Clausius–Mossotti relation

Clausius–Mossotti relation

The Clausius–Mossotti relation is named after the Italian physicist Ottaviano-Fabrizio Mossotti, whose 1850 book[1] analyzed the relationship between the dielectric constants of two different media, and the German physicist Rudolf Clausius, who gave the formula explicitly in his 1879 book[2] in the context not of dielectric constants but of indices of refraction. The same formula also arises in the context of conductivity, in which it is known as Maxwell's formula. It arises yet again in the context of refractivity, in which it is known as the Lorentz–Lorenz equation.

The Clausius–Mossotti law applies to the dielectric constant of a dielectric that is perfect, homogeneous and isotropic:[3]

 \frac{\epsilon - \epsilon_0}{\epsilon + 2\epsilon_0} \cdot \frac{M}{d} = \frac{4\pi N_A \alpha}{3}

where

Contents

Clausius–Mossotti factor

The Clausius–Mossotti factor can be expressed in terms of complex permittivities:[4][5][6]

K(\omega) = \frac{\epsilon^*_p - \epsilon^*_m}{\epsilon^*_p + 2\epsilon^*_m}
\epsilon^* = \epsilon + \frac{\sigma}{i\omega} = \epsilon - \frac{i\sigma}{\omega}

where

In the context of electrokinetic manipulation, the real part of the Clausius-Mossotti factor is a determining factor for the dielectrophoretic force on a particle, whereas the imaginary part is a determining factor for the electrorotational torque on the particle. Other factors are, of course, the geometries of the particle to be manipulated and the electric field.

Richard Feynman on the Clausius–Mossotti equation

In his Lectures on Physics (Vol.2, Ch32), Richard Feynman has a background discussion deriving the Clausius-Mosotti Equation, in reference to the Index of Refraction for dense materials. He starts with the derivation of an equation for the index of refraction for gases, and then shows how this must be modified for dense materials, modifying it, because in dense materials, there are also electric fields produced by other nearby atoms, creating local fields. In essence, Feynman is saying that for dense materials the polarization of a material is proportional to its electric field, but that it has a different constant of proportionality than for that of a gas. When this constant is corrected for a dense material, by taking into account the local fields of nearby atoms, you end up with the Clausius-Mosotti Equation.[7] Feynman states the Clausius-Mosotti equation as follows:

\mathcal N \alpha = 3\, \frac{n^2 - 1}{n^2 + 2}

Where:

  •  \mathcal N is the number of particles per unit volume of the capacitor
  •  \ \alpha is the atomic polarizability
  •  \ n is the refractive index

Feynman discusses "atomic polarizability" and explains it in these terms: When there is a sinusoidal electric field acting on a material, there is an induced dipole moment per unit volume which is proportional to the electric field - with a proportionality constant α that depends on the frequency. This constant is a complex number, meaning that the polarization does not exactly follow the electric field, but may be shifted in phase to some extent. At any rate, there is a polarization per unit volume whose magnitude is proportional to the strength of the electric field.

Dielectric constant and polarizability

The polarizability α, of an atom is defined in terms of the local electric field at the atom:

 \ \rho = \ \alpha E _{local}

Where:

  •  \ \rho is the dipole moment
  •  \ E _{local} is the Local Electrical Field at the atom

The polarizability is an atomic property, but the dielectric constant will depend on the manner in which the atoms are assembled to form a crystal. For a non-spherical atom, α will be a tensor.[8]

The polarization of a crystal may be expressed approximately as the product of the polarizabilities of the atoms times the local electric field:

Now, to relate the dielectric constant to the polarizability, which is what the Clausius-Mosotti equation (or relation) is all about,[8] you must consider that the results will depend on the relation that holds between the macroscopic electric field and the local electric field.

 P = \sum_{j}  N_j\ \rho_j = \sum_{j} N_j \alpha_j E _{local}(j)

Where:

  •  \ N_j is the concentration
  •  \ \alpha_j is the polarizability of atoms, j
  •  \ E _{local}(j) Local Electrical Field at atom sites  \ j

References

  1. ^ Mossotti, O. F. (1850). Mem. di mathem. e fisica in Modena. 24 11. pp. 49. 
  2. ^ Clausius, R. (1879). Die mechanische U’grmetheorie. 2. pp. 62. 
  3. ^ Rysselberghe, P. V. (January 1932). "Remarks concerning the Clausius-Mossotti Law". J. Phys. Chem. 36 (4): 1152–1155. doi:10.1021/j150334a007. 
  4. ^ Hughes, Michael Pycraft (2000). "AC electrokinetics: applications for nanotechnology". Nanotechnology 11 (2): 124–132. doi:10.1088/0957-4484/11/2/314. http://www.foresight.org/Conferences/MNT7/Papers/Hughes/. 
  5. ^ Markov, Konstantin Z. (2000). "Elementary Micromechanics of Heterogeneous Media". In Konstantin Z. Markov and Luigi Preziosi. 'Heterogeneous Media: Modelling and Simulation'. Boston: Birkhauser. pp. 1–162. ISBN 978-0817640835. http://www.fmi.uni-sofia.bg/fmi/contmech/kmarkov/pub/survey.pdf. 
  6. ^ Gimsa, J. (2001). "Characterization of particles and biological cells by AC-electrokinetics". In A.V. Delgado. Interfacial Electrokinetics and Electrophoresis. New York: Marcel Dekker Inc.. pp. 369–400. ISBN 0-8247-0603-X. 
  7. ^ Feynman, R. P., Leighton, R. B.; Sands, M (1989). Feynman Lectures on Physics. Vol. 2, chap. 32 (Refractive Index of Dense Materials), sec. 3: Addison Wesley. ISBN 0-201-50064-7. 
  8. ^ a b Kittel, Charles (1995). Introduction to Solid State Physics (8th ed.). Wiley. ISBN 047141526X. 

Wikimedia Foundation. 2010.

Игры ⚽ Поможем сделать НИР

Look at other dictionaries:

  • Formule De Clausius-Mossotti — La formule de Clausius Mossotti est une relation entre la polarisabilité microscopique α d un milieu diélectrique et sa constante diélectrique macroscopique ε. Elle s écrit, dans le système CGS : Cette relation provient de ce que le champ… …   Wikipédia en Français

  • Formule de clausius-mossotti — La formule de Clausius Mossotti est une relation entre la polarisabilité microscopique α d un milieu diélectrique et sa constante diélectrique macroscopique ε. Elle s écrit, dans le système CGS : Cette relation provient de ce que le champ… …   Wikipédia en Français

  • Formule de Clausius-Mossotti — La formule de Clausius Mossotti est une relation entre la polarisabilité microscopique α d un milieu diélectrique et sa constante diélectrique macroscopique . Elle s écrit, dans le système CGS : Cette relation provient de ce que le champ… …   Wikipédia en Français

  • Formule de Clausius-Mossoti — Formule de Clausius Mossotti La formule de Clausius Mossotti est une relation entre la polarisabilité microscopique α d un milieu diélectrique et sa constante diélectrique macroscopique ε. Elle s écrit, dans le système CGS : Cette relation… …   Wikipédia en Français

  • Liste Des Équations Et Formules — Ceci est une Liste des équations et formules par ordre alphabétique. Cette liste contient les équations, les formules, les relations et autres identités, égalités ou inégalités. Sommaire : Haut A B C D E F G H I J K L M N O P Q R S T U V W X …   Wikipédia en Français

  • Liste des equations et formules — Liste des équations et formules Ceci est une Liste des équations et formules par ordre alphabétique. Cette liste contient les équations, les formules, les relations et autres identités, égalités ou inégalités. Sommaire : Haut A B C D E F G H …   Wikipédia en Français

  • Liste des équations et formules — Ceci est une Liste des équations et formules par ordre alphabétique. Cette liste contient les équations, les formules, les relations et autres identités, égalités ou inégalités. Sommaire : Haut A B C D E F G H I J K L M N O P Q R S T U V W X …   Wikipédia en Français

  • Liste d'équations et formules — Ceci est une Liste des équations et formules par ordre alphabétique. Cette liste contient les équations, les formules, les relations et autres identités, égalités ou inégalités. Sommaire : Haut A B C D E F G H I J K L M N O P Q R S T U V W X …   Wikipédia en Français

  • Refractive index — Refraction of light at the interface between two media. In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum… …   Wikipedia

  • Polarization density — In classical electromagnetism, the polarization density (or electric polarization, or simply polarization) is the vector field that expresses the density of permanent or induced electric dipole moments in a dielectric material. The polarization… …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”