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Power relation


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At any instant, the product of voltage instantaneous value and current instantaneous value is called instantaneous power, which is represented by symbol P




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Instantaneous power formula




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It can be seen from the above formula that the instantaneous power of the resistor changes with time. It consists of two parts, one is constant UI, the other is cosine function with amplitude of UI and frequency of 2 £s. The curve of P is shown in Figure B above. Because the U and I phases are the same, the instantaneous power is always positive, i.e. P 0, which means that the resistance always consumes power, and the resistance element always absorbs electric energy from the AC power source and converts it into other forms of energy, which is irreversible.




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Because the instantaneous power changes periodically with time, it is inconvenient to apply in engineering. Therefore, the average value of instantaneous power in a cycle is taken to express the magnitude of AC power, that is, the average power, which is expressed by P.




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Average power formula




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Because the average power is the power actually consumed by the resistor, it is also called the active power, and the unit is watt (W) or kilowatt (kw). If the rated power of the light bulb is 100W, it means that the active power consumed when it works is 100W.

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