SummaryThe mechanical forces developed in electromagnetic systems are usually evaluated from surface integrals of the Maxwell stresses. For a large class of systems it is shown that the time average force can be expressed in terms of directly measurable circuittheory parameters. The system must be linear and loss free, but there is no restriction on the frequency of excitation.
A generalized adiabatic theorem is obtained for loss-free linear acoustic systems. The theorem includes the usual adiabatic or resonator action theorem (Boltzmann-Ehrenfest theorem) as a special case, but also includes results expressing radiation pressure forces or average radiation energy storage in terms of the phenomenological exterior behavior of the system. Some new interrelations between average energy storage and radiation pressure forces also follow from the theorem.
SummaryRadiation-pressure forces developed within an acoustic system may be calculated from a detailed knowledge of the fields. It is shown that the time average of a generalized radiation-pressure force may be expressed in terms of quantities on an arbitrary mathematical surface enclosing the system.The resulting surface integral is related to directly measurable circuit-theory parameters involving the impedance, admittance, or scattering matrices of the system. These results are the same as those obtained previously for electromagnetic systems (Smith 1961(Smith , 1964. The system must be linear and free of energy loss mechanisms but there is no limitation to the frequency of excitation.
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