Invalidation of the Kelvin Force in Ferrofluids

Direct and unambiguous experimental evidence for the magnetic force density being of the form $M\nabla B$ in a certain geometry - rather than being the Kelvin force $M\nabla H$ - is provided for the first time. (M is the magnetization, H the field, and B the flux density.)Comment: 4 pages, 4 figure

Electromagnetic Force in Dispersive and Transparent Media

A hydrodynamic-type, macroscopic theory was set up recently to simultaneously account for dissipation and dispersion of electromagnetic field, in nonstationary condensed systems of nonlinear constitutive relations~\cite{JL}. Since it was published in the letter format, some algebra and the more subtle reasonings had to be left out. Two of the missing parts are presented in this paper: How algebraically the new results reduce to the known old ones; and more thoughts on the range of validity of the new theory, especially concerning the treatment of dissipation.Comment: 10 pages, 0 figur

Electromagnetic Force and the Maxwell Stress Tensor in Condensed Systems

While the electromagnetic force is microscopically simply the Lorentz force, its macroscopic form is more complicated, and given by expressions such as the Maxwell stress tensor and the Kelvin force. Their derivation is fairly opaque, at times even confusing, and their range of validity all but a well kept secret. These circumstances unnecessarily reduce the usefulness and trustworthiness of some key quantities in macroscopic electrodynamics. This article presents a thorough yet pedagogical derivation of the Maxwell stress tensor and electromagnetic force in condensed media. It starts from universally accepted inputs: conservation laws, thermodynamics and the Maxwell equations. Simplifications are considered for various limits, especially the equilibrium, with a range of validity assigned to each expression. Some widespread misconceptions are scrutinized, and hidden ambiguities in popular notations revealed. A number of phenomena typical of strongly polarizable systems, especially ferrofluid, are then considered. In addition to enhancing the appreciation of these systems, it helps to solidify the grasp of the introduced concepts and derived formulas, and it demonstrates the ease with which the Maxwell stress tensor can be handled, inviting theorists and experimentalists alike to embrace this useful quantity.Comment: 27 pages, 7 fi

Applying GSH to a Wide Range of Experiments in Granular Media

Granular solid hydrodynamics (GSH) is a continuum-mechanical theory for granular media, the range of which is shown in this paper. Simple, frequently analytic solutions are related to classic observations at different shear rates, including: (i)~static stress distribution, clogging; (ii)~elasto-plastic motion: loading and unloading, approach to the critical state, angle of stability and repose; (iii)~rapid dense flow: the $\mu$-rheology, Bagnold scaling and the stress minimum; (iv)~elastic waves, compaction, wide and narrow shear band. Less conventional experiments have also been considered: shear jamming, creep flow, visco-elastic behavior and nonlocal fluidization. With all these phenomena ordered, related, explained and accounted for, though frequently qualitatively, we believe that GSH may be taken as a unifying framework, providing the appropriate macroscopic vocabulary and mindset that help one coming to terms with the breadth of granular physics.Comment: arXiv admin note: substantial text overlap with arXiv:1207.128