|MadSci Network: Engineering|
If you roll a 50 pound stone up a hill 5 feet, and take 10 seconds to do it, you have added 250 pound-feet of energy to the stone, and applied 25 pound-feet per second of power for 10 seconds. Power is the time rate of energy transfer. Calculus-savvy people would say power is the time derivative of energy. Direct Current (DC) came first; Alternating Current (AC) came later. In DC, you multiply voltage difference times amperes to find power. The energy is power times time in seconds. In calculus, energy is the time integral of power. For AC, there is an additional consideration. Sinewaves of voltage and current have amplitudes in volts and amperes, but the sinewaves need not peak at the same time. The phase angle difference between the peaks is called THETA. For historic reasons, the amplitudes of voltage and current are in a unit called root-mean-squared volts and amperes. Thus: Apparent Power = V*I True or Real Power = V*I*cosine(THETA) Reactive or Imaginary Power = V*I*sine(THETA) The electric watt-hour meter read by a utility is an electromechanical integrator that time-integrates the true power. Reactive power goes through the meter with no effect on the accumulated reading. But do not, by this fact, believe that utilities are not interested in reactive power and billing for its consequences. The electric company must size its transformers and other facilities based on current -- not power. Therefore (and there are ways) reactive power is often measured and taken into account for billing purposes. Hope this helps. Larry Skarin Power is to energy as speed is to distance. You can't transfer energy without having power just like you can't go a distance without having a nonzero speed in some direction.
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