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Power and Energy


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Power and energy calculations are important in circuit analysis. Although
voltage and current are useful variables in the analysis and design of
electrically based systems, the useful output of the system often is

nonelectrical (e.g., sound emitted from a speaker or light from a light bulb),
and this output is conveniently expressed in terms of power or energy. Also,
all practical devices have limitations on the amount of power that they can
handle. In the design process, therefore, voltage and current calculations by
themselves are not sufficient to determine whether or not a design meets its
specifications.
We now relate power and energy to voltage and current and at the same time
use the power calculation to illustrate the passive sign convention. Recall
from basic physics that power is the time rate of expending or absorbing
energy. (A water pump rated 75 kW can deliver more liters per second than
one rated 7.5 kW.) Mathematically, energy per unit time is expressed in the
form of a derivative, or
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