Electric power

Electric power

If a piece of electrical equipment is examined it will usually be found to have a label or engraved plate giving the working voltage and the power consumption in watts. For example, an electric lamp may be marked 240 V 60 W, or a fire 240 V 3 kW. The abbreviation kW stands for kilowatt (1 kW = 1000 W). By definition (page 82) a power of 1 watt is a rate of working of 1 joule per second, and therefore power in watts = rate of working in J/s V2t The three expressions, VIt, P Rt and R(as above) represent the work in joules done by the current in time t in seconds. Hence the wattage or power in J/s is found by dividing each expression by t.

Electric power
Electric power

The other two expressions may then be derived from this by substituting for either volts or amperes, using the formula
volts = amperes x ohms

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