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3.1: Macroscopic versus Microscopic Forces

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    28131
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    Most important in this chapter is the distinction between forces on fundamental particles and forces on macroscopic media. It is common to speak of the "force on a charge" or the "force on a current"even though what is meant is the force on ponderable material. Interest might actually be in electric and magnetic forces acting on collections of fundamental charge carriers. (Motions of electron beams in vacuum are an example. The charged particles in that case constitute the continuum, in the sense that it is the electron inertia that enters into the equation of motion.) But, more commonly, the charged particles are imbedded in media, and it is the resulting force on the material that is of interest.Examples are as obvious as the electrical force of attraction between the capacitor plates of an electro-static voltmeter or the magnetic torque exerted on current-carrying conductors in a meter movement.

    Section 3.2 develops a specific model to illustrate how momentum imparted to charged particles by the fields is transferred to the neutral media that support those particles. That macroscopic forces are more than simply an average over the forces on fundamental charges is further emphasized by considering the practical cases of polarization and magnetization forces. Force densities of engineering significance exist even in regions where the free charge and free current (and for that matter polarization charge or magnetization charge) are absent. Such forces can be associated with a microscopic picture,discussed in Sec. 3.6, in which electrical forces on
    dipoles are transferred to the media.

    Although the dipole model is useful for forming a microscopic picture of electric polarization forces, it is restricted to cases where the dipoles do not significantly interact. In the pursuit of a less restricted force density, developments in Secs. 3.7-3.8 are based on such measured macroscopic parameters as the permittivity and
    permeability. It is the business of thermodynamics to convert that information into the desired force densities. In its own way, the line of reasoning presented in Secs. 3.5, 3.7 and 3.8 exemplifies a more basic point of view than one geared to a particular microscopicmodel. Thermodynamic concepts provide a means for replacing detailed and specialized derivations by carefully defined physical measurements.

    The stress-tensor representation of electromagnetic forces which concludes this chapter will see continual application in the following chapters. The tensor concept itself, introduced in Sec. 3.9, will also be applied to the formulation of continuum mechanical and electromechanical equations.


    This page titled 3.1: Macroscopic versus Microscopic Forces is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by James R. Melcher (MIT OpenCourseWare) via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.