426
8 Electric and Magnetic Phenomena
so that the total value of the dielectric constant of HCl obtained in this way is
ε(HCl) 1.0038,
at temperature T = 273 K. If we compare our calculated value with the value
ε(HCl) = 1.0039 given in the standard CRC tables (82nd edition, 2001–2002),
we see that our calculation indeed gives a quite reasonable result.
8.4 Magnetism, I. Paramagnetism
We are all familiar with magnetism in the form of stirrer bar-magnets employed
in almost any elementary chemistry laboratory, in the form of the ubiquitous
refrigerator magnets in our homes, or from patterns made by iron filings utilized
to show the interconnectedness of electricity and magnetism in a typical firstyear physics laboratory. These examples all involve materials that have permanent
magnetic moments. The nature of magnetic materials is, however, more complex,
as there are several distinct types of magnetic material, as characterized by their
behaviours in the presence of an external magnetic field, such as that produced
by a pole magnet. For example, if a sample of a particular material brought into
the magnetic field tends to be repelled by the field, it is said to be diamagnetic,
while if it tends to be attracted into the field, it is said to be paramagnetic if it is
only weakly attracted, and (normally) to be ferromagnetic if it is strongly attracted
(we are speaking here of orders of magnitude differences in the strengths of the
attractions). The examples employed to remind us of our familiarity with magnetism
all involved ferromagnetic materials, which are readily identified by their abilities
to retain their magnetic properties in the absence of an externally applied magnetic
field. Neither diamagnetic nor paramagnetic materials can be readily characterized
in the absence of an external magnetic field.
Unfortunately, however, the old adage that life is never truly simple also applies
to the classification of the magnetic properties of materials. Magnetic properties of
paramagnetic and ferromagnetic materials depend in general upon the temperature
of the system under consideration. Ferromagnetism is associated with the ability
of the constituent atoms/molecules to achieve and maintain alignment of their
(magnetic) dipole moments even in the absence of an external magnetic field. The
fundamental distinction between paramagnetic and ferromagnetic substances is that
in paramagnetism the magnetic moments of the constituent atoms/molecules do
not interact with one another, while in ferromagnetism, there is a strong nearestneighbour interaction. Nonetheless, in both cases, the increase in the thermal
motions of the individual constituents as the temperature is increased either
progressively destroys the induced alignment of the magnetic dipoles in a paramagnetic system or disrupts the neighbour-interactions in a ferromagnetic system. At
sufficiently high temperatures, a paramagnetic system becomes diamagnetic, while
a ferromagnetic system undergoes a phase transition at a critical temperature and
becomes paramagnetic.
8 Electric and Magnetic Phenomena
so that the total value of the dielectric constant of HCl obtained in this way is
ε(HCl) 1.0038,
at temperature T = 273 K. If we compare our calculated value with the value
ε(HCl) = 1.0039 given in the standard CRC tables (82nd edition, 2001–2002),
we see that our calculation indeed gives a quite reasonable result.
8.4 Magnetism, I. Paramagnetism
We are all familiar with magnetism in the form of stirrer bar-magnets employed
in almost any elementary chemistry laboratory, in the form of the ubiquitous
refrigerator magnets in our homes, or from patterns made by iron filings utilized
to show the interconnectedness of electricity and magnetism in a typical firstyear physics laboratory. These examples all involve materials that have permanent
magnetic moments. The nature of magnetic materials is, however, more complex,
as there are several distinct types of magnetic material, as characterized by their
behaviours in the presence of an external magnetic field, such as that produced
by a pole magnet. For example, if a sample of a particular material brought into
the magnetic field tends to be repelled by the field, it is said to be diamagnetic,
while if it tends to be attracted into the field, it is said to be paramagnetic if it is
only weakly attracted, and (normally) to be ferromagnetic if it is strongly attracted
(we are speaking here of orders of magnitude differences in the strengths of the
attractions). The examples employed to remind us of our familiarity with magnetism
all involved ferromagnetic materials, which are readily identified by their abilities
to retain their magnetic properties in the absence of an externally applied magnetic
field. Neither diamagnetic nor paramagnetic materials can be readily characterized
in the absence of an external magnetic field.
Unfortunately, however, the old adage that life is never truly simple also applies
to the classification of the magnetic properties of materials. Magnetic properties of
paramagnetic and ferromagnetic materials depend in general upon the temperature
of the system under consideration. Ferromagnetism is associated with the ability
of the constituent atoms/molecules to achieve and maintain alignment of their
(magnetic) dipole moments even in the absence of an external magnetic field. The
fundamental distinction between paramagnetic and ferromagnetic substances is that
in paramagnetism the magnetic moments of the constituent atoms/molecules do
not interact with one another, while in ferromagnetism, there is a strong nearestneighbour interaction. Nonetheless, in both cases, the increase in the thermal
motions of the individual constituents as the temperature is increased either
progressively destroys the induced alignment of the magnetic dipoles in a paramagnetic system or disrupts the neighbour-interactions in a ferromagnetic system. At
sufficiently high temperatures, a paramagnetic system becomes diamagnetic, while
a ferromagnetic system undergoes a phase transition at a critical temperature and
becomes paramagnetic.