Power and Energy


Power and Energy
Power is the time rate of expending or absorbing energy, measured in watts (W).
                                                              
Hence,

                                                              


The power p in Eq. is a time-varying quantity and is called the instantaneous power. Thus, the power absorbed or supplied by an element is the product of the voltage across the element and the current through it.
Current direction and voltage polarity play a major role in determining the sign of power.

Passive sign convention is satisfied when the current enters through the positive terminal of an element and p +vi (power absorbs). If the current enters through the negative terminal, p –vi(power supply ).

In fact, the law of conservation of energy must be obeyed in any electric circuit. For this reason, the algebraic sum of power in a circuit, at any instant of time, must be zero:

Energy is the capacity to do work, measured in joules (J).
                                            


The electric power utility companies measure energy in watt-hours (Wh), where 1 Wh = 3600 J


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