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Root-power (field) quantities

When referring to measurements of root-power quantities, it is usual to consider the ratio of the squares of F (measured) and F (reference). This is because the definitions were originally formulated to give the same value for relative ratios for both power and root-power quantities. Thus, the following definition is used:

The formula may be rearranged to give

Similarly, in electrical circuits, dissipated power is typically proportional to the square of voltage or current when the impedance is constant. Taking voltage as an example, this leads to the equation for power gain level L:

where V is the root-mean-square (rms) output voltage, V is the rms input voltage. A similar formula holds for current.

The term root-power quantity is introduced by ISO Standard 80000-1:2009 as a substitute of field quantity. The term field quantity is deprecated by that standard and root-power is used throughout this article.