The electron spins
are uncoupled, consequently on applying a magnetic field the magnetic moment
generated due to spin motion align in the direction of magnetic field and
induces magnetic moment in its direction due to which the material gets feebly
magnetised. In these materials the electron number is odd.
(3) Ferromagnetic
materials : In some materials, the permanent atomic magnetic moments have
strong tendency to align themselves even without any external field.
These materials are
called ferromagnetic materials.
In every unmagnetised
ferromagnetic material, the atoms form domains inside the material. Different
domains, however, have different directions of magnetic moment and hence the
materials remain unmagnetised. On applying an external magnetic field, these
domains rotate and align in the direction of magnetic field.
(4) Curie Law : The magnetic
susceptibility of paramagnetic substances is inversely proportional to its
absolute temperature i.e. \[\chi \propto \frac{1}{T}\]\[\Rightarrow \]\[\chi
\propto \frac{C}{T}\]; where C = Curie constant, T = absolute
temperature.
On increasing temperature, the magnetic
susceptibility of paramagnetic materials decreases and vice versa.
The magnetic susceptibility of
ferromagnetic substances does not change according to Curie law.
(5) Curie temperature \[({{T}_{c}})\] :
The temperature above which a ferromagnetic material behaves like a
paramagnetic material is defined as Curie temperature \[({{T}_{c}})\].
or
The minimum temperature at which a
ferromagnetic substance is converted into paramagnetic substance is defined as
Curie temperature. For various ferromagnetic materials its values are
different, e.g. for Ni, \[{{T}_{{{C}_{Ni}}}}={{358}^{o}}C\]for Fe,
\[{{T}_{{{C}_{Fe}}}}={{770}^{o}}C\]
for CO, \[{{T}_{{{C}_{CO}}}}={{1120}^{o}}C\]
At this temperature the ferromagnetism of
the substances suddenly vanishes.
(6) Curie-weiss law : At temperatures
above Curie temperature the magnetic susceptibility of ferromagnetic materials
is inversely proportional to \[(T-{{T}_{c}})\]
i.e. \[\chi \propto
\frac{1}{T-{{T}_{c}}}\]
\[\Rightarrow \] \[\chi
=\frac{C}{(T-{{T}_{c}})}\]
Here \[{{T}_{c}}=\] Curie
temperature
\[\chi -T\] curve
is shown (for Curie-Weiss Law)
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