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alt="StartLayout 1st Row 1st Column negative bold-script upper E dot StartFraction partial-differential bold-script upper D Over partial-differential t EndFraction 2nd Column equals minus one half bold-script upper E dot StartFraction partial-differential Over partial-differential t EndFraction left-parenthesis epsilon 0 bold-script upper E right-parenthesis minus one half bold-script upper E dot StartFraction partial-differential Over partial-differential t EndFraction bold-script upper P minus one half StartFraction partial-differential Over partial-differential t EndFraction bold-script upper E dot left-parenthesis epsilon 0 bold-script upper E right-parenthesis minus one half StartFraction partial-differential Over partial-differential t EndFraction left-parenthesis bold-script upper E dot bold-script upper P right-parenthesis 2nd Row 1st Column Blank 2nd Column minus one half bold-script upper E dot StartFraction partial-differential Over partial-differential t EndFraction bold-script upper P plus one half bold-script upper P dot StartFraction partial-differential Over partial-differential t EndFraction bold-script upper E comma EndLayout"/>

      Grouping the first two, middle two, and last two terms of the right-hand side reformulates this relation as

      (2.61)StartLayout 1st Row 1st Column negative bold-script upper E dot StartFraction partial-differential bold-script upper D Over partial-differential t EndFraction 2nd Column equals minus one half bold-script upper E dot StartFraction partial-differential Over partial-differential t EndFraction left-parenthesis epsilon 0 bold-script upper E plus bold-script upper P right-parenthesis minus one half StartFraction partial-differential Over partial-differential t EndFraction left-bracket bold-script upper E dot left-parenthesis epsilon 0 bold-script upper E plus bold-script upper P right-parenthesis right-bracket 2nd Row 1st Column Blank 2nd Column minus one half left-parenthesis bold-script upper E dot StartFraction partial-differential Over partial-differential t EndFraction bold-script upper P minus bold-script upper P dot StartFraction partial-differential Over partial-differential t EndFraction bold-script upper E right-parenthesis comma EndLayout

      which, using again bold-script upper D equals epsilon 0 bold-script upper E plus bold-script upper P, becomes

      (2.62)negative bold-script upper E dot StartFraction partial-differential bold-script upper D Over partial-differential t EndFraction equals minus one half bold-script upper E dot StartFraction partial-differential Over partial-differential t EndFraction bold-script upper D minus one half StartFraction partial-differential Over partial-differential t EndFraction bold-script upper E dot bold-script upper D minus one half left-parenthesis bold-script upper E dot StartFraction partial-differential Over partial-differential t EndFraction bold-script upper P minus bold-script upper P dot StartFraction partial-differential Over partial-differential t EndFraction bold-script upper E right-parenthesis period

      Finally, grouping the first two terms of the right-hand side of this relation yields

      where w is the energy density, bold-script upper S is the Poynting vector, and upper I Subscript normal upper J and upper I Subscript normal upper K are the impressed source power densities, and upper I Subscript normal upper P and upper I Subscript normal upper M are the induced polarization power densities, respectively, which are defined by

      (2.66a)StartLayout 1st Row 1st Column w 2nd Column equals one half left-parenthesis bold-script upper E dot bold-script upper D plus bold-script upper H dot bold-script upper B right-parenthesis comma EndLayout

      (2.66b)StartLayout 1st Row 1st Column upper I Subscript normal upper J 2nd Column equals bold-script upper E dot bold-script upper J comma EndLayout

      (2.66c)StartLayout 1st Row 1st Column upper I Subscript normal upper K 2nd Column equals bold-script upper H dot bold-script upper K comma EndLayout