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Power Equation

p=vi,(1.4)
where
p=the power in watts,v=the voltage in volts,i=the current in amperes.
Equation 1.4 shows that the power associated with a basic circuit element is
the product of the current in the element and the voltage across the element.
Therefore, power is a quantity associated with a circuit element, and we have
to determine from our calculation whether power is being delivered to the
circuit element or extracted from it. This information comes from correctly
applying and interpreting the passive sign convention .
If we use the passive sign convention, Eq. 1.4 is correct if the reference
direction for the current is in the direction of the reference voltage drop
across the terminals. Otherwise, Eq. 1.4 must be written with a minus sign. In
other words, if the current reference is in the direction of a reference voltage
rise across the terminals, the expression for the power is
p=−vi.
The algebraic sign of power is based on charge movement through voltage
drops and rises. As positive charges move through a drop in voltage, they
lose energy, and as they move through a rise in voltage, they gain energy.
Figure 1.6 summarizes the relationship between the polarity references for
voltage and current and the expression for power. 
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