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Difference between Active Power and Reactive Power
The rate of work done in an electric circuit is known as electric power. In an electric circuit, there are three types of electric powers viz. active power, reactive power and apparent power. In this article, we will discuss the major differences between active power and reactive power.
What is Active Power?
In an AC or DC electric circuit, the amount of electric power which is utilized and consumed in doing a work is called active power. Active power is also known as real power or true power. Active power is usually represented by the letter ‘P’ and is measured in Watts (W). The larger units of active power are kW, MW, GW, etc.
Active power in an electrical circuit can be calculated by using the following formulae −

In a DC electric circuit −
$$\mathrm{\mathit{P\, \mathrm{=}\, V\times I}}$$

In a 1phase AC circuit −
$$\mathrm{\mathit{P\, \mathrm{=}\, V\times I\times \mathrm{cos}\, \phi }}$$

In a 3phase AC circuit −
$$\mathrm{\mathit{P\, \mathrm{=}\,\sqrt{\mathrm{3}}\times V\times I\times \mathrm{cos}\, \phi }}$$
Where, 𝜙 is the phase angle between voltage and current.
What is Reactive Power?
Reactive power is the part of total electric power which flows back and forth between the source and the load in an electric circuit. It is also known as useless power, as it does nothing in a circuit. Reactive power is only observed in AC circuits and not in the DC circuit, because in DC circuits, the imaginary circuit elements (capacitor and inductor) do not have any significance.
Reactive power is usually denoted by the letter ‘Q’ and is measured in voltamperes reactive (or VAr). The larger units of reactive power are kVAr, MVAr, etc.
Basically, the reactive power does not perform any work in the circuit, thus it is not consumed. However, it is also useful because it helps to produce the magnetic field in the electrical machines like motors, generators, etc.
The reactive power is given by the following expressions −

In 1phase AC circuit −
$$\mathrm{\mathit{Q\, \mathrm{=}\, V\times I\times \mathrm{sin}\, \phi }}$$

In 3phase AC circuit −
$$\mathrm{\mathit{Q\, \mathrm{=}\,\mathrm{\sqrt{3}}\times V\times I\times \mathrm{sin}\, \phi }}$$
Difference between Active Power and Reactive Power
The following table highlights the key differences between Active Power and Reactive Power −
Basis of Difference  Active Power  Reactive Power 

Definition  The fraction of total electric power which is used to do useful work and is consumed in the electric circuit is known as active power.  The fraction of total power which flows back and forth from source to load in the electric circuit is called reactive power. 
Alternate name  Active power is also known as real power or true power or useful power or wattfull power.  Reactive power is also called imaginary power or useless power or wattless power. 
Notation  Active power is usually denoted by letter "P".  Reactive power is usually denoted by "Q". 
Expression  $\mathrm{\mathit{P\, \mathrm{=}\, V\times I\times \mathrm{cos}\, \phi }}$  $\mathrm{\mathit{Q\, \mathrm{=}\, V\times I\times \mathrm{sin}\, \phi }}$ 
Unit of measurement  Active power is measured in Watts (W). The larger units of active power are kilowatt (kW), megawatt (MW), gigawatt (GW), etc.  Reactive power is measured in VAr (VoltAmpere Reactive). The larger units of reactive power are kVAr, MVAr, etc. 
Measuring instrument  A measuring device called wattmeter is used to measure active power in the electric circuit.  The reactive power in an electrical circuit is measured by using VAr meter. 
Direction of flow  The active power always flows in one direction, i.e., from source to load.  The reactive power flows in both directions between the source and load. 
Existence  The active power exists in both AC and DC circuits.  The reactive power exists only in AC circuits. 
Existence in pure resistive circuit  In a resistive circuit, the total power is equal to active power and is dissipated by the resistor in the form of heat.  There is no reactive power in case of a resistive circuit. 
Existence in pure inductive circuit  In a pure inductive circuit, the active power is zero, i.e., a pure inductor does not consume any power. It is zero because the phase angle between voltage and current in an inductive circuit is 90°, thus cos (90°) =0. Therefore, P = 0 Watts.  In a pure inductive circuit, there is lagging reactive power exist. It is because in the pure inductive circuit, the current lags the voltage by 90°. 
Existence in pure capacitive circuit  The active power is zero in a pure capacitive circuit. Which means the pure capacitive circuit does not consume any active power. It is because the voltage and current are 90° out of phase in a pure capacitive circuit.  There is leading reactive power in a pure capacitive circuit. It is because the current leads the voltage by 90° in a pure capacitive circuit. 
Function  Active power is the amount of power which does useful work in the electric circuit. It is the power which is converted into other forms of energy such as mechanical energy, light, heat, etc.  Reactive power does no useful work in the circuit. However, its function in the circuit is to produce the required magnetic and electric flux. 
Phase relation with voltage  Active power component is in phase with the voltage.  Reactive power component is 90° out of phase with respect to voltage. 
Applications  Active power is useful to produce torque in motors, light in lamps, heat in heaters, etc.  Reactive power is mainly used to measure the power factor of an electric circuit. It is also play a vital role in electrical appliances to produce required magnetic and electric field. 
Conclusion
From the above discussion, we can conclude that both active and reactive powers play a significant role in electrical circuits and machines. Active power is the true power which does some useful work in a circuit; whereas reactive power is the fraction of power which does not perform any useful work, but flows back and forth between the source and the load in a circuit.
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