3Active Power, Reactive Power and Power Factor

In this chapter we will consider the concept of electrical power as it applies to AC circuits. We shall see that unlike DC circuits, where the power dissipated can be found from the product of the voltage and current, in the AC case there are two forms of power to be considered, active power and reactive power, each of which must be generated somewhere within the AC network.

But before we begin, it is necessary to clearly distinguish between the concepts of power and energy. Energy (E) is added to a system the result of performing work on it; the unit of energy is the joule. For example, the energy stored within a capacitor is proportional to the square of its terminal voltage. To increase this energy, additional charge must be accumulated, and thus work must be done to increase the stored charge, against the force of the existing electric field.

Power is defined as the rate of delivery of energy, or the rate of doing work, and is therefore measured in joules per second, or watts. We can summarise this in the following simple equation:


Power is positive when work is done on a system and therefore its energy content increases, (e.g. increasing the charge on our capacitor) and negative when work is done by the system, which is accompanied by a loss of system energy (discharging our capacitor into a resistor).

Suppose a room is warmed ...

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