Debye equation

The Debye equation (named after the Dutch physical chemist Peter Debye ) links the macroscopically measurable quantity permittivity with the microscopic (molecular) quantities electrical polarizability and permanent dipole moment : ${\displaystyle \varepsilon }$ ${\displaystyle \alpha }$ ${\displaystyle \mu }$

${\displaystyle P_{\mathrm {m} }={\frac {\varepsilon _{r}-1}{\varepsilon _{r}+2}}\cdot {\frac {M}{\rho }}={\frac {N_{\mathrm {A} }}{3\,\varepsilon _{0}}}\left(\alpha +{\frac {\mu ^{2}}{3k_{\mathrm {B} }T}}\right)}$

Are in it

• ${\displaystyle P_{\mathrm {m} }}$the molar polarization (its unit is that of a molar volume , e.g. m ³ / mol)
• M is the molar mass (in kg / mol)
• ${\displaystyle \rho }$the density (in kg / m ³ )
• ${\displaystyle N_{\mathrm {A} }}$the Avogadro constant
• ${\displaystyle \varepsilon _{0}}$the electric field constant
• ${\displaystyle k_{\mathrm {B} }}$the Boltzmann constant
• ${\displaystyle T}$the absolute temperature
• ${\displaystyle k_{\mathrm {B} }T}$the thermal energy .

The Debye equation combines the temperature-dependent orientation polarization (the summand with ) and the temperature un -dependent induced polarization (the summand with ). ${\displaystyle \mu ^{2}}$ ${\displaystyle \alpha }$

For non-polar substances (permanent dipole moment only induced dipoles ) the Debye equation changes into the Clausius-Mossotti equation . ${\displaystyle \mu =0,}$

Orientation polarization can no longer be observed even with a high-frequency change in the electric field (e.g. from the microwave range), since the relatively inert permanent dipoles can no longer follow the external field. In this case the Debye equation also changes into the Clausius-Mossotti equation.

literature

• Peter Debye: Polar Molecules . S. Hirzel, Leipzig 1929.

See also

• Cole-Cole diagram