By definition, power is the rate of doing work. In electricity, work is done by electrons moving within a conductor, and is the result of the electrical potential (V) and the flow rate of the electrons (amp). Thus, in an electrical circuit, power is the product of volts x amperes. The basic units of electrical power are the watt (W) and the kilowatt (kW). In equation form:
where P = power (watts); E = voltage (V); I = current (amp).
Problem: How much power is being produced when the current is measured at 5.0 amps and 120.0 V?
Power also may be expressed in two other ways. Ohm's law states that E = IR, and (power) P = E x I; therefore the E in the power equation can be replaced with IR from Ohm's law:
Note that the use of either of these equations requires knowledge of a value for the resistance. When the resistance and one of the other quantities are known, power can be determined.
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