The portion of the total power that is successfully delivered to the load during one period of alternating current is called active power. It is equal to the product of the root mean square (RMS) values of current and voltage multiplied by the cosine of the phase angle between them.
In direct current (DC) circuits, the instantaneous and average power values over a given period coincide, and the concept of reactive power does not exist. In alternating current (AC) circuits, this occurs only if the load is purely resistive, such as an electric heater or an incandescent lamp. With such a load in an AC circuit, the voltage and current phases coincide, and all the power is transferred to the load (active power). In radio communication, this mode of operation is achieved in an untuned feeder when working with a matched load. The transmission line operates in the traveling wave mode.
If the load is inductive (transformers, electric motors), the current lags behind the voltage; if the load is capacitive (various electronic devices), the current leads the voltage. Since current and voltage are out of phase (reactive load), only a part of the total power that could be delivered to the load if the phase shift were zero (purely resistive load) is actually transferred to the load.
For non-sinusoidal current circuits, the electrical power is equal to the sum of the corresponding average powers of individual harmonics. Active power characterizes the rate of irreversible conversion of electrical energy into other forms of energy (thermal and electromagnetic). Active power can also be expressed in terms of current, voltage, and the active component of the circuit resistance or its conductance.
In any electrical circuit, whether with sinusoidal or non-sinusoidal current, the total active power is equal to the sum of the active powers of individual parts of the circuit; for three-phase circuits, the electrical power is determined as the sum of the powers of individual phases.
The unit of active power is the watt (W). For a microwave electromagnetic signal in transmission lines, the equivalent of active power is the power absorbed by the load.
Related articles: Reactive Power, SWR, TWR.