The Senftleben–Beenakker effect is the dependence on a magnetic or electric field of transport properties (such as

viscosity Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...

and

heat conductivity The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa and is measured in W·m−1·K−1. Heat transfer occurs at a lower rate in materials of low thermal conduct ...

) of polyatomic gases. The effect is caused by the

precession Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In o ...

of the (magnetic or electric)

dipole In physics, a dipole () is an electromagnetic phenomenon which occurs in two ways: * An electric dipole moment, electric dipole deals with the separation of the positive and negative electric charges found in any electromagnetic system. A simple ...

of the gas molecules between collisions. The resulting rotation of the molecule averages out the nonspherical part of the collision

cross-section Cross section may refer to: * Cross section (geometry) ** Cross-sectional views in architecture and engineering 3D * Cross section (geology) * Cross section (electronics) * Radar cross section, measure of detectability * Cross section (physics) ...

, if the field is large enough that the precession time is short compared to the time between collisions (this requires a very dilute gas). The change in the collision cross-section, in turn, can be measured as a change in the transport properties. The magnetic field dependence of the transport properties can also include a transverse component; for example, a heat flow perpendicular to both temperature gradient and magnetic field. This is the molecular analogue of the

Hall effect The Hall effect is the production of a voltage, potential difference (the Hall voltage) across an electrical conductor that is wikt:transverse, transverse to an electric current in the conductor and to an applied magnetic field wikt:perpendicul ...

and Righi–Leduc effect for electrons. A key difference is that the gas molecules are neutral, unlike the electrons, so the magnetic field exerts no

Lorentz force In electromagnetism, the Lorentz force is the force exerted on a charged particle by electric and magnetic fields. It determines how charged particles move in electromagnetic environments and underlies many physical phenomena, from the operation ...

. An analogous magnetotransverse heat conductivity has been discovered for

photons A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless particles that ...

and

phonons A phonon is a collective excitation in a periodic, Elasticity (physics), elastic arrangement of atoms or molecules in condensed matter physics, condensed matter, specifically in solids and some liquids. In the context of optically trapped objects ...

. The Senftleben–Beenakker effect owes its name to the physicists

Hermann Senftleben Hermann Senftleben (April 8, 1890, in Bremen – 1975 in Recklinghausen) was a German physicist and physical chemist. Education and life Senftleben was born in Bremen. After graduating from the König-Wilhelm-Gymnasium in Breslau, Senftleben st ...

(Münster University, Germany) and

Jan Beenakker Joannes Joseph Maria Beenakker (February 1, 1926, in Koog aan de Zaan – July 23, 1998, in Leiden), more often known as Jan J. M. Beenakker or Jan Beenakker, was a Dutch physicist and the rector of the Leiden University. Education and career B ...

(Leiden University, The Netherlands), who discovered it, respectively, for

paramagnetic Paramagnetism is a form of magnetism whereby some materials are weakly attracted by an externally applied magnetic field, and form internal, induced magnetic fields in the direction of the applied magnetic field. In contrast with this behavior, ...

gases (such as NO and O₂) and

diamagnetic Diamagnetism is the property of materials that are repelled by a magnetic field; an applied magnetic field creates an induced magnetic field in them in the opposite direction, causing a repulsive force. In contrast, paramagnetic and ferromagn ...

gases (such as N₂ and CO). The change in the transport properties is smaller in a diamagnetic gas, because the magnetic moment is not intrinsic (as it is in a paramagnetic gas), but induced by the rotation of a nonspherical molecule. The importance of the effect is that it provides information on the angular dependence of the intermolecular potential. The theory to extract that information from transport measurements is based on the Waldmann–Snider equation (a quantum mechanical version of the

Boltzmann equation The Boltzmann equation or Boltzmann transport equation (BTE) describes the statistical behaviour of a thermodynamic system not in a state of equilibrium; it was devised by Ludwig Boltzmann in 1872.Encyclopaedia of Physics (2nd Edition), R. G ...

for gases with rotating molecules). The entire field is reviewed in a two-volume monograph.Frederick R. W. McCourt, Jan J. M. Beenakker, Walter E. Köhler, and Ivan Kuščer, ''Nonequilibrium Phenomena in Polyatomic Gases'' (Oxford University Press, 1991).

References

External links

Historical remarks
on the experiment by Jan J. M. Beenakker.
Historical remarks
on the theory by Siegfried Hess (a student of

Ludwig Waldmann Ludwig Waldmann (June 8, 1913 in Fürth – February 9, 1980) was a German physicist who specialized in transport phenomena in gases. He derived the Waldmann-Snider equation.Hess, 2003 Career Waldmann completed his Ph.D. under Arnold Sommerfeld a ...

). {{DEFAULTSORT:Senftleben-Beenakker effect Gases

See also

References

External links