Category : JEE Main & Advanced
For ferromagnetic
materials, by removing external magnetic field i.e. \[H=0\]. The
magnetic moment of some domains remain aligned in the applied direction of
previous magnetising field which results into a residual magnetism.
The lack of retracibility as
shown in figure is called hysteresis and the curve is known as hysteresis loop.
(1) Retentivity : When H is reduced, I reduces but
is not zero when \[H=0\]. The remainder value OC of magnetisation when \[H=0\]
is called the residual magnetism or retentivity.
The property by virtue of
which the magnetism (I) remains in a material even on the removal of
magnetising field is called Retentivity or Residual magnetism.
(2) Corecivity or
corecive force : When magnetic field H is reversed, the
magnetisation decreases and for a particular value of H, denoted by \[{{H}_{c}}\],
it becomes zero i.e., \[{{H}_{c}}=OD\] when \[l=0\]. This value of H
is called the corecivity.
Magnetic hard substance
(steel) \[\to \] High corecvity
Magnetic soft substance
(soft iron) \[\to \]Low corecivity
(3) When field H is
further increased in reverse direction, the intensity of magnetisation attains
saturation value in reverse direction (i.e. point E)
(4) When H is
decreased to zero and changed direction in steps, we get the part EFGB.
Thus complete cycle of
magnetisation and demagnetisation is represented by BCDEFGB. This curve
is known as hysteresis curve
Comparison
between soft iron and steel
Soft iron | Steel |
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The area of hysteresis loop is less (low energy loss) | The area of hysteresis loop is large (high energy loss) |
Less relativity and corecive force | More retentivity and corecive force |
Magnetic permeability is high | Magnetic permeability is less |
I and \[\chi \] both are high | I and \[\chi \] both are low |
It magnetised and demagnetised easily | Magnetisation and demagnetisation is not easy |
Used in dynamo, transformer, electromagnet tape recorder and tapes etc. | Used for making permanent magnet. |
Property | Diamagnetic substances | Paramagnetic substances | Ferromagnetic substances |
Cause of magnetism | Orbital motion of electrons | Spin motion of electrons | Formation of domains |
Explanation of magnetism | On the basis of orbital motion of electrons | On the basis of spin and orbital motion of electrons | On the basis of domains formed |
Behaviour In a non-uniform magnetic field |
These are repelled
in an external magnetic field i.e. have a tendency to move from high
to low field region.
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These are feebly
attracted in an external magnetic field i.e., have a tendency to move
from low to high field region
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These are strongly
attracted in an external magnetic field i.e. they easily move from low
to high field region
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State of magnetisation | These are weekly magnetised in a direction opposite to that of applied magnetic field | These get weekly magnetised in the direction of applied magnetic field | These get strongly magnetised in the direction of applied magnetic field |
When the material in the form of liquid is filled in the U-tube and placed between pole pieces. |
Liquid level in
that limb gets depressed
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Liquid level in
that limb rises up
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Liquid level in
that limb rises up very much
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On placing the gaseous materials between pole pieces | The gas expands at right angles to the magnetic field. | The gas expands in the direction of magnetic field. | The gas rapidly expands in the direction of magnetic field |
The value of magnetic induction B | \[B<{{B}_{0}}\] (where \[{{B}_{0}}\] is the magnetic induction in vacuum) | \[B>{{B}_{0}}\] | \[B>>{{B}_{0}}\] |
Magnetic susceptibility \[\chi \] | Low and negative \[|\chi |\approx 1\] | Low but positive c » 1 | Positive and high \[\chi \approx {{10}^{2}}\] |
Dependence of \[\chi \] on temperature |
Does not depend on
temperature (except Bi at low temperature)
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On cooling, these
get converted to ferromagnetic materials at Curie temperature
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These get
converted into paramagnetic materials at Curie temperature
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Relative permeability \[({{\mu }_{r}})\] | \[{{\mu }_{r}}<1\] | \[{{\mu }_{r}}>1\] | \[{{\mu }_{r}}>>1\] \[{{\mu }_{r}}={{10}^{2}}\] |
Intensity of magnetisation \[(l)\] | I is in a direction opposite to that of H and its value is very low | I is in the direction of H but value is low | I is in the direction of H and value is very high. |
I-H curves |
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Magnetic moment (M) | Very low \[(\approx 0)\] | Very low | Very high |
Examples | Cu, Ag, Au, Zn, Bi, Sb, NaCl, \[{{H}_{2}}O\] air and diamond etc. | Al, Mn, Pt, Na, \[CuC{{l}_{2}},\,\,{{O}_{2}}\] and crown glass | Fe, Co, Ni, Cd, \[F{{e}_{3}}{{O}_{4}}\] etc. |
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