BUOYANCY FORCES IN MAGNETIC FLUIDS

The ponderomotive force on a macroscopic body in a magnetic fluid is calculated by a hydrodynamic approach. The resulting equations are generally valid, neither small susceptibilities nor stationarity are assumed. The simple and widely-used formula V(M-M(bg))del H is recoverd in linear order of chi; magnetostrictive effects are shown to contribute in the order chi3. The expressions derived here are definite and unambiguous, they do not depend on whether one starts from a theory in terms of H, or in terms of B: the correct evaluation of the contribution integral dV[ - delp] resolves the apparent contradiction between the force density expressions mu0 MdelH, or MdelB.