ELECTROMAGNETIC ENERGY AND FORCES In this chapter we will consider the storage and transformation of energy in electromagnetic
In the evolution of physical theory, two of the most durable principles have been that
of conservation of charge and that of conservation of energy. Although the latter has in
this century been generalised to the concept of conservation of mass-energy, we are not
in this course concerned with mass-energy transformations, and our view of the principle
will be simply that of conservation of energy.
The principle of conservation of energy, and the fact that electromagnetic fields can
exert forces which can do work, inevitably leads to the idea that energy can be stored in
electromagnetic systems. Pursuing mathematical formulae for the amount of energy stored
leads to the discovery that very general expressions for that quantity can be developed
in terms of the field variables, rather than in terms of the distribution of sources. A
simple way of looking at this result is that the energy is stored within the field itself.
This viewpoint is very commonly adopted in the study of energy and power flow in
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